Phenotypic Variation and Speciation in Marine Fringe Snails

Understanding the capacities of animals to adapt to different environmental circumstances allows insight into species' responses to future human-driven environmental change. Gastropod molluscs living in marine fringes such as intertidal zones and estuaries experience extremes in environmental conditions, presenting excellent opportunities to explore their adaptive capacities.

Ongoing collaborative studies involving international researchers and Universiti Brunei Darussalam (UBD) students aim to understand genetic, physiological and morphological attributes of marine fringe gastropods. A study by Professor David J Marshall and Hussein Taha from the Faculty of Science (FOS) considered these differences in supratidal and intertidal rocky-shore snails (Echinolittorina malaccana) that modify behaviours and lifestyles in line with their shoreline distributions.

Supratidal snails spend most of their lifetime inactive in air, as they are infrequently wetted and only during high swells and storm seas, whereas intertidal snails are regularly active and feed when wetted by the incoming tide. It was found that compared to the supratidal ecotype, the intertidal ecotype was more heavily melanised (relating to feeding in the sun), possessed a broader shell (relating to rapid growth), and was less effective in limiting evaporative water loss.

Despite these phenotypic differences, both ecotypes were found to be genetically similar, which implies that the observed changes occur during the lifetimes of individual snails (phenotypic plasticity). This was even more remarkable because the adults in intertidal and supratidal populations exhibit reproductive isolation, which is typically the primary mechanism for speciation. In other words, through their behaviours and lifestyles, supratidal snails are only able to mate with other supratidal snails and likewise for intertidal snails.The only mechanism that can thus explain these circumstances is that snails in the supratidal zone are recruited from pelagic swimming intertidal larva which settle in this zone during the infrequent high swells and storm seas. This study has now been published in the Biological Journal of the Linnean Society, UK.

In a second study, the researchers from FOS considered speciation in the gastropod genus, Indothais, that belongs to the Muricidae, a family of ornate whelks that functionally predominate in hard rocky-shore or coral reef ecosystems. However, in Brunei this genus transcends a steep salinity gradient from the open sea rocky shores of the South China Sea coastline (33 parts per thousand salinity) to deep into the Sungai Brunei estuary (< 3 ppt), where snails inhabit both muddy and rocky surfaces. We undertook a morphological and genetic study of four morphotypes of this genus and show that they represent three different species.

By overlaying the phylogenetic relationships of these species with their ecological distributions, we were able to confirm that speciation in this cohort is primarily driven by increased tolerance of low salinity water from the basal oceanic species, I. rufotincta to the deep estuarine I. gradata and I. javanica. The most recent species, I. gradata, showed unique niche expansion by feeding in both hard and muddy substrata; the latter probably relates to ephemeral lucrative food resources that become available and a lack of competitors in mud sediments. This study advances understanding of speciation and habitat utilisation of this important Southeast Asian gastropod genus, and has been accepted for publication in the Journal of the Marine Biological Association, UK.

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