Ontogeny of Respiratory and Growth Responses of Larval Mud Crabs Rhithropanopeus harrisii Exposed to Different Temperatures, Salinities and Naphthalene Concentrations

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Abstract
Larval mud crabs R. harrisii were exposed continuously from hatching through the 1st crab stage to sublethal concentrations of naphthalene (0, 75, 150 or 300 .mu.g/l) at several combinations of salinity (S) and temperature (5, 15 or 25.permill. S and 20.degree., 25.degree. or 30.degree. C, respectively), the experimental design consisting of a complete 3 .times. 3 .times. 4 factorial. Respiration rates were determined under all treatment combinations for the 2nd and 4th zoeal stage, megalops and 1st crab. In addition to a steady-state respiratory response at the rearing salinity, an osmotic-shock respiratory response was also assessed by respirometry immediately after transfer of larvae reared at 15.permill. S to 5.permill. S (hypoosmotic shock) or 25.permill. S (hyperosmotic shock). Respiratory rates increased with temperature for all stages. Increased respiration rate of larval stages, especially zoeal stages, also occurred in low salinities. The increase due to low environmental salinity was least in the juvenile crab, probably because of development of the osmoregulatory capacity of the gill. Respiratory response to hypoosmotic shock was similar to that observed for steady-state trials at 5.permill. S but the pattern for hyperosmotic shock was intermediate between the steady-state response at 15 and 25.permill. S. Naphthalene exposure usually resulted in an increased respiration rate (of zoeal stages) in low salinities. When physical conditions were not stressful, naphthalene effects generally were not obvious. The weight of megalops from the various trials (environmental factor combinations) was determined as an indication of growth during zoeal development. Growth was greatly affected by higher temperatures and lower salinities and slightly by lower salinity-higher naphthalene combinations. At optimal salinity, 15.permill. S, no consistent effect of naphthalene on growth was apparent. This partial energy budget approach was seen as a useful method for assessing long-term sublethal stress in marine invertebrates.