If Nemo's kids can't find home

The transition from a pelagic to a benthic lifestyle is a crucial phase in the life cycle of many marine organisms. During early stages of development, dispersal of planktonic larvae may be mostly driven by currents; however, larvae progressively develop behavioural and physiological competencies that allow them to locate, orient towards and selectively settle in suitable benthic habitat. Competencies acquired during development include increased body size, development of functional fins and strong musculature, as well as sensory capabilities necessary for navigation and habitat selection. This set of adaptations means that settlement is far from a stochastic event.

Research has shown that acoustic cues help in the settlement of marine fishes and some invertebrates. Electrophysiological studies also revealed that the sense of hearing becomes more important throughout larval development and that at this particular life stage they are able to hear particular frequencies, an ability lost later in life. It is also known that some fish species have a very narrow window of competency for settlement. It seems likely that both findings are connected and that hearing guides little fish toward their future home.

A new study published today provides some more proof for the assumption that the interpretation of normal ocean sound cues helps juvenile fish to find an appropriate home. However, tests conducted as part of the study also showed that fish were completely confused under the levels of CO2 predicted to be found in oceans by the end of the century as a result from Ocean acidification:

Here we show that larvae of a catadromous fish species (barramundi, Lates calcarifer) were attracted towards sounds from settlement habitat during a surprisingly short ontogenetic window of approximately 3 days. Yet, this auditory preference was reversed in larvae reared under end-of-century levels of elevated CO2, such that larvae are repelled from cues of settlement habitat. These future conditions also reduced the swimming speeds and heightened the anxiety levels of barramundi. Unexpectedly, an acceleration of development and onset of metamorphosis caused by elevated CO2 were not accompanied by the earlier onset of attraction towards habitat sounds. This mismatch between ontogenetic development and the timing of orientation behaviour may reduce the ability of larvae to locate habitat or lead to settlement in unsuitable habitats.