All Is Not Quiet on the Western Front: High Host–Parasite (Echinodermata and Caenogastropoda) Diversity Revealed at an Australian Marine Transition Zone

While substantial eulimid diversity has been revealed in the Indo-West Pacific marine diversity hotspot, many neighbouring areas are still unexplored, including in Western Australia. The Houtman Abrolhos are a unique chain of islands in a well-characterised marine transition zone where tropical and temperate waters meet along the mid-west coast. During a biodiversity survey of the islands in 2025, sixty-two eulimids from 15 stations were collected, a family of marine gastropods never-before documented from this region. Here we incorporate newly collected and legacy material from the Western Australian Museum to illustrate 23 new eulimid morphospecies records for the Houtman Abrolhos. Sixteen hosts, representing all five classes of Echinodermata, were identified. Most eulimids were attached externally to their hosts, though Stilifer utinomii and two unidentified species of Melanella were found embedded in Disasterina longispina and Actinopyga mauritiana, respectively. Apicalia angulata, Peasistilifer nitidula and Stilifer utinomii are newly recorded for Western Australia, and Thyca ectoconcha and Vitreobalcis tripneusticola are new Australian records. The biogeographic affinities of these symbionts, like other marine life surveyed in the Houtman Abrolhos islands, are overwhelmingly tropical in nature, representing in many instances the southernmost records of otherwise widespread Indo-West Pacific species.