Sea as Biological Environment

8/10/2019 Sea as Biological Environment

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CHAPTERW

The Sea as a Biological Environment

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I nt hefor egoing ch apt er s a n a ccount ha s been given of t he ch em ica l

and physica l a speet sof the elemen ts tha t together cons t itu te the inorgan ic

ma rin e envir onmen t-n amely , (1) t he sea wa t er it self, a n d (2) t he ocean

floors. The chemica l const it uent s a nd t he physica l proper t ies of t he

sea w a t er , t oget her w it h t heir dist ribut ion , con cen tr a t ion s, a n d cy clic

cha nges, t he movement of t he w a t er , a nd t he na t ure of t he ocea n floors

a r e d ecisive fa ct or s in t he hw tor y a n d fa t e of a per plexing a r ra y of livin g

t hin gs. Her ein a r e h eld man y secr et e of r a cia l developmen t, a n d h er ein

must be sough t the underst and ing of the delica te ly ba l anced main tenance

of life and of t he pot en t ia l it ies of fu ture developmen t .

Ma rine orga nisms a re t o be considered a pa rt of t he sea a s it exist s

today.

J ust a s sea w a t er includes t he va rious sa lt s, bot h conserva t ive

a nd nonconserva t ive (biologica lly cha nged), so a lso it includes t he

mult it ude of orga nisms w hich a re bound t o t he sea for t heir exist ence

a n d wh ich, by or igin, a r e a pa r t of t he sea bot h r a cia lly a n d in dividua lly .

The orga nisms, like t he sa lt s, a re subject t o t he na tura l la w s of t he sea

a nd a re

a

pa r t of t he perpetua l cycle of inorgan ic and organ ic subs t ances

so import ant in ma ny a spect s of ocea nogra phy. The cha nges t ha t a re

a ppa r en t in concen tr a t ion r epr esen t only pa t t er ns t ha t a r e inh er en t in

t he ph a ses of t he cy cle or t ha t r esu lt fr om ot her ca u ses, such M cur ren ts

and processes of mixing.

The aqua t ic envir onmen t offer s t he g rea t es t in t imacy between it self

a n d t he or ga n ism swhich it ba t hes bot h over t he body sur fa ce a r id w it hin

open or pa r t ia lly closed cav it ies a s , for example, t he in terna l s ys tems of

coelent era t + echinoderms, a nd t unica t es. B eca use of t he st a bilit y of

t he ph ysica l ch a ra ct er ist ics of t he sea w a t er a n d of t he composit ion a n d

concen tr a t ion of t he d is solved s a lt s t h e or gan isms , in gener a l, h a ve not

developed highly specia lized int egument s a nd regula t ory sy st ems t o

prot ect t hemselves aga in st sudden and in t en se environmen t a l ch anges ,

ss ha ve most la nd a nima ls. I t follow s t ha t sma ll cha nges in t he a qua t ic

medium a re prompt ly brought t o pla y upon it a popula t ion. I t should

be borne in m ind, a lso, t h a t t he or gan isms t h emselves, bein g a pa r t of t he

dynamic envir onment , modify pa r ticula r ly it s chemica l cha r a ct er by

w ithdr aw ing or adding subs t ances a s socia t ed w ith the act ivit ies of life.

267


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268 THE SEA AS A BIOLOGICAL ENVIRONMENT

In subsequent cha pt ers w e sha ll discuss the rela t ion of some of

the measura bleenvironmenta l f actors to such phenomena a s dis t r ibut ion ,

pr opa ga t ion, surviva l, a nd specia l a da pt a t ions, but fir st some fa ct s of

genera l appl ica t ion must be considered .

Physicaland Chemical Characteristicsof the Marine Environment

Wa ter is essent ia l t o t he ma int ena nce of a ll life. I t const it ut es

80

per cent or more by weight of a ct ive protopla sm. I t is t he

most

e icient of all solvents

and ca r r ies in solu t ion the necessa ry ga ses , oxygen

a nd ca rbon dioxide, a s w ell a s t he miner a l subst a nces necessa r y t o t he

grow th of pla nt s a nd a nima ls, a nd it is it self one of t he essenfia l ra w

materials

in t he manufa ct ure of foods by pla nt s.

Organ isms living in the t er res t ria l environmen t have dev ised means ,

such a s imper vious in tegumen ts, t o con ser ve w a t er , a n d t he la n d pla n t s

ha ve root s a nd specia l va scula r syst ems for t ra nspor t of w a t er t o a ll

gr ow in g pa r t s. I n t he ma r in e envir onmen t t h er e is jr eedom from dessica-

kion,

except a t h igh-t ide levels , and therefore no Klghly specia l ized means

a r e pr ovid ed for con ser va t ion of w a t er or for it s t ra n spor t in pla n ts.

Also of biologica l impor ta n ce a r e t he high h ea t ca pa cit y of w a t er a n d

it s

high latent heat of

evapor a tion ,bot h of wh ich obvia t e t he da n ger t ha t

might result from ra pid cha nge of t empera t ure in t he environment a l

medium . Ow ing t o t he high degree of

transparency

of wa t er it is pos sible

for t he sea t o sust a in pla nt life t hroughout a rela t ively deep la yer , a nd

in a nima ls t he development of orga ns of vision a nd of or ient a tion ha s

prog res sed to a marked deg ree.

Sea w a ter is a bq ered solution; t ha t is, cha nges fr om a cid t o a lka line

condit ion, or vice versa , a r e r esist ed (p. 195). This proper ty is of vit a l

import ance t o t he -ma rine orga nisms, ma inly for t wo rea sons: (1) a n

abundant supply of ca r bon can be av a ila b le in t h e form of ca r bon d ioxide

for t he use of pla nt s in the synthesis of ca rbohydra tes w ithout dk-

turba nce to the a nima l life t ha t ma y be sensit ive t o sma ll cha nges in

pH , a nd (2) in the slight ly a lka line ha bit a t the ma ny orga nisms tha t

con st r uct shells of ca lcium ca rbon a t e (or ot h er ca lcium sa lt s) can ca r r y

on t h is fun ct ion much more efficien t ly t han in a neu tr a l s olu tion .

The suppor t offer ed t o t h e bod ies of ma r in e or gan isms by t he specific

grav ity of the surroundingmedium obvia t es the need of specia l support ing

skelet a l st ruct ur e in man y forms.

S tr ikhg examples of t hese a re t he

jelly fish es , una rmored mollu scs , una rmored d inofla g ella t es, a nd even

t he la r ge ma rin e mamma ls w it h t heir hea vy skelet on s, which could n ot

survive in t heir present bulky st a te except in a n a qua t ic ha bit a t. The

h a rd shells of cr a bs , clams, sna ils , a n d so on , doubt les s s er ve a s suppor t ,

especia lly in some bur row ing a n d in ter tid a l forms, but t hese ha r d pa r t s

ma y be looked upon a lso a s prot ect ive a nd a s a framew ork for a t t ach-

men t of muscles used in d igging, creeping, or sw imming.


