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The functional morphology and ecology of the spatangoid genus Brisaster Gray Gibbs, Peter Edwin

Abstract

The functional morphology, taxonomy and ecology of the Spatangoid genus Brisaster Gray (Family Schizasteridae) from Howe Sound, British Columbia, have been investigated. Brisaster predominantly inhabits a mud substratum and burrows to a depth of 1 cm., constructing both a respiratory funnel and a double sanitary apparatus. Burrowing and feeding activities resemble those of Spatangids. The absence of a sub-anal fasciole in Brisaster correlates with its shallow burrowing habit. Despite the lack of a sub-anal fasciole, the ciliary current pattern of the test is similar to that of the Spatangids which possess this fasciole. This suggests a common ancestral form (perhaps for all Spatangoids) in which the basic ciliary pattern had evolved; thus the different types of fascioles appear to have evolved as superimpositions on the basic ciliary pattern rather than the reverse. The zoogeography and taxonomy of the genus is briefly reviewed and the need for qualification of certain taxonomic criteria stressed. The latero-anal fasciole, for example, is an unreliable taxonomic character since it disappears with growth to varying degrees in different species. This possibly reflects changes in niche since the isolation and speciation of the genus took place. Biometrical analyses of the lengths of the ambulacral petals and the test height indicate that only a single species of Brisaster is present along the west coast of North America. Formerly two species had been described. Taxonomic priority is given to the species Brisaster latifrons; B. townsendi thus becomes a synonym. Synonymy is also suggested for the Japanese species B. owstoni, in view of paleogeographic evidence. The gonads of Brisaster do not develop until the second year and, as the ova appear to require two years to develop to maturity, the females probably first spawn in their third year. On this basis, the first, second and third (and older) year classes were identified in the populations occurring in Howe Sound. These populations showed marked differences in their size-frequency distribution but a similar age class composition. The size differences of individuals appear to correlate with differences in the population density, larger individuals being found in less crowded areas. It is suggested that these density differences are a result of the irregular settlement of a restricted pelagic larval stage. The differences in the size of individuals can therefore be related to differences in the individual growth rate. Passive interference, both inter- and intraspecific may be responsible and two possible mechanisms have been suggested.

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