A BIOCHEMICALLY BASED MODEL OF THE GROWTH AND
DEVELOPMENT OF CRASSOSTREA GIGAS LARVAE
E.A. Bochenek, J.M. Klinck, E.N. Powell, E.E.
Hofmann-2001
Journal of Shellfish Research, 20(1): 243-265
Abstract:
A biochemically based model was developed to simulate
the growth, development and metamorphosis of larvae of the Pacific oyster,
Crassostrea gigas. The model is unique in that (1) it defines larvae in
terms of their protein, neutral lipid, polar lipid, carbohydrate, and ash
content; (2) it tracks weight separately from length to follow larval
condition index; and (3) it includes genetic variation in growth efficiency
and egg quality to better simulate cohort population dynamics. The model
includes parameterizations for larval filtration, ingestion, and
respiration, which determine growth rate, and processes controlling larval
mortality and metamorphosis. The initial biochemical content of the larva is
determined by the composition of the egg. Changes in the initial ratios of
protein, carbohydrate, neutral lipid, and polar lipid occur in response to
the biochemical composition of available food as the larva grows. Modelling
the process of metamorphosis requires a series of size-based and
biochemically based triggers: (1) larvae become potentially competent to
metamorphose at 275 µm, following a decrease in filtration rate at 250 µm;
(2) larvae become competent to metamorphose when a daily decline in neutral
lipid of 25% or more occurs; and (3) larvae metamorphose successfully if
neutral lipid exceed polar lipid stores. Although based on simple
biochemistry, the model succeeds in simulating such basic characteristics of
C. gigas larval development and metamorphosis as larval life span and size
structure at metamorphosis and the influence of egg and food quality and
food quantity on survival. These results suggest that simple biochemical
constructs may encompass the biochemical transitions most prominent in
determining cohort success. Simulations of larval development show that for
the smallest larvae, assimilation does not provide adequate resources to
explain observed growth, although measured filtration rates would indicate
otherwise. Egg lipid stores are needed to sustain the larva. The simulations
also identify egg sizes in the range 37-73 µm to be viable, very similar to
observations. Egg sizes outside this range are predicted to be non-viable
due to lipid deficiencies in early larval life. Similarly, simulations
identify upper and lower genetic limits on growth efficiency beyond which
larvae cannot acquire sufficient neutral lipid stores to successfully
metamorphose. As food supply declines, animals with high growth efficiencies
are selected in the simulation. Low-protein food diets are predicted to
increase larval survival. High-protein diets result in insufficient
carbohydrate and neutral lipid to cover metabolic
and storage needs. Thus, the influence of growth efficiency is
non-randomly distributed across egg size and respiration rate and the
influence seems to be mediated in part by food quantity and to a larger
measure by food quality.
(Haskin Shellfish Research Laboratory, Rutgers
University, 6959 Miller Ave., Port Norris, New Jersey 08349, USA)