Reproductive Isolation? Interannual Variability in the Timing of Reproduction in Sympatric Sea Urchins, Genus Pseudechinus

Abstract

Reproductive cycles of 3 sympatric species of the echinoid genus Pseudechinus, found in southeastern New Zealand, were examined over 27 months from October 1989 to December 1991. Analysis of periodic changes in relative gonadal size was used to help define the reproductive cycle of each species. Although P. huttoni reproduced primarily during the early (austral) summers of 1990 and 1991 (December-January), mature gametes were present in the gonads throughout much of the year. Conversely, P. novaezealandiae reproduced primarily during the autumn and early winter (May-July). P. albocinctus had an extended reproductive period in 1990, which peaked in late spring/early summer (December-January), followed by a gradual spawn-out to late autumn (May-June). The pattern was different in 1991, with a secondary peak in reproductive potential noted in late autumn (June). For all 3 species, males tended to be competent to spawn gametes for a longer period than were females. These observations were all corroborated by analysis of changes in overall gonad histology and in the distribution of oocyte sizes in the ovaries of each species throughout the year. P. huttoni and P. novaezealandiae thus appeared to exhibit temporal reproductive isolation from each other. Intraspecific as well as interannual variability in the timing of reproduction, however, suggests that such temporal isolation may not always be complete. In contrast, P. albocinctus was not temporally isolated from either of its 2 sympatric congeners. These results were examined in the context of seasonally variable environmental conditions. Additional key words: Echinoidea, Echinodermata, El Ninio, hybridization, reproductive isolation The echinoid genus Pseudechinus (MORTENSEN 1921) comprises 11 species, of which 6 are endemic to New Zealand waters (McKnight 1969) and 3 cooccur throughout much of their range in southern New Zealand. P. albocinctus (HUTTON 1872), P. huttoni (BENHAM 1908), and P. novaezealandiae (MORTENSEN 1921) are restricted primarily to waters off southern North Island, South Island, and subantarctic islands south of mainland New Zealand (Mortensen 1943; McKnight 1969). All 3 species have been recorded in sympatry in nearshore waters off the southeast coast of South Island from Oamaru (North Otago) to Foveaux Strait. The presence of 2 or more closely related species living in sympatry provides an interesting situation: closely related species may have similar life histories, possibly sufficiently similar to allow them to interbreed. How is the genetic integrity of a species maina Author for correspondence. E-mail: dmcclary@unitec.ac.nz tained if hybridization with congeneric species can occur? Is it maintained? A number of isolating mechanisms have been suggested, including microhabitat separation (Nishihara et al. 1991), differences in reproductive periodicity (Pearse & Cameron 1991), and gametic incompatibility (Strathmann 1981; Lessios & Cunningham 1990; Palumbi & Metz 1991). As these mechanisms are not likely to result directly from selection (see Butlin 1987; Endler 1989), they may be applied equally to both conand hetero-specific fertilization events; we will refer to them as constraints on fertilization. While there are many reported instances of 2 species of closely related echinoids living in sympatry (Mayr 1954), the co-occurrence of 3 or more congeneric echinoid species is somewhat unusual. Where it does occur, differences in the timing and/or place of reproduction have usually been noted between the species. Temperate-water echinoids typically reproduce annually or biannually for a relatively short period of time (Lawrence 1987; Strathmann 1987). As well, reThis content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 04:36:26 UTC All use subject to http://about.jstor.org/terms