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THE SEA AS A B1OLC3GICALENVIRONMENT

269

SEAWATERANDTHEBODY

FLUI S

S ea w a t er is a mos t a ppr opr ia t e

environment for liv ing cells , s ince it con t a ins a l l of the chemica l elemen ts

es sen t ia l t o t he g row th and ma in t enance of plan t and an ima l prot opla sm .

I t ha e been show n t ha t sea w a ter is a solut ion of a la rge number of sa lt s,

a nd it is import a nt her e t o consider how it is rela t ed a s a n ext erna l fluid

medium t o t he f int er na l medium

namely, the body fluids (blood, *

coelomic fluid, a nd so on) of the orga nisms. The ra t ios of the ma jor

sa lt s t o ea ch ot her , a nd usua lly t heir t ot a l con cen tr a tion a lso, a r e st r ik-

in gly sim ila r in sea w a t er a n d in t he body fluids of m a rin e in ver tebr a tes.

The s imila r i t y of compos it ion is not con fined to mar ine an ima l s, however ,

but is a lso in evidence in modified form in bot h t errest ria l a nd fresh-

w a t er a nima ls, including t he low er a nd higher ver tebr a tes, a s is shown

in t a ble 55, wh ich is fr om da t a compiled by P an t in (1931) a nd expan ded

by

Dakin

(1935).

OSMOTICRELATIONSHIPS I t is w ell know n tha t w hen solut ions of

d ifferen t osmot ic pres su re a re sepa r a t ed by a semipermeab le membrane

t ha t a llow s t he pa ssa ge of w a ter but not of t he solut es, t here is a move-

ment of t he w a ter t hrough t he membra ne int o t he more concent ra ted

solu t ion . The cell membranes of organ isms a re just such semipermeab le

membra n es t hrough which a movement of fluids occur s inw a rd or out -

w a rd, depending upon w hether t he osmot ic pressure of the ext erna l

medhm is less (hypokmic) or grea ter (hypertonic) tha n the int erna l

medium . The in ter na l a n d ext er na l media a r e isot on icwh en t hey a r e of

equal osmot ic pressure.

The osmot i c pressureof a solu t ion can be computed from the free~ ing-

poin t depression (p. 67). This comput a t ion is possible beca use t he

sa lt s t ha t increa se t he osmot ic pressure of a solut ion a lso depress it s

freez ing poin t . The freez ing-poin t depression below OC has been des ig-

na t ed by A@f (p. 67), but w ill here be a bbrevia t ed t o A. S ea w a ter ha v-

ing a sa hit y of 35.oo /00 freezes a t -1.91 , ow ing t o depression by t he

subst ances in solut ion. In ot her w ords, t he va lue for A is 1.91 . S imi-

la rly , w e obta in a A of 0.56 for huma n blood w ith a freezing point of

0.56C.

On the ba sis of A va lues, t he osmot ic rela t ions of t he body fluids

of ma rine a nd fresh-w a ter a nima ls t o t heir externa l environmenta l

med ium a re compa r ed in t a b le 56, fr om da t a compiled by Dakin (1935),

t o w hose review the rea der is direct ed for much grea ter detnil a nd

his tor ica l t rea tment .

F rom t he few examples in t he t a ble it is evident t ha t t he body fluids

of mar ine inver t ebr a t es a re isot on ic or nea r ly so w i th their flu id environ-

ment , w her ea s in t he fresh-w a t er forma t he body fluids a re hypert onic

t o the dilut e ext erna l medium. For this rea son the ma rine environ-

ment in it s osmot ic rela t ions fa ils t o exa ct of it s inha bit a nt s a s grea t a n

expendit ur e of ener gy in ma in t a in in g t he proper con cen t ra t ion of body


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270

THE SG4 AS A BIOLOGICAL ENVIRONMENT

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THE SEA AS A BIOLOGICAL ENVIRONMENT

271

fluids m dam the fresh-w a t er environment . The exa et i meoha rdsm

whereby the f resh-wa ter an ima ls a re independen t of the externa l med ium

a nd a re a ble t o ma int a in a

honzoiiowwtic

condit ion (t h a t is , st ea d y va lue

for A) in t he presence of t he hy pot onic w a t er is not known (see A for t he

eel

An@la anguilla

in fresh a nd sa lt w a t er , t a ble 56). Their exist ence

under t hese condit ions, how ever, req uires a const a nt expendit ure of

en er gy in elim in a t in g, t h rough t h e kidney s and ot h er excr et or y or gan s,

t he excess w a t er t a ken in by osmosis. Ma rine inver t ebra t es a re

poihil-

osmotic (A cha nging w ith tha t of t he ext erna l medium) only w it hin

ra t her na rrow lim it s (D a kin , 1935); hence, t hey, t oo, must ha ve some

regul ting mecha nism.

E xcept in est ua r ine condit ions, how ever, t he

ra nge of sa lin it y in m ost pa r t s of t he sea is perha ps w it hin t he limit s of

poikilosmot icit y of t he invert ebra t es living t here. F or example, t he

lugworm, Areniicota marina, in Helgola nd w a ters w it h a A1.72 ha s a n

in t er na l med ium Al. 7, bu t in t he B a lt ic Sea w ith a w a t er of AO.77 the

same species ha s a A va lue of 0.75 for t he in tern a l medium .

I t sh ould be men tion ed h er e t ha t t he t eleost (bony ) fish es in ma rin e

w a t ers a re defin it ely hypot onic a nd, t herefore, in order t o keep t heir

body fluid s down t o t he requir ed osmot ic pr essur e for t he species, t hey

secr et e ch lor id e t hr ough t he ch lor id e cells of t he gills (Key s, 1933).

Th is funct ion is a r egu la t ion t owa r d a low osmot ic pr ess ur eof t h e blood ,

a s opposed t o regula t ion t ow a r d a high on e a s per formed by t he kidn ey s

of a nima ls in fresh-w a ter environment s. Tha t t his group of a qua t ic

a n im a ls ha s a chieved a m a rked degree of independence of t he osmot ic

pr essu re of t he ext ern a l medium is eviden ced especia lly by such forms

a s t he sa lmon an d eel, bot h of wh ich, t hough pr a ct ica lly h omoiosmot ic,

spen d t heir lives pa r t ly in h ypot on ic a n d pa r t ly in h yper ton ic environ -

ments .

The ela smobr anch s-n amely , t h e sha rks and r ay sare is oton ic

w it h sea w a ter , but in t hese t he high osmot ic pressure of t he blood is

due not only t o t he presence of such sa lt s a s occur in sea w a ter , but a lso

t o high ur ea con ten t.

For fu rther d is cuss ion of s a lin it y t -isan envir on -

men t a l fa ct or , see a l so p. 839.

Other Characteristicsof the Environment

In a ddit ion to the chemica l a nd physica l proper t ies of sea w a t er ,

cer t a in ot h er biolog ica lly impor t a nt ch a r a ct er ist ics a r e in her en t in t h e

ma rine environment a s a w hole. These result from t he ma gnit ude of

t he ocea n it self, it s gr ea t dept h, a n d it s expa n se.

