Effects of spine damage and microhabitat on resource allocation of the purple sea urchin Strongylocentrotus purpuratus (Stimpson 1857)

Abstract

Though phenotypically plastic responses in echinoids are well studied, the majority of the literature examines the effect of altered diet. Some species, however, such as the purple sea urchin, Strongylocentrotus purpuratus, occur over a large geographic range and thus experience a variety of substrate types and wave forces. To determine whether these factors impact resource allocation in adult intertidal S. purpuratus, a laboratory experiment was conducted using two types of microhabitat based on extremes inhabited by sea urchins in the field (flat substratum vs. concave pit), and two levels of spine damage to simulate wave exposure (monthly clipping vs. control). A suite of measurements were taken to determine the effects of these two treatments including feeding and movement behavior, test morphometrics, demipyramid (jaw) allometry, spine growth cycles, and mass and stereological counts of gonad sections. When given the opportunity, sea urchins remained in their pits for the duration of the experiment, which limited their ability to feed. This behavioral response caused a decrease in weight and volumetric growth, and an increase in jaw growth relative to test growth. Sea urchins in pits also grew more spheroid in shape (increased height:diameter ratio), while those on flat substrates decreased in height:diameter ratio. Further, both males and females in pits showed altered gametogenesis. While spine damage did not affect test growth, sea urchins with clipped spines showed decreased gonad index and altered production of mature oocytes. Altogether, these results are congruent with the life history strategy of long living species like S. purpuratus, in that it spreads reproductive efforts over multiple seasons, while diverting resources toward repairing damage, acquiring suitable refuge, and maximizing attachment area. Therefore, microhabitat and spine damage, by impacting movement, feeding, growth, and reproductive output, likely have important indirect impacts on the intertidal community by way of altered sea urchin grazing rates and susceptibility and value to predators.