Emergence of Small Invertebrates in the Shallow Subtidal Zone: Investigations in a Subtropical Sea at Iriomote-jima in the Ryukyu Islands, Japan

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Abstract: Many invertebrates inhabit the shallow subtidal zone in subtropical regions, and some of them also emerge at times into the water column. To detect any synchrony of their emergence patterns with the day/night or tidal cycle, and to monitor their abundance in the surface and bottom waters, small invertebrates were collected continuously for one month at Iriomote-jima in the Ryukyu Islands, Japan. Two pumps were set to collect surface and bottom water, respectively. The daily patterns of emergence in 24 major taxa, mostly crustaceans, were analyzed by visual inspection and statistical methods. A notable feature was a clear nocturnal pattern in all the taxa except Corycaeus sp.; however, the nocturnal pattern varied among taxa with regard to its strength and the influence of the tidal cycle upon it. The emergence patterns of each taxon were much the same between the surface and bottom. Some animals (e.g. Labidocera pavo and Lucifer hanseni) were more abundant in the bottom water, but other animals (e.g. crab zoeas comprising several species) in the surface water. Almost all the animals collected by the pumps were also collected from the bottom substrate in the daytime. Therefore, absence of these invertebrates by day from pump samples does not imply their escape from the area where the pumps were installed, but merely that by day they hide within the bottom substrat or just above it, where very few specimens could be collected even by the bottom pump. Dispersing or swimming into the water column at night, they could then be collected by both pumps. ‘Daily emergence/dispersal’ may be a better term than ‘daily vertical migration’ for this behavior pattern in subtropical marine invertebrates. Furthermore, because of similar patterns of periodic activity, the distinction between ‘benthos’ and ‘plankton’ is blurred for small subtropical invertebrates.

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  • BENTHOS RESEARCH

    BENTHOS RESEARCH 54 (2), 59-70, 1999

    日本ベントス学会

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