In consider ing t he ocea n in it s ent iret y a s a n environment , w e a re

a t fir st impr essed by t he w ide r a nges of living con dit ion s, t he sa lin it ies

va r y ing fr om those of d ilu t e estu a r ian wa t er s t o concen t r a t ion s of 37 0/00

or more in the open sea , t empera tures from 30*C t o freezing point ,

light in ten sit ies from brillia n t sunlight a t t he sur fa ce t o a bsolut e a nd

perpet usd da rkness in t he deeper la yers, a nd pressures from a single


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THE SEA AS A BIOLOGICAL

ENVIRONMENT

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273

a tmosphere a t t he sur fa ce t o a bout 1000 a tmospheres in t he grea t est

oceanic deeps.

Impres sive a s these r anges may be, never theles s ver y uniform condi-

t ions do preva il over ext ensive a rea s of t he. environment , a nd ma ny

orga nism s m a y, by rea son of t he monot ony of t hese ext ensive a rea s, be

ver y delica t ely a t t uned t o t h e pr eva ilin g unva r yin g condit ion s. Hen ce,

it follow s t h a t fa una I a r ea s ch a r a ct er ized by specific forms can be r ecog-

n ized . On th e ot h er h and, a w id er a nge of condit ion s may be en count er ed

in more r es t rict ed a r ea s , especia l ly in coz st a l r eg ion s. These condit ion s

ma y be due t o the physiogra phic cha ra ct er of t he coa st line, depth t o

bot tom, t opogra phy a nd na t ure of t he bot tom, inflow of la nd dra ina ge,

met eor ologica l condit ion s, a n d so for t h. Specia I ly a d apt ed and t oler an t

forms occu r h er e in pr ofu sion , for , a s w ill be shown in la t er ch apt er s, t h e

sha llow depths a nd va rying condit ions a re frequent ly fa vora ble t o

abundan t product ion of pr ima r y food .

I t must not be over looked t ha t t he gra dient s of sa lin it y, light , a nd

t empera t ure t ha t exist in t he sea a re fa vora ble t o a number of sensit ive

a nima ls t ha t possess t he a bilit y, t hrough sw imming or ot herw ise, t o

ad just themselves to opt imum cond it i ons.

DEPTHANDLIGHT Inherent in t he ver t ica l ra nge or dept h of t he

open-sea ha bit a t a re a number of import ant fea tures of fa r-rea ching

biologica l ef fect .

Of pr im e import a nce is t he rela t ively grea t ver t ica l

r ange of the euphot ic zone ava il ab le for product ion of f loa t ing microscopic

pla nt s. B ut the gra dient of light , both a s t o qua nt it y a nd qua lit y ,

result ing from dept h of w a ter a lso a llow s a djustment of ma ny a nima ls

t o the opt imum condit ion w ith respect t o th is fa ct or a nd, indeed, is

a ssocia t ed w it h diurna l migra t ions of ma ny forms t o light er or da rker

situations.

PRESSURE

P r essur e in it self does n ot exclude life fr om t he a by ssa l

regions of t he sea , for w a ter is but lit t le compressed a nd equilibr ium

exist s betw een t he inn er a n d out er pr essur e a ffect in g t he body t issu es.

However , pr es su remay lim it t h e ver t ica l r ange of mot ile forms, a l though

some euryba t hic a n im a ls a ppa rent ly a re not ser iously a ffect ed a nd a re

known t o make da ily ver tica l w a n der in gs of up t o 400 m , cor respon din g

t o pressure va ria t ions up t o 40 a tmospheres. H arpooned w ha les a re

sa id t o sound t o a dept h of 800 m, a nd t he sperm wha les must descend

norma lly t o grea t dept hs, since t he la rge sq uids upon w hich t hey feed

in ha bit ver y deep w a t er .

WATER MOVEMENTS The sea must be view ed a s a n environment

t ha t for t he most pa r t is in con st a n t mot ion w it h bot h r egula r a n d ir regu-

la r pa t t er na of flow . The pr incipa l b iolog ica l benef it s der ived from the

circula t ion a re (1) oxygena t ion of subsurfa ce w a ter , (2) dhpersa l of

w a st es result ing from processes of m et a bolism , (3) disper sa l of pla n t

nut rient s a nd ot her va ria ble element s essen t ia l t o pla nt a nd a nima l


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274


grow t h, a nd (4) d~ persa l of spores, eggs, la rva e, a nd a lso ma ny a dult s.

On t he whole, t he circula t ion of w a t er is of direct benefit , y et in st a nces

may be not ed wher e some adver se condit ion s r esu lt eit h er a s in cid en t a l

or a s perma nent fea tures. Incident a l disturba nces ma y be due t o

unsea s on able s hift s in t h e r egula r cu rr en t sy st em , such a s give r is e t o t h e

a ppea ra nce of E l Nifio off t he west coa st of South Amer ica (p.

704). In t his inst a nce, w a rm w a ter of t he E qua t or ia l C ount ercurrent

is ca rr ied sout hw a rd a long t he coa et s of E cua dor a nd P eru, w hich a re

norma lly ba t h ed by cold cu rr en t s. The r esu lt is a wholesa le dest r uct ion

of a nim a l life a long t he coa st , including ma ny gua no birds t ha t depend

upon th e sea for food .

P ermanen t or semipermanen t fea t u res of cur ren t

sy st em s t ha t t a ke a regula r t oll of life a re found where t he moving w a t er

ca rr ies t he inha bit a nt s int o a rea s of less fa vora ble living condit ions.

For example, G ulf S t rea m inha bit a nt s ult ima tely per ish a s they a re

sw ept nor thw a rd into regions w here the t empera ture of t he w a t er is

low ered by a dmixt ure of cold w a ter or by cooling in higher la t it udes.

La rva e of ner it ic forms a re freq uent ly dispersed t o offshore or ot her

loca t ions uninhabit ab le to the adu lt an ima ls . Sur facecur ren t s somet imes

st rew t he shores w it h defunct bodies of norma lly ocea nic or offshore

forms such as the coelen t er a t e

Velte2a

or t h e pela g ic sn a il J a n t hin a .

EXTENT OF THE MARINE ENVIRONMENT

Tha t pa r t of t he ea rth

w hich is ca pa ble of sust a in ing life, bot h pla nt a nd a nima l, is know n a s

the Mosphere. The biosphere is subdiv ided in to three pr incipa l d iv is ions

or ha bit a ts know n a s Mocyct es.

Th ese a r e t he t er rest r ia l, t he ma rin e,

a nd the fresh-w a t er biocycles. E ach ha s it s cha ra ct er ist ic t ypes of

ecologica l fea tures a nd a ssocia t ions of pla nt s a nd a nima ls. A few

a nima l species ma y a t t imes migra te freely from one t o a not her , a s is

w it n essed especia lly by t h e sa lmon or t h e eel.

