Upload
xinngoh
View
315
Download
4
Embed Size (px)
Citation preview
3/1/2010
1
MEROPLANKTON:
ICHTHYOPLANKTON
Fish Eggs and Fish Larvae
- important part of meroplankton
Reasons why ichthyoplankyon surveys are done:
i. Surveys are often directed towards a single target species(or a group of closely related species) in order to use their
distribution and abundance of pelagic eggs to obtain an estimate of the biomass of the adult spawning population;
ii. Larvae of the target species are studied in order to estimate the success of the year brood resulting from its
spawn and hopefully to understand the factors underlying fluctuations of survival (recruitment);
iii. Surveys are used to evaluate fish resources in general.
3/1/2010
2
- some fish attach their eggs to substrates
- most marine fish release free-floating planktonic eggs that
are fertilized externally and float individually near the sea
surface(ex. Sardines, anchovy, tuna & many other commercially
harvested species)
Planktonic eggs typically:
•spherical (some ovoid to oblong)
•transparent
•small (usually 1-2 mm in diameter; range 0.5-5.5 mm)
•contain varying amounts of clear yolk which is the food for developing embryos and newly hatched larvae
•contain one or more spherical oil globules which aside from aiding flotation, may also provide nourishment (eggs without oil globules are equally buoyant); newly fertilized eggs usually float with the oil globule uppermost.
Appearance in plankton:
• dependent on spawning cycles of adults• linked to environmental change
Rate of egg development:• species-specific
• closely tied to ambient seawater temperature (hatching delayed in colder waters)
• hatching generally occurs within a few days to a few
weeks after the eggs are spawned
Egg Number & Survival• fecundity high: 250,000; 500,000 or over 1 million
(no. of eggs spawned per fish per spawning season)
• survival is low – food for holoplankton & adult fish
Egg Number & Size
• inverse correlation between egg size & number
Large eggs but small number
• because of size & energy restrictions• large eggs with more yolk hatched young larger
• larger young higher survival rates� too large to be eaten
� more active� better able to evade predators
Small eggs in large numbers
•with little or no nutritive material for the developing embryo•hatch at small size vulnerable to predators begin
feeding immediately
•high mortality but compensated for by large numbers
3/1/2010
3
Yolk sac
•first few days after hatching retain yolk in sac under body•rely on yolk sac as food until mouth & gut develop
•yolk exhausted begin to feed totally dependent on
suitable food in the plankton
Plankton as Food•feed on plankton for several months
•until large enough (nekton) actively seek feeding areas
independent of current drift
Fish larvae as Plankton – vulnerable to pelagic predators –both large zooplankton & nekton
Mortality very high�Cod – mortality = 99.999%
Identification of Fish Eggs
Characters used for identification:
•presence or absence of oil globules
•homogeneous or segmented egg yolk•size of perivitelline space
•egg membrane with smooth or sculptured surface•size of the egg
•shape of the egg
•in late stages of development�presence or absence of pigmentation on yolk sac or
oil globule�degree of pigmentation in the eyes
�pigmentation pattern of the embryo
�presence or absence of yellow or red pigment when� examining living eggs
3/1/2010
4
In late stages of development
�presence or absence of
pigmentation on yolk sac or oil globule
�degree of pigmentationin the eyes
�pigmentation pattern of
the embryo
�presence or absence of
yellow or red pigmentwhen examining living
eggs
FERTILIZATION TO HATCHING
Released egg(protected by fairly tough chorion or egg case. Within the chorion is the cytoplasm & yolk covered by a vitelline membrane. Often 1
or more oil globules are present)
Fertilization(spermatozoa penetrates egg through microphyle resulting in fusion
of egg & sperm nuclei)
Development = Embryogenesis(Vitelline membrane separates from chorion creating a perivitelline
space & the microphyle is plugged – preventing spermatozoa from entering; chorion hardens to protect the egg. Chorion remains
permeable to water & small molecules; most species telolecithal –yolk concentrated at vegetative pole & cytoplasm at animal pole)
The zygote period.
A: The zygote within its uplifted chorion, a few minutes after fertilization.
B. The dechorionated zygote with the animal pole to the top, about 10 min after fertilization. Yolk-free cytoplasm has begun to segregate to the animal
pole. Scale bar: 250 µm.
3/1/2010
5
Cleavage & Morphogenesis(cells divide, form layers and then organs)
Blastulation(Holoblastic cleavage = entire egg divides to form smaller cells or
micromeres at animal pole and macromeres at vegetative pole; Meroblastic cleavage = cleavage at animal pole leads to blastoderm
or cap of cells; blastoderm overgrows the yolk (epiboly) eventually enclosing it to form
Gastrula(a hollow sphere of cells containing yolk with a small opening in the
perivitelline space = the blastopore)
Fig. 4. Embryos during the cleavage period. Face views, except for B, which shows the embryo twisted about the animal-vegetal axis, roughly 45 degrees
from the face view. A: 2-cell stage (0.75 h). B: 4-cell stage (1 h). C. 8-cell stage (1.25 h). D: 16-cell stage (1.5 h). E: 32-cell stage (1.75 h). F. 64-cell stage (2 h). Scale bar: 250 µm.
Cleavage & Morphogenesis(cells divide, form layers and then organs)
Blastulation(Holoblastic cleavage = entire egg divides to form smaller cells or
micromeres at animal pole and macromeres at vegetative pole; Meroblastic cleavage = cleavage at animal pole leads to blastoderm
or cap of cells; blastoderm overgrows the yolk (epiboly) eventually enclosing it to form
Gastrula(a hollow sphere of cells containing yolk with a small opening in the
perivitelline space = the blastopore)
Fig. 8. Face views of embryos during the blastula period. A: 256-cell stage (2.5 h). B: high stage (3.3 h). C. transition between the high and oblong stages (3.5 h).
