EFFECTS OF REDUCED SALINITIES ON DEVELOPMENT AND BIOENERGETICS OF EARLY LARVAL SHORE CRAB, CARCINUS-MAENAS

The shore crab, Carcinus maenas L. (Portunidae), is a coastal and estu arine species, which can live and reproduce under brackish water condi tions; freshly hatched larvae have been observed in the field at salin ities below 15 parts per thousand. In the present laboratory study, th e tolerance of hypo-osmotic stress was experimentally investigated in early larvae of a marine (North Sea) population of C. maenas reared at four different salinities (15, 20, 25, 32 parts per thousand). Two an d 4 days after hatching, the Zoea I larvae were moult-staged microscop ically, and their rates of respiration and growth (changes in dry weig ht, W, carbon, C, nitrogen, N, and hydrogen, H) were measured. Surviva l and development were monitored until the megalopa was reached: 15 pa rts per thousand did not allow for development beyond the first zoeal stage, while metamorphosis to the megalopa was reached at salinities g reater than or equal to 20 parts per thousand. At 20 parts per thousan d, development was significantly delayed and mortality enhanced as com pared with 25 and 32 parts per thousand. Rates of growth and respirati on decreased during exposure to reduced salinities less than or equal to 25 parts per thousand. Hence, the suppression of growth could not b e explained as a consequence of enhanced metabolic losses per larva. I nstead, a partial C budget indicates that the Zoea I larvae suffered f rom decreased capabilities of assimilating ingested and subsequently c onverting assimilated matter to tissue growth. Net growth efficiency ( K-2, C-based) was at 25 and 32 parts per thousand initially high (> 60 % during the postmoult and intermoult stages of the Zoea I moult cycle ), but decreased during the later stages (down to less than or equal t o 30% in premoult). An inverse pattern of C partitioning was observed at less than or equal to 20 parts per thousand, with initially low K-2 values (less than or equal to 21% during the first 2 days of the moul t cycle), and a later increase (up to greater than or equal to 46% in premoult). Thus, larval growth was initially suppressed under conditio ns of reduced salinity, but this was later (during premoult) partially compensated for by an increase in C assimilation and K-2. Our observa tions indicate that Zoea I shore crab larvae react during the late sta ges of their moulting cycle less sensitively against reduced salinitie s than during postmoult and intermoult. This suggests that the transit ion between moult cycle stages C and D-0 may be a critical point for e ffects of hypo-osmotic stress, similarly as already known in relation to effects of nutritional stress. Negative effects were found also whe n freshly hatched Zoea I shore crab larvae were exposed only transitor ily (for 24-72 h) to 20 parts per thousand, with significantly lower r ates of survival, development, growth, respiration, and K-2. These eff ects increased with increasing duration of initial exposure to reduced salinity. (C) 1998 Elsevier Science B.V.