Influence of the Atlantic Ocean thermal anomaly on the Longsnout seahorse Hippocampus reidi in a Brazilian estuary.

One of the consequences of climate change is an increase in the temperature of the oceans, which is considered to be one of the greatest impacts on biodiversity. Fish may respond to this impact in several ways, including shifts in their patterns of occurrence. The present study investigated the variation in the structure of a H. reidi population between 2015 and 2017 in the northern Guaíba Island area, highlighting a possible relationship to thermal anomaly associated with the El Niño phenomenon. The seahorse population monitoring was performed monthly, recording sex ratio, abundance, juvenile and adult proportion, depth of occurrence, total length and the holdfast which the seahorse were found attached. The influence of the El Niño event on the study population was evaluated by the correlation of the thermal anomaly data reported for the Tropical South Atlantic Index. Seahorse density on northern Guaíba island was positively and significantly correlated with water temperature, but the sex ratio and number of juveniles were not. The diversity of holdfasts used increased over the study period and was inversely proportional to the thermal anomaly. These results suggest that the thermal anomalies caused by the El Niño in the South Atlantic might trigger migration behaviour in the study species, providing a large aggregation during that period in Guaíba island.

Shining in the dark: First record of biofluorescence in the seahorse Hippocampus reidi.

Marine environments are visual domains restricted regarding light characteristics. Overall, blue monochromatic spectrum prevails in offshore areas especially below 15m depth, since long wavelengths are quickly attenuated. Light intensity is even more constrained in coastal waters, particularly those of tropical estuaries and bays, because further scattering through dissolved and suspended materials. Biofluorescence, which is the ability of organisms to absorb light and reflect it in a different wavelength, has been reported for many marine fish. In this paper, biofluorescence was recorded for the first time for the longsnout seahorse Hippocampus reidi, under natural conditions at Ilha Grande bay, Brazil, and both adult, juvenile and fry individuals kept in captivity. Although displaying the same colour emissions, seahorses differed in relation to body lighting, colour patterns, and age wherein fluorescence occurs. Newborn seahorses exhibit green biofluorescence only in the eyes and stomach. Further experiments are necessary to address whether H. reidi can change the patterns of biofluorescence emission for sensorial and social purposes.