The embryonic life history of the tropical sea hare Stylocheilus striatus (Gastropoda: Opisthobranchia) under ambient and elevated ocean temperatures.

Rael Horwitz,Suzanne C Mills,Matthew D Jackson

doi:10.7717/peerj.2956

Abstract

Ocean warming represents a major threat to marine biota worldwide, and forecasting ecological ramifications is a high priority as atmospheric carbon dioxide (CO2) emissions continue to rise. Fitness of marine species relies critically on early developmental and reproductive stages, but their sensitivity to environmental stressors may be a bottleneck in future warming oceans. The present study focuses on the tropical sea hare, Stylocheilus striatus (Gastropoda: Opisthobranchia), a common species found throughout the Indo-West Pacific and Atlantic Oceans. Its ecological importance is well-established, particularly as a specialist grazer of the toxic cyanobacterium, Lyngbya majuscula. Although many aspects of its biology and ecology are well-known, description of its early developmental stages is lacking. First, a detailed account of this species’ life history is described, including reproductive behavior, egg mass characteristics and embryonic development phases. Key developmental features are then compared between embryos developed in present-day (ambient) and predicted end-of-century elevated ocean temperatures (+3 °C). Results showed developmental stages of embryos reared at ambient temperature were typical of other opisthobranch species, with hatching of planktotrophic veligers occurring 4.5 days post-oviposition. However, development times significantly decreased under elevated temperature, with key embryonic features such as the velum, statocysts, operculum, eyespots and protoconch developing approximately 24 h earlier when compared to ambient temperature. Although veligers hatched one day earlier under elevated temperature, their shell size decreased by approximately 20%. Our findings highlight how an elevated thermal environment accelerates planktotrophic development of this important benthic invertebrate, possibly at the cost of reducing fitness and increasing mortality.

Highlights

Increasing anthropogenic carbon dioxide (CO2) emissions are changing ocean conditions at an accelerating rate (Hoegh-Guldberg & Bruno, 2010)
The present study focuses on the tropical sea hare, Stylocheilus striatus (Gastropoda: Opisthobranchia), a common species found throughout the Indo-West Pacific and Atlantic Oceans
This species is relatively well-studied, including many aspects of their taxonomy (Bebbington, 1974), anatomy (Bebbington, 1977), physiology (SwitzerDunlap, 1978; Carefoot, 1987), reproductive patterns (Switzer-Dunlap & Hadfield, 1979), diet-derived chemical defense (e.g., Paul & Pennings, 1991; Nagle, Camacho & Paul, 1998; Arthur et al, 2009) and sensitivity to noise pollution (Nedelec et al, 2014). The important role they play in benthic reef ecology is well-established, as a specialist grazer of the toxic cyanobacterium, Lyngbya majuscula (Thacker & Paul, 2004), that prevents the settlement of coral larvae (Kuffner & Paul, 2004) and can cause phase shifts from coral to algal dominated reefs (Thacker, Ginsburg & Paul, 2001)

Summary

Introduction

Increasing anthropogenic carbon dioxide (CO2) emissions are changing ocean conditions at an accelerating rate (Hoegh-Guldberg & Bruno, 2010). This species is relatively well-studied, including many aspects of their taxonomy (Bebbington, 1974), anatomy (Bebbington, 1977), physiology (SwitzerDunlap, 1978; Carefoot, 1987), reproductive patterns (Switzer-Dunlap & Hadfield, 1979), diet-derived chemical defense (e.g., Paul & Pennings, 1991; Nagle, Camacho & Paul, 1998; Arthur et al, 2009) and sensitivity to noise pollution (Nedelec et al, 2014) The important role they play in benthic reef ecology is well-established, as a specialist grazer of the toxic cyanobacterium, Lyngbya majuscula (Thacker & Paul, 2004), that prevents the settlement of coral larvae (Kuffner & Paul, 2004) and can cause phase shifts from coral to algal dominated reefs (Thacker, Ginsburg & Paul, 2001). Their embryonic developmental life history has not been investigated and there is a lack of basic information about their initial life stages (but see EJ Armstrong, T Allen, M Beltrand, V Dubousquet, JH Stillman, SC Mills, 2012 & 2015, unpublished data)

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Conclusion