D. transition between the oblong and sphere stages (3.8 h). E: dome stage (4.3 h). F. 30%-epiboly stage (4.7 h). Scale bar: 250 µm.
3/1/2010
6
Cleavage & Morphogenesis(cells divide, form layers and then organs)
Blastulation(Holoblastic cleavage = entire egg divides to form smaller cells or
micromeres at animal pole and macromeres at vegetative pole; Meroblastic cleavage = cleavage at animal pole leads to blastoderm
or cap of cells; blastoderm overgrows the yolk (epiboly) eventually enclosing it to form
Gastrula(a hollow sphere of cells containing yolk with a small opening in the
perivitelline space = the blastopore)
Fig. 11. Development during the gastrula period. Left
side views, except where noted, with anterior up and
dorsal to the left. A: 50%-epiboly stage (5.25 h). B.
Germ ring stage (5.7 h). C. Animal pole view of the
germ ring stage; the arrow indicates the germ ring; the
embryonic shield will probably developed from the
flattened region of the ring at the lower right. D: Shield
stage (6 h). The embryonic shield, marking the dorsal
side is visible as a thickening of the germ ring to the
left. E: Animal pole view of the shield stage; the arrow
indicates the embryonic shield. F: 70%-epiboly stage
(7.7 h). The dorsal side of the blastoderm, to the left, is
thicker than the ventral side, to the right. The anterior
axial hypoblast, or prechordal plate, (arrow) extends
nearly to the animal pole. G: 70%-epiboly stage, ventral view, but tipped slightly forwards anteriorly to reveal the
now well delineated axial hypoblast (arrow) of the
prechordal plate. H: 75%-epiboly stage (8 h). The arrow
indicates the thin evacuation zone on the ventral side. I:
80%-epiboly stage (8.4 h), dorsal view. The arrows
indicate the boundaries between axial mesoderm in the
midline, and the paraxial mesoderm flanking to either
side. J: 90%-epiboly stage (9 h). The tail bud (arrow)
becomes visible in some embryos at this stage. K:
90%-epiboly stage, ventral view. The anterior
prechordal plate (compare with G) enlarges as the
polster. L: Bud stage (10 h). The arrow shows the
polster, and the arrowhead shows the tail bud. A
distinctive region just ventral to the tail bud (i.e. just to
the left in this view) shows where the yolk disappears as epiboly ends. Scale bar: 250
Fig. 14. Fate map of the deep cell layer (DEL) at gastrula onset, at the 50%-epiboly stage before formation of the germ ring and hypoblast. The
blastoderm now has the shape of an inverted hemispherical cup overlying the yolk cell
Organ Formation / Organogenesis(The optic cups (the future eyes) and the heart are the first organs to
be identified and in some cases the eyes become pigmented & functional; the embryonic fish often wriggles & rotates within the
chorion)
Neurula stage(embyonic axis laid down in relation to dorsal lip of blastophere =
future head, spinal cord and body musculature soon visible; the tail region moves away from the neurula & coils round inside the
perivitelline space)
3/1/2010
7
Fig. 15. Development during the segmentation period. Fig. 15. Development during the segmentation period.
Softening of the Chorion(chorion softened by enzymes secreted by glands on the head)
Hatching(the embryo breaks away from the chorion)
Larva
Postlarva
Fry(young fish is capable of active swimming
so that its strictly planktonic life is over)
pla
nk
ton
ne
kto
n
3/1/2010
8
3/1/2010
9
LARVA
• early larva – yolk sac prominent; about half of body length
• short incubation – eyes not pigmented, mouth not functional & anus not open
• marginal primordial fin – no fin rays yet – whole length of
body, from crown of head in dorsal to caudal to ventral side
• some species – pigmentation pattern + absence/presence
of oil globules & their position = identification• living specimens – coloured pigmentation other than black
melanophores – on body only; on primordial fin; yolk sac
& oil globule
• eyes become fully pigmented, mouth & anus open (position
of anus useful character for identification)
3/1/2010
10
• during development gradually use up yolk sac & oil
globule • yolk sac completely gone all organs necessary for
searching & devouring food are fully functional;
• availability of right food organisms is critical
complete utilization of yolk = end of larval period
POSTLARVA
Anatomic & morphometric features of postlarva
POSTLARVA
• most of essential organs are functional
• able to catch food• gradually assumes adult characteristics
• earlies postlarval stages = characteristic pigmentation
pattern for species has appeared• pigmentation pattern persists until adult meristic
characteristics are developed – pigmentation becomes
diffuse & silvering occurs
Melanophore pigmentation of postlarva
3/1/2010
11
3/1/2010
12
FRYactive swimming
= nekton
• remain pelagic &
form shoals
• become benthic
(on seafloor)
• in inshore waters
• associate with jellyfish
Amali 8
Meroplankton: Fish Eggs & Larvae
March 11-12 (Thursday to Friday)
• Report to Makmal BioD at 9:30 P.M. on Thursday.• Work in groups of 3 students.
• Each group needs a digital camera, a log book, and acompound microscope.
• Calibrate your microscope first for measurement of
egg & larvae dimensions.• You are not allowed to leave the lab until 8:00 A.M. of
Friday, so bring your food & drinks when you come on Thursday night.
• Behave in the lab by not making too much noise.
• Bring references on fish embryogenesis.
3/1/2010
13
Examples of Fish Egg & Larvae Report