The ocea ns cover some 71 per cent of t he ea rt hs sur fa ce. Thus,

t he a rea of t he ocea ns is a bout tw o a nd one ha lf t imes the a rea of t he

la nd, but , w hen consider ing t he spa ce in w hich life m igh t conceiva bly

exist , a ccount ha s t o be t aken of t he rela t ive ver t ica l ra nge provided

by t he t w o ma in environment s, t he t er rest ria l a nd t he ma rine. On t his

ba sis it is est im a ted (H esse, Allee, a nd S chm idt , 1937) t ha t t he m a rine

environmen t actua l ly prov ides abou t th ree hundred t imes the inhab it ab le

space provided by the t er res t ria l and the fr esh -wa t e r b iocycles t ogether ;

for , wher ea s the t er res t ria l env ir onmen t prov ides space on ly in a sha l low

zone ma inly a t t he immedia t e surfa ce a nd t o a dept h of a few feet a t t he

mos t , t h e mar ine hab it a t pr ov ides liv able space for

at

lea s t some form of

life from the surfa ce even t o the a byssa l dept h of severa l miles. The

fr esh -w a t er biocy cle con st it ut es on ly a sma ll fr a ct ion of t he ot her tw o.

The a er ia l por t ion of t he globe is not properly considered a sepa ra te

biocycle, since ent ra nces int o it by birds, insect s, a nd so for t h ma y be

considered main ly as temporary journeys.


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THE SEA AS A BOIQGKJAL

Ow ing t o the Wieult ies a t t enda nt on

ma r in e blocy cle is t h e I ea et known of a ll.

ENVIRONMENT

275

the study of t he ocea ns, t he

Classification

of

the Marine Environment

In order t o fa cilit a te a study of the ma rine environment a nd it s

ir ihab lt an te ,the former may}e conven ien t ly d iv ided broad ly in to pr imary

and secondary biot i c dkis ions based upon physica l -chemica l a t t r ibutes or

upon the na ture of the blot s.

Th e bounda r ies between t hese biot ic

Fig. 67. Thema indivisionsof th ema rineenvironment.

diviions, w hich a re dia gra mma tica lly show n in fig. 67, a re in some

in st a nces well defin ed, but mor e fr equent ly t her e is a good dea l of over -

la pping. Thus, a lthough t he pr ima ry divisions a re definit ely set off

fr om ea ch ot her on phy sica l ba s es, a n d t he t ypica l subdivision s of t hese

ha bit a ts ca n be clea r ly recognized bot h blot iea lly a nd a biot icdly , yet

there are no well-defined boundar i es between them.

The t w o prima ry division s of t he sea a re t he

ertthi

a nd t he pek@.

The former in cludes a ll of t he ocea n floor , wh ile t he la t t er in cludes t he

whole ma ss of w a t er .

THE BENTHICBIOTIC ENVIRONMENTAND

ITS SUB DIVIS IONS.

This

divhion in cludes a ll of t he bot t om t ei?m in fr om t he wa ve-w a shed sh or e

line

at

flood -t id e level t o t he gr ea t es t d eeps. I t s uppor t s a ch a r a ct er is tic

type

of life t ha t not only lives upon but cont r ibut es t o a nd ma rkedly

modifies the cha ra ct er of t he bot tom. E kma n (1935) discusses the

bounda r ies of t he ver tica l zones fr om a zoogeogr a phic st a ndpoin t, a nd

we follow ma in ly t he scheme employed in h is t ext .


10/19

276


The bent hic division ma y be subdivided int o t wo ma in syst ems

namely , t h e littoral a nd t he deep-sea s@ems. The div id ing line between

t hese h a s been set a t a dept h of a bout 200 m on t he a r bit ra r y supposit ion

t ha t t his represent s t he a pproxima t e dept h of w a ter a t t he out er edge

of t he con tinent a l sh elf (p, 20), a n d, roughly, a lso t he dept h sepa r a t in g

the light ed from the da rk port ion of the sea . The lit t ora l syst em is

subcWded int o t he eulittoral a nd the wblittorat zones.

The deep-sea

syst em is divided into a n upper (archibenthic) a nd a low er (abyssal-

benthic) zone.

The lim it s of t he ben t hic subd ivision s a r e h a rd t o defin e,

and are var iously placed by d ifferen t au thor s because uniform boundar ies

t ha t w ill fit a ll req uirement s ca nnot be dra w n. F or genera l biologia d

s tud ies , the d ifferen t boundar ies must be based on the pecu lia r i t iesof the

endem ic pla nt a nd a nim a l dist ribut ion a nd should follow t he region of

mos t d is t in ct f auna l and flor a l ch ange.

The biot i c zones thus delinea t ed

w ill be ch a ra ct er ized by a mor e or less clea r ly defin ed ra n ge of ext er na l

ecolog ica l fa ct or s wh ich have given cha r a ct er t o the popu la t i on .

The eulit tora l zone ext ends from t he high-t ide level t o a dept h of

a bout 40 to 60 m. The low er border is set roughly a t t he low est limit

a t w hich t he more a bunda nt a t ta ched pla nt s ca n grow . The sublit tora l

zone ext ends from t his level t o a dept h of a bout 200 m , or t he edge of t he

con tin en t a l sh elf. The d ividin g lin e bet ween t h ese subd ivision s va r ies

gr ea t ly between ext r emes, sin ce it is d et ermin ed by penet r a t ion of ligh t

su fficien t for phot osynt h esis. I t w ill be r ela t ively sh a llow in t h e h igh er

la t it udes a nd deep in the low er la t it udes. In the upper pa r t of t he

eu lit t or a l zon e a r ela t ively well-d efin ed t id a l or intertidal zone tha t is

bounded by t he high- a n d low -w a t er ext remes of t he t ide is r ecognized.

~ S ome a uthors confine t he eulit tora l zone t o t his na rrow sect ion a nd

con sid er t he sublit t or a l t o begin a t t h e low -t id e level (cf. G islen , 1930).

The ver t ica l r a nge of t he in t er t id a l zon e, t hough r a t her well d efin ed for

a ny given a r ea , va r ies grea t ly in cWferent sect ion s of t he wor ld , for it is

d et ermin ed by t he t id a l r a nge (see ch apt er XIV). I n t h e upper r ea ch es of

the B a y of Fundy the zone ma y ha ve a ver t ica l ra nge of over 15 m,

w hile in the G ulf of Mexico it is less tha n 0.7 m, a nd in a rea s like the

Med it er ranean a l ong the sou thwes t coa s t of I t a ly the r ange is yet sma l ler ,

only 10 t o 30 cm. On exposed coa st s subject ed t o direct ocea n w aves

a nd sw ells t he upper ra nge is somew ha t ext ended t o include a ra ther

we ll-defined supra t ida l spr ay zone wi th a spar se popu la t ion of especia l ly

resist a nt form s among which a few a nima ls, such a s t he isopod Lig~da,

a ppea r t o be in the process of becoming t errest r ia l in ha bit . Ma ny

species of a nima ls a re found only in the t ida l zone a nd ma y be limit ed

ver t ica lly in ma ximum dist ribut ion even t o cert a in levels w it hin t he

zone-for example, Ligpda a nd t he ga et ropods Ldtorina scutalata, L.

planaxis, Acrnaea digitalis, a nd others found a t Monterey B a y only

a bove t he 0.76-m t ida l level (H ew a tt , 1937). Thus, in t he t ida l zone


11/19

THE SEA AS A BIOLOGICAL ENVIRONMENT 277

in wh ich t h e r ange in ext er na l fa ct or s is gr ea t est we fin d a mor e r est r ict ed

ver t ica l ra nge of specific a n ima ls.t ha n is obvious a nywhere else in t he

ben thic r eg ion of the sea .

