Comparative morphological and ecological study of two species of the sea urchin genus Echinometra in Hawaii Echinometra in Hawaii

Kelso, Donald Preston
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Two species or varieties of the sea urchin Echinometra occur in Hawaii. The relationship between the two may be that of polymorphs, polyphenotypes, or separate species. One form (E. oblonga) has black spines, usually has stout, blunt spines, rarely has spindle-shaped spicules in the gonad, has a higher average number of pore-pairs per are, and has a smaller mean and maximum test length. The other form (E. mathaei) has non-black (usually pink and/or green) spines, rarely has heavy spines, almost always has spindle-shaped gonad spicules, has a lower average number of pore-pairs per are, and has a larger mean and maximum test length. The abundance of the black form is greatest in areas of surf action, where it numerically dominates the pink/green form. The pink/green form predominates in areas of less turbulence in tidepools, on shallow reef flats, and in deeper water. No significant differences were detected in tolerance to insolation, desiccation and heating, extreme salinity changes, or spine strength per unit cross-sectional area. However, the larger mean diameter of the black spines does make the breaking resistance of the average black spine greater than that of the average pink/ green spine. Polyphenotypic variation under the different ecological conditions was investigated with respect to spine color, spine shape, and gonad spicules and judged not responsible for the observed morphological differences between the two forms. Monthly samples collected for 2 years indicate that the spawning period of both forms occurs in the spring (March-June) with a second, minor spawning in December. Mature gametes were found in a few individuals at all times of the year. The December spawnings were perhaps induced by extremely heavy rainfalls. Initiation of the spring spawning season appeared to be correlated with increasing temperature and internal nutrient supply. Homogametic fertilizations (sperm and eggs from the same color form) were over 94% successful. Heterogametic fertilizations (sperm and eggs from different color forms) were about 10% successful. Sperm from an unusual urchin with intermediate morphological characteristics produced 64% fertilizations with one form and 100% fertilizations with the other. Larvae from all crosses survived normally for 6 days, after which they died from starvation. No attempt was made to rear larvae to metamorphosis. Examination of the morphological variation in Echinometra in the rest of the Pacific Ocean shows less distinction between the varieties, with more overlap in morphological features. stout, blunt spines and an absence of spindle-shaped gonad spicules are correlated with each other, but spine color differences (black versus non-black) are not consistent with this separation. Monomorphic populations of Echinometra occur in the Gulf of Suez, Western Australia, Japan, and the eastern Pacific. The definite habitat specialization, extensive non-ecophenotypic morphological distinctness, and the gamete incompatibility strongly indicate that the two forms in Hawaii are separate species. The varieties of Echinometra found in the South Pacific are evidently polymorphs of a single species. The distinctness of the two Hawaiian species is probably indicative of character displacement.
Bibliography: leaves [106]-112.
ix, 112 l illus., maps, graphs, tables
Sea urchins -- Hawaii
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Zoology; no. 299
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