Combined effects of radiation and food on vertical migration of crab larvae Cyrtograpsus altimanus

R.D.HERNÁNDEZ MORESINO; M.S. VALIÑAS; M. V. FIORDA GIORDANINO; M. L. SPINELLI; R. J. GONÇALVES

Conferencia; CERF 2012. The Changing Coastal and Estuarine Environment: A Comparative Approach; 2012

Coastal & Estuarine Research Federation

Many studies have documented the negative effects of the increased ultraviolet radiation (UVR, 280-400 nm), especially ultraviolet radiation-B (UV-B, 280-315 nm), in aquatic ecosystems.  To cope with this potential threat, organisms may respond either behaviorally, developing repair systems, and/or accumulating photoprotective compounds.  Many species of zooplankton seem to cope with high UVR levels by altering their depth in the water column by means of diel vertical migration (DVM).  DVM may be caused by biotic (predation, food) and abiotic (solar radiation, temperature) factors as well.  Planktonic crab larvae are able to actively swim and perform DVM, and in particular Zoea I stage (ZI) of Cyrtograpsus spp. is known to be the sensitive to UVR exposure from our previous experiments. Adult crabs of C. altimanus are a transient component along the Patagonian coast, and A. tamarensis is found in high concentrations in these waters and could be an important food item for their crab larvae during the early developmental stages.  Furthermore, A. tamarensis is known to be toxic for some crustacean species, therefore its presence could affect the vertical distribution of herbivorous zooplankton.  Thus our goal was to determine the response of ZI of the crab Cyrtograpsus altimanus under different conditions of radiation and food.  Three replicated experiments were performed in which larvae were exposed to a solar simulator to configure two radiation treatments (i.e., visible light + UVR or visible light only) and two food treatments (with or without food supply) The food supply consisted of the dinoflagellate Alexandrium tamarensis (2000 cells/ml).  Our results show that the presence of UVR inhibits the tendency of larvae of C. altimanus to swim towards the surface (i.e., compared to larvae exposed to visible light only). In the UVR treatment, vertical distribution shifted downwards.  This helps explaining why larvae are not concentrated in the surface even when they quickly react and swim actively towards the light in the laboratory.  On the other hand, the presence of A. tamarensis didn?t have a significant effect on the vertical distribution of larvae.  Thus, diel vertical migration appears to be a very important strategy of C. altimanus to cope with high UVR irradiances which typically occur in Patagonia during the hatching seasons (spring-summer).