Many mot i le an ima ls , f or example crust a ceans

a nd fishes, move regula r ly int o t he in ter t ida l zone t o feed during high

t ide, a nd t he sm a Il pela gic fish known a s grunion m igra t e int o t he zone

dur in g cer ta in high spr in g t id es t o d eposit t heir eggs in t he sa n d.

The eu lit t or a l zon e gives r is e t o many biot opes, for it is gr ea t ly va r ied

a s t o t ype of subst ra t umfor example, rocky, sa ndy, or muddya nd

a lso a s t o ch a ra ct er of sh or e lin e a n d degree of exposur e. Th e over ly in g

w a ter ma y be slight ly or grea t ly reduced in sa lin it y. These va r ia t ions

a re direct , decisive fea tures cont rolling t he t ype a nd a bunda nce of

ses sil el it t or a l forms (cf. Shelford et a l , 1935). The plen t ifu l pr imary food

in t h is zon e is derived fr om bot h pela g ic a n d a t t a ch ed pla n t s.

At t empt s t o est a bliih such zones a s F ucus zone, L amina r ia n zone,

a n d so on , ba s ed on t h e depth s a t wh ich t h ese plan t s a r e ch a r a ct er ist ica l ly

a t t a ched, ha s the disa dva nt a ge tha t t he pla nt s a re very frequent ly

absen t a l ong vtw t s t ret ches of t h e coa s t , ow ing to unfavor able subs t ra tum

or ot her ecologica l fa ct or s; n ever th eless, such cla ssifica t ion ma y be of

useful loca l a ppl ica t ion .

Though the bounda ry betw een the sublit t ora l a nd the deep-sea

syst ems is set a t a dept h of 200 m, E kma ns compila t ions ba sed on t he

fauna in dica t e t h a t in mos t r egion s t h e bounda r y maybe loca t ed between

200 a nd 400 m. Light a nd t empera ture a re import an t fa ct ors, a nd in

high la t it ud es t hese fa ct or s oper a t e t oget her t o sh ift t he bounda r y in to

sha l lower wa ter .

The upper d ivision of t he d eep-sea sy st em is ca lled t he archibnthic, .

a

word in tr oduced by Alexa n der Aga ssi~ , but t he t erm is un fort un a te in

t ha t it implies t he beginning of t he bent hos from t his region. The zone

is a lso ca lled t he

coru w l deep-sea zone,

but t his gives r ise t o grea t er

confusion, since t he t erm cont inent a l fa una som et imes used must

inolude

also

the li t t or a l fauna un les s specifica l ly ca l led con t inenW- lope

or deep-sea fa una . The urchibenthic zone ext ends fr om th e sublit t or a l t o

a dept h bet w een 800 a nd 1100 m .

The aby=ssal-benthiczone compr is es a l l of t h e deep-sea ben thic s ys t em

below t he a rchibent hic zone. I t is a region of rela t ively uniform condi-

t ions. Tempera tures a re uniformly low , from 5 t o 1 C , a nd sola r

light is wan t in g.

There are no sezsons, and hence the seasona l b iolog ica l

ph en omen a a s socia t ed w it h t he lit t or a l zon e a r e suppr essed. S t a gn a nt

con dit ion s do n ot pr eva il in t he open ocea n , however, for t her e is ample

circula t ion t o supply w ell-a era t ed w a t er result ing from deep ver t im d

movemen ts in t he high la t it udes (p. 138). No pla n ts a re produced, a nd

t he ext ent t o w hich a ut ot rophlc ba ct er ia pla y a pa r t in t he m a nufa ct ure

of food is not know n. The a nima ls a re ca rnivorous, feeding ma inly

upon orga n ic d et r it us wh ich in it s in it ia l or ga n ic st a t e must ha ve or igi-


12/19

278


na ted in the pla nt s of the surfa ce w a ters. The a byssa l zone, though

not sha rply ma rked off a t it e upper limit s from t he a rchibent hic zone,

h a s it s own cha r a ct er is t icpopu la t ion , a s w i ll be brough t ou t in a follow ing

chapter.

The berAhk environment from shore sea w a rd t o a byssa l dept hs is

cover ed , t o a gr ea t er or les s d egr ee, by sedimen ta r y d epos it s t h a t may be

cla s sified a s t er r ig enous depos it s , or gan ic or pela g ic oozes , and red cla y .

A det a iled discussion of t he deposit s w ill be found in cha pt er XX, a nd

t he na t ure of t he dist ribut ion is shown in fig. 253. As fa r a s t he biology

of bent hic a nima ls is concerned, t he most impor t ant fea tures of t hese

oozes are the ir phys ica l cons is tenciesa nd the amount of d iges t ib le organic

ma ter ia l they cont a in. Most deep-sea benthic forms a re det r it us

ea t ers and mainly dependent , therefore, upon the ra in of pelag ic organisms

tha t fa l ls t o t he bot t om . The product ion of pela g ic food usua lly decrea ses

ma rked ly w i t h increa s ingd is t ance fr om the coa s t , a nd the amount r ea ch-

ing the bot t om in a rea s of very deep w at er is fur ther reduced by it s

d is in tegra t ion whi le s inking .

H ence, t he lit tor a l muds a re most r ich in

food, a nd t he red cla y a t grea t dept hs a nd fa r from shore is t he poorest .

Th is differ en ce is r eflect ed in t h e number of a n ima ls a ct ua lly collect ed

fr om d iffer en t a r ea s ( f. p.

806).

THE PELAGICENVIRONMENTAND ITS SUBDIVISIONS

The pela g ic

division in cludes a ll of t he ocea n wa t er s cover in g t he ben th ic division .

Hor izonta l ly , the pelag ic d iv is ion is subdivided into an open-sea (oceanic)

province, and an inshore (ner it ic) province.

Vert ica lly , t he ocea nic province ha s a n upper lighted zone a nd a

low er da rk zone w it h no w ell-ma rked bounda ry bet ween t he t w o, F or

convenience t he bounda ry is a rbit ra r ily set a t 200 m , since t his w ould

cor respond w it h t he a r bit ra r ily set dept h for t he edge of t he con tin en ta l

shelf a nd a t t he sa me t ime pla ce t he lit t ora l syst em and the nerit ic

pr ovin ce in a r ea s defin it ely w it hin t he ligh ted por tion . Act ua lly, ligh t

cha nges gra dua lly in both qua nt it y a nd qua lit y from the very surfa ce

dow nw a rd t o dept hs w here it is no longer detect able (p. 82), a nd th is

depth va r ies w i t h la t i t ude, s ea son , amount of suspended ma t er ia l , living

or dea d, a nd t herefore a lso w it h dist a nce from shore. Th ese va r ia bles

of t he pela g ic envir onmen t a r e of profound impor t ~nce t o t he popu la t i on

of t he sea , a s w ill be point ed out la t er .

The out st a nding fea tures of t he ocea nic province a re the broa d

spa tia l expa nses a nd the grea t ra nges of depth . As dist inguished

fr om t he n er it ic pr ovin ce t he wa t er s a r ea s a r ule ver y t ra n spa r en t, w it h

lit t le or no det r it u s of t er r es t ria l or ig in . These wa t er s a r e predominan t ly

blue in color a nd support t he blue surfa ce fa una t o be discussed more

fu lly in chapt er XVI I .

