A rare example of non-native chitons: Broad intertidal habitat range and large densities of sypharochiton pelliserpentis show no evidence of habitat engineering effect in South Australia

Abstract

There are numerous examples of non-indigenous rocky-intertidal mobile invertebrate species, but there have been very few instances of introductions of chitons despite their often high abundance in fouling assemblages where non-indigenous species originate. In 2016, it was observed incidentally that the snake-skin chiton Sypharochiton pelliserpentis, native to eastern Australia and New Zealand, was occurring in Coffin Bay in western South Australia, which is a sheltered bay used for oyster farming and far outside the previously documented S. pelliserpentis range. The main coastal water currents in the region flow from west to east, meaning natural larval dispersal into Coffin Bay from eastern Australian populations is unlikely. Surveys of population and community dynamics done in 2018 revealed that a large population has established with the non-native chiton far outnumbering a comparable native chiton (Plaxiphora albida) that may occupy a similar niche space. The chitons occurred on exposed bedrock habitats as well as cryptic habitats underneath boulders; both species were equally abundant between these habitats except at one site where S. pelliserpentis densities specifically underneath boulders were 10 times greater than other habitat-types or sites. Sypharochiton pelliserpentis in its native range can largely impact sessile assemblages; here we tested the hypothesis that sessile assemblages would differ on boulders with versus without the non-native chiton, but no evidence of any such effect was found. Sypharochiton pellisperpentis is a common epibiont of oyster-reefs and the origin of its introduction into South Australia may involve transport of oyster-industry materials, which was how introductions of four other non-native benthic invertebrates occurred previously into another Australian region. The geographic extent of S. pelliserpentis in South Australia is unknown at this stage but the chiton was not found during surveys in another nearby bay. Knowledge that oyster-associated chitons may be spreading outside their native ranges, and that they can establish primarily in habitats that are largely hidden from view, such as underneath boulders, can inform monitoring and management practices for ecology of non-indigenous intertidal species.