Stenophagous, herbivorous sea slugs attack desiccation-prone, green algal hosts ( Codium spp. ): indirect evidence of prey-stress models (PSMs)?

Abstract

Several congeneric species of intertidal green algae inhabit shady, low-desiccation microhabitats. The N.E. Pacific green alga Codium setchellii occurs on sand-influenced rocky shores, particularly on shady, vertical surfaces on low intertidal rocky benches. The S. Pacific congener C. convolutum shows a similar, though less restrictive, distribution. The algal distributional patterns were inversely related to potential desiccation stress. Thallus abundance and/or size increased lower on the shore and on shady aspects. Because some thalli persist in desiccation-prone habitats, C. setchellii and C. convolutum can persist in alternative areas. Spatial patterns of specialist herbivore abundance indicated that stenophagous sea slugs that fed selectively on Codium were disproportionately more frequent on desiccation-prone algal hosts. On Oregon shores, USA, the sea slug Placida dendritica was more frequent on the alga C. setchellii (i) at the upper end of the alga's tidal distribution than at the lower end and (ii) on desiccation-prone, eastern aspects than on those with more benign, western aspects. Populations of slugs on Oregon shores peaked in spring and early summer during the period of peak desiccation to low intertidal species. The seasonal, habitat-associated pattern of slug distribution on algal hosts was geographically and taxonomically robust. On N.E. New Zealand shores, two species of ascoglossan slugs were significantly more frequent on thalli of C. convolutum at the upper end of the alga's tidal distribution than at the lower end. Increased larval settlement of slugs on desiccation-prone algal hosts contributed to this pattern whereas spatial variation in predator abundance did not. Larval metamorphosis experiments with the ascoglossan Elysia viridis from Scottish shores demonstrated that the frequency of larval metamorphosis was significantly greater in the presence of desiccation-prone algal hosts than unstressed ones. Thus, variable algal susceptibility and differential slug settlement and metamorphosis both contribute to this striking pattern of targeted slug attack. In contrast to the wealth of terrestrial examples, this is one of the few marine examples of prey-stress models.