Although sola r l ight penet ra tes rela t ively deeper

tha n in inshore w a t ers, t he grea t depth of the w a ter included in th is

province resu lt s in complete el imina t ion of sola r l ight b the deeper por t ion


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THE SEA AS A BIOLOGICAL E?WRONMENT 279

of t he pr ovin ce, a nd a s a r esult on ly ca r nivor es a n d det rit us feeder s oa n

exist in t he ver y deep la y er s.

The chem ica l compos it ion of t he offshor e wa t er is r ela t ively st a ble.

S aEn it y is un iformly h igh , w it h on ly sma l l f lu ct ua t ion s in spa ce and t ime

(p, 123), a nd pla n t nut rien ts a r e fr eq uen tly r ela t ively low in t he upper

la y er a nd on ly slow ly r epla ced.

The ver t ica l bor der sepa r a t in g t h e ner it ic pr ovin ce fr om the ocean ic

is set a t the edge of the cont inent a l shelf; hence a ll w a t er of U epths

sha llow er tha n 200 m w ould fa ll w ith in the ner it ic province, w hich

a ccor din gly ma y ext en d fa r seaw a r d in in st a nces wher e t he con tin en ta l

sh elf is br oa d, a s off t he ea st coa st of t he Un it ed S t a tes, or be ver y n a rr ow ,

a s off t he w est coa st of S out h Amer ica .

Although biolog ica l ly and chemica l ly the border between the ocean ic

a nd t he ner it ic provinces is not st rict ly defina ble, yet a s w e a pproa ch

the coa st the pla nt a nd a nima l life t akes on cha ra ct er ist ics not found

in t he t ypica lly ocea n ic pr ovin ce wher e blue-sea forms pr eva il. The

chem ica l const it uen ts of t he sea w a t er in t he nerit ic province a re more

va ria ble t ha n in t he ocea nic. S a lin it ies a re usua lly low er , somet imes

ma rked ly , and undergo sea sona l or spor ad ic flu ctu a t ions such tha t many

of t he inha bit ant s a re more or less euryhdine in na ture-t ha t is , a ble t o

endure w ide ra nges of sa lin it y.

River w a ter ma y bring in nut r ient s

a nd may Als oexer t a s ta b ilizin g influence on t he tu rbulen t mot ion , being

a t t imes , t her efor e, in st r umen t a l in in it ia t in g pla n t gr ow t h in t he upper

la yer s (p. 789). P l an t nut r ien t s, n it r a t es , phosphorus, and soon a remore

readily a va ila ble in t he sha llow er in shore w a t er beca use of t he grea t er

possib ilit y of retu rn by ver t ica l cu rren t s a f t er they have been regenera t ed

from t he dish tegra t in g or ga n isms on t he bot tom or in t he deeper w a t er

(ch a pt er VI I). Th is fa ct or is of t he utmost impor ta n ce t o pr oduct ion of

dia t oms, foremost of t he pr ima ry food of t he sea . Therefore, per unit

a rew of t he sea , the nerit ic province is fa r more product ive tha n the

ocean ic prov irwe and is eohaequen t ly the reg ion of g rea t e st impor t ance to

ma rine life in genera l. H ere fish of grea teet economic import ance a re

t a ken, not only beca use of grea ter a va ila bilit y , but a lso beca use it is

t heir na tur a l h abit a t .

OTHER BIOTIC UNITS The a bove cla ssifica tion of t he ma rine

environment s i s baeed ma inly on broad geographica l , physica l , chemica l ,

a nd biologica l ch a ra ct er ist ics t ha t cir cum scr ibe mor e or less clea r ly

t h e s epa r a t e zones.

Wit hin ea ch of t hese ext en sive zones w e observe

many and var ied sets of ecolog ica l condit ions resu lt ing from dif ferences in

subst ra tum, proximity to shore, depth and chemica l-phys ica l condit ion of

t he w a ter , a nd so for t h.

The prima ry t opogra phic unit used in ecologica l cla ssifica t ion

of t he envir onmen t is t h e biot ope, or n iche,w h ich is d efin ed a s a n a r ea of

w hich t he principa l ha bit a t condit ions a nd t he living forms which a re


14/19

280


a dapt ed t o t hem a r e un iform (Hesse, Allee, a nd S chmidt , 1937). S in ce

in any given t ype of biot ope the habit a t condit ion s makespecific demands

on the inha bit a nt s, it follow s tha t a n a na logous development of the

inhab it an t s is fr equen t ly reflect ed in the popu la t ion , and those not fit t ed

for t he ha bit a t a re elimina t ed from it .

Obviously , some organ isms a re

not so na rrow ly bound t o t he biot ope a s a re ot hers of more specia lized

nature.

Thus, w it hin a biot ope ma y be found some genera lized forms

such a s cert a in cepha lopods a nd fishes t ha t w a nder more or less fr eely

from one type of biot ope t o a not her .

The more specia lized a biot ope

becomes w it h respect t o living condit ions, t he more uniform w ill t he

inha bit ant s become, so t ha t only a few species w it h la rge numbers of

individua ls ma y exist .

The sma ller ha bit a t a noma lies found w it hin

the biot ope a re ca lled ja cies. The number of orga nisms t ha t ca n live

in a ny given biot ope ma y in specia l ca ses be det ermined by a va ila ble

suit a ble spa ce, but mor e fr equent ly it w ill depen d upon t he food supply

t ha t ma y be produced w it hin t he biot ope or be ca r ried t o it from out side

by cu rr en t s. The communit y of forms in a biot ope is ca lled a biocoenos is,

B iot opes ha ving cer ta in ch a ra ct er ist ics in commonfor example,

proximit y t o the coa st or estua rine loca lit ya re unit ed int o la rger

d iv is ions known as

Mochores.

General Character of Populationsof the Primary BioticDivisions

Under t he previous hea dings w e ha ve dea lt w it h t he cla ssifica t ion

of t he ma r ine

environment.

For purposes of fu ture d is cuss ion it is des ir -

ab le a t t h is poin t t o ou t line br iefly a broad , h ighly pr act ica l cla s s ifica t i on

of the ma rine

population

inhab it ing the above pr imary b iot ic d ivis ion s,

a cla s s ifica t i on based not on na tu ra l phylogenet ic or t axonomic rela t ion -

ships, a s given on p. 282, but ra ther on a n a rt ificia l ba sis, grouping

heterogeneous assor tments of organ isms depending upon common habi t s

of locomot ion and mode of life and upon common ecolog ica l d is t r ibut ion .

On t hese gr ounds t he popula t ion of t he sea ma ybe divided in to t hr ee

la rge groupsnamely, the benthos, nekton, a nd pla nkt on, t he first

belonging t o t he ben th ic r egion a nd t he ot her t wo t o t he pela gic r egion .

