Life at the edge: population dynamics and salinity tolerance of a high intertidal, pool-dwelling ascoglossan opisthobranch on New Zealand rocky shores

Abstract

The population dynamics of the pool-dwelling ascoglossan (= sacoglossan) opisthobranch Stiliger felinus were examined from September 1992 to August 1993 on the North Island of New Zealand. The small (≤13 mm long), stenophagous herbivore associates with and feeds on the filamentous green alga Chaetomorpha aerea (Dillw.) Kuetz. in high intertidal rocky pools. The ascoglossan's phenology and demography differs between N.E. and N.W. New Zealand shores. In the Hauraki Gulf (N.E. shore), S. felinus inhabits pools all year, occurring in 26% of the small, shallow Chaetomorpha pools surveyed. Recruitment peaks in summer, and mean slug length peaks in winter. At Piha (N.W. shore), S. felinus is common and large during winter and spring; the species' absence during summer is not attributable to food availability or physical conditions of tidepools. Slugs are tolerant of a wide range of salinities (9 to 60 ppt). Egg mass production and tenacity of adults and developmental rate of encapsulated embryos decline at extreme salinities. Thus, suboptimal tidepool conditions delay ascoglossan embryonic development and larval hatching until pools are flushed with fresh seawater and planktonic larvae can emigrate. This strategy differs markedly from the accelerated development of larval chironomids and amphibians inhabiting transient rain pools.