Life Science Mass Spectrometry Facility

Sea bed scrapers and shapers: Urchins in control?

Echinoid grazing can play a crucial role in the structuring of many intertidal and subtidal habitats (1,2). They have been established as keystone herbivore in some environments for their role in controlling algae populations and preventing overgrowth of reef building corals (3). The majority of previous work, however, has concentrated on their herbivory, and it is now accepted that many populations are total or opportunistic omnivores (1,4,5,6,7,8).

Adam Hughes and Maeve Kelly of the Scottish Association of Marine Science, are investigating the feeding ecology of two species of sea urchin which inhabit the west coast of Scotland at Loch Creran. Psammechinus miliaris and Echinus esculentus, are both known to be omnivorous and can be highly abundant. P. miliaris is the smaller urchin, with a maximum test diameter of approximately 40mm; it occurs in a wide range of habitats including the low intertidal on boulder shores, sub tidal soft bottom and on rocky reefs (7). E. esculentus, in contrast, has a maximum test diameter of 160mm. Diet can effect the growth and reproductive fitness of urchins, and a diet high in animal material has resulted in enhanced somatic and gonadal growth of P. miliaris in experimental conditions (9,10). In echinoids, the gonad can be used as a storage organ, so gonadal development is an indicator of nutritive status as well as reproductive output (11).

Populations of P. miliaris from within Scottish sea lochs are known to exhibit different levels of gonad development. Those occurrin<13 September, 2005than those of the subtidal. It has been hypothesized that this inter population variation may be due to predation of sessile and encrusting invertebrates by the intertidal population (11), as opposed to mainly algal diets of the subtidal populations. Difficulties are encountered when attempting to establish the diet of wild urchins, which have a wide choice of natural foodstuffs. Through stable isotope analysis, we will be able to test this theory in natural populations. Determination of d13C and d15N will help distinguish dietary patterns and to indicate the relative trophic position of contrasting populations of urchins.



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