In the

benthos

(Gr.,

deep

or deep-sea)ar e included the sessile, creeping,

a nd burrow ing orga n isms found on t he bot tom of t he sea . Represent a -

t ives of t he gr oup ext end fr om t he high -t ide level down in to t he a by ssa l

depth s. The ben thos compr ises (1) ses sile a n ima ls , such as the sponges ,

ba rna cles , mus sels , oy st er s, cr in oid s, cor a ls , h ydroid s, br yozoa , s ome

of t he worms , a ll of t he seaweed s and eel g ra s ses ,a nd many of t he d ia t oms,

(2) creeping forms, such as crabs, lobsters , cer t a in copepods, amphipods,

a nd ma ny ot her crust a cea , ma ny prot ozoa , sna ils, a nd some biva lves

a nd fishes, a nd (3) burrow ing forms, including most of t he clams a nd

worms, some crust acea , and ech inoderms.


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281

The nekton (G r ., ew inwnhg) is composed of sw imsnhg an ima ls found

in t he pela g ic d ivision .

In t his group a re included most of the a dult

squids, fishes, a nd w ha les-namely, a ll of the ma rine a nima ls tha t a re

a ble t o m igr a t e fr eely over con sid er a ble d is ta n ces . Obviously , t her e a r e

no pla n t s in t h is gener a l gr oup.

In the pkw ddon (G r., w anderer) is included a ll of the floa t ing or

drift ing life of t he pela gic. division of the sea , The orga nisms, both

pla n t a n d a n ima l, of t his division a r e usua lly m icr oscopic or r ela t ively

sma l l; t hey floa t more or les s pa s sively w i t h t he cu rr en t s and a re t herefor e

a t the merey of preva iling w a ter movement s. Ma ny of t he a nima ls a re

a ble t o ma ke some progress in sw imming, a lthough their orga ns of

locomot ion a re r ela t i vely weak and ineffect ive. The plankt on is d iv ided

int o tw o ma in dkisions, the

phytoptinkton and

t he

zooplankton.

The

former comprises a l l of the f loa t ing plants , such as d ia t oms, d inof lage lla tes j

coccolithophores, a nd sa rga ssumweeds.

In the zooplank ton a re included

(1) myria ds of a nima ls t ha t live perma nent ly in a floa ting st a te, a nd

(2) count less number s of h elples s la r va e and eggs of t he an ima l ben t hos

a nd nekton. S ince the pla nkt on a nd nekton occupy the sa me biot ic

rea lm a nd a re pa rt of t he sa me comrnunity, it is necessa ry a lw ays t o

r emember t h a t t he d ist in ct ion is one ba s ed pr ima r ily on r ela t ive siz e a nd

speed of sw imming, a nd does not signify a divergence of ecologica l

relationship.

E a ch of t h ese t h ree ecologica l gr oups w i ll be mor e fully d iscu ssed in

la t e r chap ter s.

Development of life in the Sea

Let us review briefly the observa t ions tha t indica te the rela t ive

a nt iquit y of the ma rine environment a s a biologica l rea lm. I t is not

pos sible t o know when life a r os e in t he sea , but t he clos e sim ila r it y of t he

chemica l composit ion of body fluids a nd sea w a ter ha s led t o the sup-

posit ion tha t the sea w as a lrea dy sa line a t t ha t ea r ly t ime a nd tha t,

beca use of t he in tim a cy of pr im it ive or ga n ism s w it h t he fluid en vir on -

men t, t he elemen ts pr esen t en ter ed in to t he fmdamen ta l composit ion

a n d mode of met a bolism of t he pr im it ive or ga n ism s a nd a r e ma in ta in ed

in presen t -d ay forms wi t h cer t a in mod ific@ions in t he propor t ions of t he

primipa l ions, especia lly ma gnesium (t able 55). These int erest ing

rela t ionshipshave led to much specu la t ion rela t ive to the development of

or gan isms and the chemica l compos it ion of pr im it ive sea s , bu t we cannot

ent er furt her upon t ha t pha se of t he a ct ion of t he environment . P eame

(1936) h a s given some r eviews and lis ted lit er a t ur e per t a in in g t o t h ese

quest ions a nd to t he theory of migra t ion of a nima ls from sea t o la nd.

The pa rt pla yed by t he sea in the dist ribut ion a nd ma int ena nce of

presen t -d ay life upon ou r g lobe is a v it a l one. The sea it s elf is abundan t ly

popula ted, a nd no life could exist on la nd w ere it not for t he perpet ua l


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282 THE SEA AS A BIOLOGICAL ENVIRONMENT

wa t er cy cle of evapor a t ion , pr ecipit a t ion , a nd dr a in a ge between s ea and

la nd. Only in t he sea w ould it be possible t o a pproa ch any degree of

self-sufficiency a s a biologica l r ea lm, and h is t or ica l ly t he sea ha s a ct ed a

pr incipa l r ole in t he developmen t of an ima l life.

Tha t the sea is the origina l environment of a nima l life is st rongly

indica ted by cert a in fa ct s tha t point t o the grea ter a ge of ma rine life

a s compa r ed t o t er rest ria l a n d fr esh -w a t er fa un a s t o wh ich it h a s seem-

in gly given r ise.

E vidence point ing t o a grea ter a ge of ma rine fa una

over t he t er rest ria l a nd fresh-w a t er fa una s is ma inly a long four lin es:

(1) genera l composit ion of present -da y fa una s, (2) simila r it y in t he

chemica l composit ion of body fluids a nd sea w a ter , (3) life hist or ies,

and (4) pa leontolog ica l rela t ionships .

(1) The whole a nima l kingdom is divided in to a number of pr ima ry

d iv is ions , each known as a [email protected].

Each phylum is composed of an ima ls

hav ing cer t a in fundament a l morpholog ica l s imila r i t ies not possessed by

a ny a nima ls of ot her phyla .

Thus, a na t ura l, a s opposed t o a r t ificia l,

rela t ionship is indica ted. Ea ch phylum is then divided int o na tura l

bu t more res t r ict ed g roups known as cla s ses ,and these in tu rn a re followed

by ot her y et lower d ivision s in t he follow ing manner :

Phylum

class

Order

Family

Genus

Species

S pecies a re formed of in dividua ls, a nd t he morphologica l fea t ures

by wh ich ea ch species is cha r a ct er iz ed a re les s fundamen t a l and presum-

a bly of more recent or igin t ha n t hose cha ra ct er izing t he genera . S imi-

la r ly , t h e gener ic st r uct ur es a r e less fundamen ta l t ha n t hose of fam ilies,

a nd so on t o t he highest division, w hich is ba sed on st ruct ures of grea t

ant iqui ty .

A review of a ll t he higher or ma jor divisionsnamely , the phyla

a nd cla s ses of a n ima l life-r evea ls t h e s tr ikin g pr eponder a nce of ma rin e

gr oups. All of t he seven teen phyla (usin g t he t a xonom ic r a nkingof H . S .

P r a t t , 1935, in

Manual oj Invertebrate Animals)

a r e r epr esen t ed in t h e

sea , a nd most , if not a ll, a re believed t o ha ve origina ted there. The

follow ing five a re exclusively ma rine: C tenophora ,

.Echinodermatu,

Phoronidea, Brachiopoda, Chaetogaatha. Some au thor s r ecogn ize fewer

t ha n sevent een phyla , but t his ha s only t he effect of increa sing t he pre-

ponderance of purely mar ine cla s ses .

