lnstar Increments in Copepod Growth

Alan R. Longhurst Department oi Fisheries and Oceans, Bedhrd institute of Oceanography, Darm?oilth, N.S. B2Y $ A t

Longhurst, A. R. 1386. lnstar increments in copepod growth. Can. 1. Fish. Aquat. Sci. 43: 1671 -1674.

There is a difference in the growth patterns of large and small copepods as indicated by lengths at each instar for 55 species of copepods from all latitudes. Large species put on a greater proportion of their adult size relatively late in life compared with small species. This confirms an earlier suggestion based on a comparison of only two species.

La determination des longueurs, a chaque stade de developpement, de 55 es@ces de copkpodes de tsutes Bes latitudes montre Ifexistence d'un k a r t entre Bes modes de croissawce des grog et des petits sop5podes. Conepara- tivement aux petites espPces, les grosses especes atteignent la taille adulte relativement tard au sours de leur vie. Cela sonfirme une hypothese deja formulee qui reposait sur la cornparaison de seulernent deux espPces.

Received january 25, 5 9635 Accepted April 28, 1986 (SSOSS,

C opepods, like a11 other arthropods. grow by size incre- ments at each ecdysis, and it is these increments, to- gether with the duration of each instar, that determine the pattern o f their growth. Considering the relative

importance of copepods in plankton dynamics, it is surprising that neither sf these growth characteristics is comprehensively described; in fact, uncertainty concerning their quantification has clouded recent discussion of how to estimate copepd production (e.g. Tremblay and Roff 1983; McLaren and Corkett 1 984).

There have been attemps to derive a comprehensive model for instar duration from analysis of Acarria growth, stated as the "isochronal rule" (Miller et al. 19771, although it is clear from later studies of additional species (Landry 1983) that instar duration is not uniform even in species producing several generations during one growth season. Further, in species or stocks having csntsgenetic migrations or periods of diapause, some growth stages extend for relatively very long periods; for some arctic copepods9 it has k e n suggested that diapause may even extend over more than B yr (e.g. Dawson 1978).

Landry's study appears to establish a sensible first apprsxi- mation to a comprehensive model of instar duration, capable of being modified by diapause, but we still lack a similar approx- imation for increments at each ecdysis. It is the purpose of this note to present an analysis of this fundamental characteristic of copepod growth: since copepods, apparently like all crustacea, do not entirely fill the cavity of their new, larger exoskeleton with body tissue after each ecdysis, it may be argued that instar lengths are not really a useful characteristic of their growth trajectories. However, length is very much more rapidly mea- sured than weight, and since Klein Breteler et al. 4 1982) have shown that length and weight are directly related for several species throughout all larval stages, we can indeed use lengths as a useful indication of the time-course of organic growth. We must remember, however, that length more properly represents mid-instar, rather than initial or terminal weights.

Results. and Discussion The data presented in this short note come entirely from

published sources, as indicated in Table I , which presents lengths at instar for 55 species or populations of cyclspoid and calanoid copepods from both warm and cool oceans. For 26 species, lengths for all instars from the first nauplius to the adult are available, while for the remainder there are only copepodite lengths. This body of data probably represents copepods quite well, and the distribution between large and small species may also reflect the distribution of large and small species in nature. Examination of comparable data sets for weight (e.g. those sf Shrneleva 1965) shows these to be much more variable, as indeed they must be unless growth in weight were as discontinuous as growth in length. For this reason, and for questions of availability, the present analysis is based on length data.

Figure I presents the descriptive statistics of BarvaH instars for all 55 species in terms of their relative approach to the adult size, and an indication of the reBative growth in length achieved at each instar. Relatively large growth increments are achieved at each instar during both the early nauplii and copepodites, and between the Bast nauplius and the first copepodite. Exam- ination of the growth trajectories of species for which data are available for all larval stages suggests that, although there is much variation, the trajectories leading to the larger adults differentiate from the remainder only during the middle cope- podite stages. This appears to reflect the results of Ivanova (%973), who showed that naupliar growth could be described by an exponential function and copepodite growth by a power function.

Figure 2 shows the regression of the growth increment from 633 to C4 (as a percentage sf C3) against adult size; the line fitted by least squares shows a positive correlation between relative increase in length at this ecdysis and the final adult size ( r = 0.677h the value of r 2 suggests that almost half the total variability in relative growth increment is accounted for by adult size (P < 0.8001). The growth increment at this ecdysis is described by y - 0 .857~ + 0.182, as a function of adult size. Finally, Fig. 3 shows that there is also a general corre$ation between the size of early nauplii and their final adult size, while Fig. 4 demonstrates that there is no cs~elation between rela-

Can. .I. Fish. Aquaf. Sci., Vol. 43, 1986 8671

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MAUPLIAR STAGES I COPEPODITE STAGES

PIG. 1. Mean, standard deviation, and range of larval lengths as percentages s f adult female length, and mean relative increment in length at each larval ecdysis. Data for 55 species listed in Table 1.

y = .05? x + .I82 R - SQUARED: , 4 5 8

0

FIG 2. Regression of relative growth increment f r ~ m C3 to C4 (increment relative to C3 length) against length of adult female.

I s L L 1 I I 1 I

0 1 2 3 4 5 6 7 8 9

ADULT SIZE (mmB

Re. 3. Regression of lengths of first naupliar stage against length of adult female.

FIG. 4. Regression of relative growth increment from N2 to N3 (increment relative to N2 length) against length of adult female.

the size increase at mid-nauplliar ecdysis and final adult size. Thus, these data confirm the suggestion of Mclaren and

Corkett (1984), based on only two species, that Barge species of copepods accumulate more growth in their later instars than small ones. As McEaren (1969) reminds us, 'bBrooks Law" suggests that a 25% increase in length between instars is a useful rule of thumb for crustaceans, and that this implies an approximate doubling of volume (hence weight) in an iso- metrically growing arthropod. Figure 1 shows that the values for instar increments are distributed within 110- 15% sf a mean value of 26.25%. The largest available block of instar weight data is that of Shrneleva (1965); this suggests that for 16 species of copepods the growth in weight is close to a doubling at each ecdysis. Mean weight increments are Cl -C2 = 9596, C2-C3 = 84%, C3-C4 = 11296, C4-C5 - 76%, and GS-Adult = 71%.

The relative length increments (Fig. 1) characteristic of each instar may also be interpreted alongside the analysis of Landry (1983), who found that first-feeding nauplii and C5 instars are no longer than nonfeeding early nauplii and

Can. J . Fish. Aquab. Sck., Vok. 43, 1986

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C1 -C4. Clearly, the iwstars with the longest duration are not those which acc~rnplish the largest increment in length at their terminal ecdysis.

1 am grateful to Ian McLaren, Ken Mann, Bob Conover, and Mike Mullin for lasefu'ul comments on this note and to Madhu Paranjape for some references from his apparently inexhaustible store. i gratefupally acknowledge the permission of my employer, the Department of Fisheries and Oceans, to ~mndertake the study.

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Can. 9 . Fish. Aquat. Sci., Vol. 43 , 1986

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