Gonad growth in the sea urchin, Strongylocentrotus purpuratus (Stimpson) (echinodermata: Echinoidea) and the assumptions of gonad index methods

In central Oregon intertidal populations, the echinoid Strongylocentrotus purpuratus (Stimpson) first begins to produce mature gametes at a diameter of 25 mm during the second year of its life. All individuals above 40 mm in diameter have mature gametes during the mid-winter spawning season. Gonad size increases rapidly relative to body size until the animals are ≈45 mm in diameter and have a total dry weight of ≈ 15 g. The ratio of gonad to body size reaches its maximum in animals between 40 mm and 50 mm in diameter; thereafter the ratio drops slightly with increasing size. The curvilinear relation between gonad size and body size is not related to a relative difference in body proportions with size. Internal volume, height, and diameter increase linearly with size. Body proportions did not differ between the two populations studied. Differences between populations in the ratio of gonad to body size extend to all sizes in the populations and the direction but not the amount of the difference is consistent from year to year. These differences between habitats are probably related to food supply. The use of gonad ratio methods to normalize gonad size assumes that, within a given population, animals of different body size have the same ratio of gonad size to body size. This assumption is not correct for four species of sea urchins. Gonad index or ratio methods are useful for comparing animals of the same size through time within a population or from different habitats. Restriction of the size range of animals used and use of samples of sufficient size permit gonad ratios to be used to assess the capacity of different habitats to support gonad production in sea urchins and other animals.