Of t he fort y-seven cla sses (where only subphyla w ere given under

phyla , t hey a re here ra t ed a s cla sses) of invert ebra t es a s given by P r a t t ,

t wen ty -one, or 43.7 per cen t , a r e exclusively ma rin e, a nd on ly t h ree, or


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283

6.2 per cer it , a r e exclu sively n onma r in e. Of t h e subphylum Ver t ebr a t e,

mdy member s of t h e cla s s Amph ibia a r e nonma r in e, wh ile t h e ot h er fou r

cla sses sha re member s in bot h m a rine a nd nonma rine environment s.

The fishes a re predomina nt ly ma rine, w hile t he rept iles, birds, a nd

m amma ls a re predomina nt ly t er rest ria l. The amphibia ns represent

the h ighes t nonmar ine group.

Theee divisions demonst ra te t he a st onishing va riet y of ma rine

a n im a ls, a s fa r a s t he m a jor phy logenet ic groups a re concerned. H ow -

eve~ the t er res t ria l envir onmen t h a rbor s the g rea t e st number of

species,

ma inly ow ing t o t he la rge number of species of one rest rict ed group,

t he insect s, w hich a re a lmost t ot a lly a bsent from t he sea . The presen ce

in t he sea of so ma ny ma jor groups, ma ny of w hich a re rest rict ed t o t he

sea , in dica t es t he grea t t endency on t he pa r t of t he ma rin e en vironmen t

t o pr eser ve t h e gr oups t h a t h a ve on ce become evolved .

I t should be not ed a lso t ha t , in a ddit ion t o t he rem a rka ble diversit y

of ma rine life in t he ocea n, t here is a conspicuous pr imit ive element ,

a s judged by simplicit y of st ructure, in the groups represent ed. In

t h e sea t h er e is a mor e complet e developmen t a l ser ies of a n ima l life t han

exist s a nyw here else, beca use of w hich, a nd a lso beca use of t he na t ura l

and i rit ima te rela t ionsh ipsof the organ isms to the sea-wa ter med~ um,the

s tudies is su ing f rom the mar ine b iolog ica l labor a tor ies have con t r ibuted

va s t ly t o in forma t ion on biolog ica l pr oblems dea lin g w it h developmen t

and ma in t en ance of life.

Th e r ela t ive un iformit y of t he ma rin e en vir onmen t h a s been in st ru-

men t a l n ot on ly in pr eser vin g t h e d iver sit y of forms bu t a lso in r et a in in g

a genera lly more pr im it ive cha ra ct er a s compa r ed w it h t errest ria l a nd

fresh-w a ter a nima ls. I t is t rue tha t in the sea w e do find a ssocia ted

w it h t he low er forms a n umber of high ly developed a n ima ls t ha t must be

considered ma r ine beca use of t heir dependence on t he sea . These a re

t h e sea l s, wh a les, cer t a in r ept iles , fish es, a n d bir ds. All of t h ese gr oups,

h owever , ha ve ha d a la r ge pa r t of t heir r a cia l developmen t in t he t er res-

t ria l a nd fresh-w a t er ha bit a t . They ha ve more recent ly rever t ed t o t he

sea a nd ha ve only seconda r ily become a da pt ed t o it . The t eleost fish es,

wh ich a r e believed t o h ave evolved t o t h eir pr esen t st a t u s in fr esh wa t er ,

were or ig ina l ly der ived f rom mar ine s tock .

(2) The rela t ion of body fluids t o sea w a t er ha s a lrea dy been die.

cussed (p. 269).

(3) A study of th e life h is t or ies of inver t ebr a t es suggest s th e an t iqu it y

of ma rine liie. During the ea rly hist ory of t he individua ls of some

an ima l gr oups t he la r va l st a ges a r e ma rkedly differ en t in st ruct ur e a n d

ha bit from the ma ture pha se. The la rva l st a ges, w hich somet imes

resemble t he ma t ure st a ges of ot her groups or oniy t he la rva e of ot her

groups, a r e t hough t t o r eflect a st ruct ur a l sim ila r it y t o a n cest ra l st ock.

Whet her or not t hk is a rea l reca pit ula t ion of ra cia l h ist ory or only a n


18/19

284


expr ession of individu a l la r va l a d apt a t ion t o a common envir onmen t is

impor t a nt in seeking a n underst a nding of the simila rit y. Wha tever

t he t rut h ma y be, it is w ell known t ha t most ma rine invert ebra tes pa ss

through a n ea rly st age during w hich t he la rva e in no w a y st ruct ura lly

suggest t heir pa rent a ge, but ma y even ha ve st r iking funda ment a l

simila rit y t o exist ing la rva e of other groups. From this it ha s been

possible t o est a blish t ypes of la rva e-for example, t he tmchophore of

t he Annelida a nd Mollusca , a nd t he nuuplius of t he crust a cea n groups

(fig . 80, p. 321).

There is a t endency for some a ggressive a nima l groups t o desert

the sea for fresh-w a t er or la nd ha bit a ts. Thw is show n by the crust a -

cea n s, among wh ich t her e a r e forms such a s t he pr awn, Zh-iocheir, which

enters fresh w a ter a t a young st age but w hen ma ture ret urns t o the sea

t o spa wn. The la nd cra bs, Ca rdisoma , Gecarciwa, a nd so for th , a lso

go t hr ough a fr ee-sw immin g la r va l st a ge in sea w a t er .

(4) I t is well kn own t ha t a n im a l fossils occur rin g in t he oldest kn own

fos silifer ou s rocks of t he ea r t h s crus t a r e ma in ly ma r ine forms.

Ma rine an ima ls were a bundant and became fos siliz ed in t he Cambr ian

period (500 million yea rs a go), w hen cert a in port ion s of t he la nd how

above s ea level formed a pa r t of t h e sea bot t om a long t he coa s t s of a n cien t

sea s. S evera l invert ebra te phyla w ere a lrea dy developed, a nd such

forms a s t r ilob it es and br a ch lopods were pa r t icu la r ly abundan t .

The chief roles of the ma rine a nd t errest ria l environment s in the

development of life ma y be summa rized by sa ying t ha t t he grea t pa rt

pla y ed by t he former is ch iefly in t he developmen t a nd ma in ten a nce of a

w ide d iver sit y of lower forms, wh ile in t he la t t er t he in fluence of t he more

rigorous ha bit a ts ha s produced less diversit y of form but a higher t ype

of complexity .

The a rea w here t hese t wo grea t environment s meet , t he in t er t ida l

zone, is in a n in t ermedia te posit ion a nd subject t o ra pid a nd ma rked

vicissitudes, a nd it is from here tha t much of t he migra t ion to la nd is

supposed t o h a ve t a ken pla ce.

Bibliography

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1929.

Ionendurchlass igkei tder Korper flache von wirbel losen

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Ekman , Sven . 1935. Tiergeographiedes Meeres . Akad . Ver lagsgesellsch .,

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H ew at t, Willis G . 1937. E cologica l st udies on select ed ma rine intert ida l

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Documents

Sea as Biological Environment