Adult mud crabs, Rhithropanopeus harrisii, were held in the laboratory for 14 months at densities of 20 or 40 crabs per 0.5 m2 and at female: male ratios of 2:1, 4:1, and 6:1. Females spawned for 9 months during the calendar year and entered a refractory period from November through January. Field populations in North Carolina spawn for only 5 months of the year. Spawning increased with increasing crab density and female: male ratio. Ovigers were most abundant in habitats containing 40 crabs at a 6:1 sex ratio (8.3 ovigers per week), and were least abundant in habitats containing 20 crabs at a 2:1 ratio (2.8 per week). However, the highest proportion of crabs spawned in habitats containing 20 crabs at a 6:1 sex ratio. Neither density nor sex ratio affected female mortality. Crabs at the lowest sex ratio (2:1) and highest density (40 crabs) produced larvae that survived poorly (45% survival to first crab stage). However, larval viability was good at all other densities and sex ratios. Maintaining crabs at high densities and female: male sex ratios in the laboratory, even during a portion of the nonbreeding season, augmented egg production. These methods can facilitate studies of reproduction and development of R. harrisii.
Abstract Ophryotrocha diadema is stenohaline. The complete life cycle has only been observed at salinities of 30, 35 and 40 ‰. At an optimum salinity, 35 ‰, reproduction occurs between 11 and 27 °c. Experiments were performed in 5 ‰ steps of salinity from 15 to 45 ‰ combined with temperatures of 15, 18,21 and 25 0c. At salinities of 15 and 20 ‰ all animals, both adults and larvae, died within two days. In the remaining 20 salinity/temperature combinations adults were kept for 3 weeks and larvae for a minimum of one month or until they were sexually mature. For adults, mortality, number and size of egg masses, and reproductive success were recorded. Corresponding parameters for larvae were mortality, growth rate and time to beginning of reproduction. Optimum conditions for survival were obtained at 35 ‰ salinity and 18 ac. The optimum for egg production (reproductive rate) was 35 ‰ and 25 °c. The maximum growth rate was observed at 30 and 35 ‰ salinity and 25°C. These two combinations also represented the shortest time of development to sexual maturity. The number of eggs per egg mass was significantly salinity dependent, with an optimum at 35 ‰, but was not affected by temperature, except at 40 ‰ salinity, or by salinity/temperature interaction. Both adults and larvae demonstrated good survival at 25 ‰ salinity, but fertilization and/or early development was blocked. This was also true at 45 ‰ where the rate of survival was much lower than at 25 ‰. At low temperatures, 15 and 18°C, survival was better at 40 than at 25 ‰ salinity. At 21 and 25 °C the relations are reversed. The growth rate figures indicate a similar change, which occurs between 15 and 18 ac. At 15 ac the larvae maintained at 40 ‰ salinity were superior, but at higher temperatures, the higher survival was observed for larvae reared at 25 ‰. The coefficient of population growth, r, was calculated from the exponential equation Nt = N0 × ert for the four most favourable salinity/temperature combinations. The highest value was obtained at 25°C and 35 ‰ salinity.
The larval development of the sand crab Emerita talpoid (Say) was studied under similar laboratory conditions using larvae hatched from eggs produced in a single locality during spring and summer. Two developmental sequences were found, larvae hatched in May went through five or six developmental stages while those hatched in August showed six or seven stages and took longer to metamorphose. These developmental sequences were ascribed to environmental conditions to which adults and embryos were exposed. The role of factors such as temperature, salinity, amount of yolk in the eggs are discussed in relation to ecological models of larval development.
Abstract : The review considers the staff, general research programs, and research facilities in the two major Turkish institutions associated with the marine sciences, the Hydrobiological Research Institute and the Department of Navigation and Hydrography of the Navy, and the changes which have occurred since 1963.
Eyestalk removal accelerated the molt cycles of megalopal and juvenile (first through fifth crab instars) Rhithropanopeus harrisii. Eyestalkless crabs also demonstrated a greater increase in size at each ecdysis. The growth rate of eyestalkless crabs was approximately twice the rate measured in control crabs. Epidermal cell density measurements showed that the cell density was the same in intermolt fifth instar control and eyestalkless crabs. The results demonstrate that growth in juvenile crabs is under the influence of eyestalk neurosecretory centers and that growth is a result of epidermal cell proliferation and not cell enlargement.
20-Hydroxyecdysone (20-HE) stimulated the initiation of premolt (apolysis) in third and fourth instar Rhithropanopeus harrisii larvae in vivo and in spines explanted from third and fourth instar larvae in vitro. The premolt-initiating activity of 20-HE, in vitro, was inhibited by extracts of third or fourth instar zocae, megalopae, and shrimp eyestalks. The results suggest that apolysis in crab larvae may be controlled by both 20-HE and MIH.
