Five-lined snapper (Lutjanus quinquelineatus) aggregate under an overhanging wall, Similan Islands, Andaman Sea.

Five-lined snapper (Lutjanus quinquelineatus) aggregate under an overhanging wall, Similan Islands, Andaman Sea. There are a number of closely related, blue lined snapper, that occur, mostly on coral reefs, in the tropical and subtropical Indo-Pacific. The common bluestripe snapper (Lutjanus kasmira) and the Bengal snapper (Lutjanus bengalensis) all have similar patterns and overlapping ranges, including the Andaman Sea. Differentiation is not always easy, and the academic literature has at times been confusing. Modern genetic techniques are helping clarify relationships, to a degree at least. Perhaps a more important question, and one harder to answer, is why very similar species that appear to occupy similar ecological niches, and with broadly overlapping ranges, actually exist. What is the driver for speciation, genetic and reproductive isolation? This is a harder problem to investigate because studying the behaviour of highly mobile species inhabiting deeper waters on offshore reefs is difficult, time consuming and expensive. Colour patterns involving horizontal stripes is a feature that has evolved separately in many species of fish, especially schooling fish (e.g. fusiliers, snapper). Various hypothesis have been proposed as to why horizontal stripes might be advantageous. It may be difficult for a potential predator to identify individual fish in a tightly formed school, all horizontally striped, as the stripes may be more obvious than the outlines of individual fish (often called the confusion effect). It may be difficult to judge the speed of horizontally striped prey as swim past if they are swimming in the same orientation as the stripes are aligned.  Others have suggested that that stripes help co-ordinate movement in schooling fish; as the school changes direction so does the orientation of the stripes, providing a strong visual cue.  Zebrafish are used in many laboratory studies, partly due to their ease of rearing and fully sequenced genome. Working with striped and unstriped zebrafish morphs, scientists found that striped individuals showed the strongest tendency to form schools, and would preferentially form schools with individuals that most closely resembled themselves. How much of this behaviour transfers from zebrafish, fish that inhabit the shallow streams, ponds and rice paddies of South-east Asia, to large schooling fish on deep, offshore reefs. Who knows? Deeper offshore reefs don’t easily lend themselves to manipulative experiments where confounding factors can be controlled. Observational studies can be conducted in large public marine aquaria, but extrapolation to truly natural behaviour must be done with care. It is likely that much of our understanding of behaviour and interactions in such difficult habitats will continue to be opportunistic, conducted from recreational dive boats, with some coming from big-budget natural history documentary filming, such as the BBC’s Blue Planet series, where the money and expertise exists to overcome some of the major logistical problems. There is so much we don’t know about the how and why of the ecology of such reefs, and that situation is unlikely to change anytime soon.

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