Glassfish switch feeding from thalassinid larvae to crab zoeae after tidal inundation of saltmarsh

Abstract

Saltmarsh-dwelling grapsid crabs release free-swimming larvae (i.e. zoeae) into ebbing tides during spring-tide cycles that inundate saltmarshes, where initial inundation is a cue for larval release on subsequent inundations. In a saltmarsh environment, crab zoeae are the main food for fish (including the glassfish, Ambassis jacksoniensis), which ‘fast’ at other times. This saltmarsh-feeding model was tested by obtaining glassfish from near saltmarshes in a reasonably unmodified tributary of a large temperate estuary on flood and ebb tides during the night in two spring-tide events in austral autumn of 2009. Glassfish fed only on ebbing tides, with stomachs being similarly full on both spring-tide events. Thalassinid larvae (including Trypaea australiensis) dominated the dietary volumes, especially on the night before saltmarsh inundation, presumably being released during inundation of intertidal mud and sand habitats. Although glassfish progressively ‘switched’ to feeding on greater volumes of crab zoeae (presumably released after inundation of a saltmarsh) over both spring-tide cycles, such zoeal contributions never exceeded those of thalassinid larvae. The above differences highlight that, although ebb tides trigger feeding by glassfish, this ambassid focuses on different prey in a reasonably unmodified environment. The ability of glassfish to switch prey, and thus accommodate environmental differences, helps explain their high abundance in estuaries of this region.