Skeletal changes in the test and jaws of the sea urchin Diadema antillarum in response to food limitation

When food-limited, Diadema antillarum increases the relative size of Aristotle's lantern compared to the size of the test. This is accomplished primarily by decreasing the size of the test: the demipyramid grows at a reduced rate as the test shrinks. These results suggest that an altered relationship between demipyramid and test size can provide evidence for food-limitation in field populations. The sizes of the lantern and test of museum and fossil specimens can be investigated for the presence of food limitation and biotic interactions in recent and past times. limited conditions appears to be a general phenomenon. The present study elucidates the morphometric changes in the skeleton and provides evidence of these changes under experimental manipulations of population density and resource availability. Materials and methods Two experiments were conducted to determine how population density and food availability affect the growth of Diadema antillarum (for details, see Levitan 1989). The first was an 8-mo field experiment in which individuals from two size classes (25.0 to 30.0 and 35.0 to 40.0 mm test diameter) were placed in subtidal cages (0.25 m2 in area) at two densities (12 and 24(m2). Each treatment group had eight replicate cages (total of 144 individuals). The second was a 6-mo laboratory experiment in which individuals from one size class (40.0 to 45.0 mm) were given one of four levels of food (4, 2, 1, and 0 9 wet weight of the green alga U/va /actuca, every 4 d). The laboratory experiment had two levels of crowding (one and three individuals per container), with both levels receiving the same per capita food level. Each treatment group had eight replicates (total of 128 individuals). Crowding did not affect growth (Levitan 1989), and for the present study these data were pooled by food level. In both these experiments, test diameter and the length of the demipyramid were measured for each individual. Test diameter was measured by picking up the echinoid with tongs, turning it oral side up, and placing the tips of needle-nose vernier calipers between the spines from one interambulacral region across the mouth to the opposite ambulacral region. Each individual was measured on three different diameters to 0.1 mm and the mean diameter was used as the datum (the mean standard error of the three measurements was 0.18 mm). Each individual was measured at the start of the experiment and every 2 wk thereafter for a total of 32 wk in the field experiment and 23 wk in the laboratory experiment. In the laboratory experiment, each echinoid was individually identified, and only those echinoids that survived to Week 22 were included in the analysis (those echinoids which died in the last week were kept and their skeletal measurements recorded). In the field experiment, individuals could not be identified within a cage, and thus differential mortality might have influenced measurements of changes in size. However, in this latter experiment, mortality rate was not significantly different between treatments, and in three out of four treatments the final sizes were outside the initial size range (Levitan 1989). In analyses where initial sizes were needed, only data from the laboratorY experiment were used. Introduction Food limitation in echinoids results in an increase in the relative size of Aristotle's lantern compared to the size of the test (Ebert 1980, Black et al. 1982, 1984, Edwards and Ebert 1991). Ebert proposed the "plastic resource allocation" hypothesis when he noted this relationship in two species (Strongylocentrotus purpuratus and Diadema setosum). He suggested that food-limited echinoids increased the size of the jaw apparatus (measured by the length of the demipyramid) which increased their capacity to graze. Black et al. (1982) noted a similar relationship between demipyramid and test size dependent on population density in the echinoid Echinometra mathaei; as population density increased so did the relative size of the jaws. Black et al. (1984) suggested that a relatively larger Aristotle's lantern was associated with increased grazing ability. However, since these studies did not provided information on initial sizes (Ebert 1980, Black et al. 1984) or did not follow single individuals over time (Edwards and Ebert 1991), it is unknown whether this relationship is a result of an increase in demipyramid size and/or a decrease in the size of the test. The altered relationship between the size of the test and demipyramid under food* Present address: Department of Zoology, University of california at Davis, Davis, California 95616-8755, USA @ Springer-Verlag 1991 432 D. R. Levitan: Skeletal plasticity in sea urchins