Population Variability and Resource Utilization Patterns of Caribbean Comatulid Crinoids (Echinodermata).
Liddell, William David
1980
Abstract
Studies dealing with resource utilization patterns of tropical marine organisms have tended to support the view that this system is characterized by fine niche separation brought about by competition at equilibrium. Recently, however, this reasoning has been challenged and it has been suggested that, due to physical disturbance, predation, and other factors, competitive equilibrium might rarely be achieved in many natural systems, including the tropical marine environment. One consequence of this would be the "tolerance" of considerable overlap in resource utilization by co-occurring species. This study of resource utilization patterns of reef-dwelling crinoids from the Caribbean evaluates the applicability of this non-equilibrium model to this invertebrate group. The study involves: (1) determination of taxonomic relationships of co-occurring crinoids, (2) evaluation of food resources acquisition, and (3) determination of habitat utilization patterns. The shallow-water Caribbean crinoid fauna examined includes five genera which are distributed among four families. The majority of the genera are monospecific while the genus Nemaster includes at least three and probably four species which exhibit a considerable amount of variation. Multivariate analysis of morphological characters as well as electrophoretic enzyme patterns were employed in the assignment of taxonomic affinities. The crinoid species comprise three behavioral groups: (1) nocturnal or functionally so, (2) continuously exposed, and (3) cryptic. Analyses of feeding mechanisms indicate that the fauna may also be subdivided into forms with planar filtration fans and those with tetrad/radial arrays. Minor differences (length, spacing, thickness) at the level of the primary capture units, the tube feet, do occur and appear to reflect both systematic as well as environmental control. Despite such behavioral and morphological differences, for the most part, the crinoids studied exhibit few differences in qualitative composition or size of food items captured. Preliminary analysis of potential caloric contributions suggest that planktonic food items may contribute relatively little to the overall energy budget of the crinoids. In contrast to the great similarities in food resources utilized, many of the crinoid species show considerable differentiation along the parameters of bathymetry and , particularly, microhabitat. However, two of the Nemaster species, N. discoidea and N. rubiginosa, overlap considerably with each other and to a lesser degree with certain other crinoid species in habitats occupied. These appear to be generalists, occupying any sites which provide protection for the oral disk and elevation into the water column. It is likely that the niche of a species can be defined by relatively few parameters. Analysis of times of activity indicates separation of the various crinoid species into two well defined groupings. That little divergence has occurred in food resources utilized might be expected since planktonic food items may well not be a limiting resource for these reef-dwelling suspension feeders. It is uncertain whether or not the differentiation of many species by habitats occupied actually reflects the outcome of past competitive interactions (i.e. character displacement) or other factors. N. discoidea and N. rubiginosa, however, overlap considerably along time, food, and habitat niche axes. The present study suggests that competitive equilibrium is not reached by these populations, thereby permitting a great deal of niche overlap. Predation is regarded as the agent responsible for the non-attainment of equilibrium conditions. A high proportion of suitable habitat types are not occupied by individuals of either species and much indirect evidence of fish predation upon these crinoids suggests that their populations are maintained at artificially low (i.e. non-competitive) levels.Types
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