Responses of echinoid larvae to food patches of different algal densities |
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Authors: | A Metaxas C M Young |
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Institution: | (1) Department of Larval Ecology, Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Fort Pierce, Florida 34946, USA, US |
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Abstract: | High densities of larvae have been found in areas of high primary production, but it remains unclear whether this is the
result of hydrodynamics or of larval aggregative behaviour in the presence of food. In this study, we examined changes in
the vertical distribution and swimming patterns of four-armed larvae of the sea-urchin Echinometra lucunter (Linnaeus) around food patches of a range of microalgal densities. We reared larvae in the laboratory in a high or low concentration
of either single (Isochrysis galbana) or mixed (I. galbana, Dunaliella tertiolecta, Thalassiosira weissflogii) microalgal species. In Plexiglas cylinders, we experimentally constructed haloclines in which the salinity of the bottom
water-layer was 33‰ and that of the top water-layer was 24‰. In a thin layer in the middle of the halocline, we inserted a
food patch that consisted of 0, 2500, 5000 or 10 000 T. weissflogii cells ml−1. The presence of a food patch had a pronounced effect on the vertical distribution of larvae. This effect depended upon the
algal density of the food patch and varied with dietary conditioning. The number of larvae that were above or within the patch
decreased with increasing algal density, and was greater if larvae were reared in low-ration or single-species diets than
in high-ration or mixed-species diets. Tracking of individual vertical swimming paths showed that within a few minutes, larvae
swam into the patches of low algal density, and to positions just below the patches of the two higher algal densities, and
remained there until the end of the experimental period. The greater number of algal cells in the digestive tracts of larvae
from treatments with a food patch than in those without a patch confirmed that larvae were feeding on the microalgal cells
of the patch. To our knowledge, this is the first study to experimentally show an aggregative behavioural response of invertebrate
larvae to a food patch. Such a response may reduce the probability of food limitation and therefore enhance larval survival.
Received: 14 February 1997 / Accepted: 24 September 1997 |
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