Reproductive output in the hatchery-reared california sea hare at different stocking densities.

Although the California sea hare (Aplysia californica) is well known from neurobiological studies and is raised in the laboratory for this purpose, the life history of this species in the laboratory is less well studied. Therefore we conducted a study of the reproductive period of hatchery-born and -raised A. californica in which sibling animals were reared at stocking densities of 2, 5, 10, 15, and 20 animals per 16-liter cage. Temperature was controlled at 13 to 15 degrees, and the photoperiod was a 14:10-h light:dark cycle. Seawater O2 concentration, pH, and salinity were optimized by seawater flow through the animal cages. Compared with scheduled feedings, an ad libitum algal diet produced early sexual maturity. Despite different growth rates at different animal densities per cage, the age at first sexual maturity (defined as the first egg mass) did not differ significantly among the different densities and averaged 210 +/- 15 days of age. Although there was no difference in the total spawn weight per cage as a function of animal density from 2 to 20 animals per cage, the number of spawning episodes per animal decreased as the cage density increased. The average weight of spawn per animal over its reproductive lifetime decreased as cage density increased. When expressed per day of reproductive maturity, spawn weight per animal decreased as the cage density increased. Finally, larger animals produced more eggs during the 10 spawns at the peak of the spawning period. The largest animals corresponded to the cages containing the fewest animals. Stocking densities between 2 and 20 per cage had little influence on time to maturity and on average number of days of reproductive life. However, stocking density (and thus animal size) strongly influenced so-called lifetime fecundity.

The effect of stocking density on growth rate and maturation time in laboratory-reared california sea hares.

California sea hares (Aplysia californica) were reared from the late juvenile period (approximately day 100 posthatch) to senescence in a laboratory study of growth and maturation at different stocking densities. Temperature, light, and food were controlled, and other seawater parameters such as O2 concentration, pH, and salinity, although not controlled, were optimized by the flow-through design of seawater through the cages. Stocking densities evaluated were 1, 2, 5, 10, 15, and 20 animals per 16-liter cage. Food availability is likely to be a limiting factor to growth in wild populations of A. californica, but in our experiments, algal diet was ad libitum at all densities and presumably was not a controlling factor. The animals maintained at each of the various densities grew at different rates but reached sexual maturity (defined as the age at the appearance of the first egg mass) at approximately the same age, 204 +/- 4 days (mean +/- standard error), for densities higher than 2 animals per cage. Age at sexual maturity for 2 animals per cage was 274 days. Growth rates were highest in cages with the lowest stocking densities and lowest in high-density cages, ranging from 3.72 g live weight/day in animals housed individually to 1.06 g live weight/day for those housed 20 per cage during the period 100 to 200 days of age. Growth differed significantly among the various stocking densities beginning at 9 weeks of growth (age, 167 to 174 days). In summary, we show that stocking density has an important influence on growth and is a key factor for consistently rearing Aplysia as an animal model under hatchery conditions.