The Cardiidae (Mollusca: Bivalvia) of tropical northern Australia: A synthesis of taxonomy, biodiversity and biogeography with the description of four new species

WA Museum Records and Supplements | Updated 6 years ago

ABSTRACT – Cardiids or marine cockles are one of the best known families of marine bivalves in the world, with a long history of study and collection given their large and accessible shells, a tractable level of biodiversity globally (~290 species) and a solid fossil record extending to the late Triassic. In Western Australia, the cardiids are well known relative to other marine bivalve families, having been the subject of focussed treatment and consist of 31 species. However, given the proximity to the Indo-West Pacific, sheer size of the state and almost unparalleled number of habitats, the diversity is likely underestimated. Over the last 40 years, ongoing expedition work coupled with taxonomic study by the authors, has resulted in a doubling of the cardiid fauna of Western Australia (WA). Current cardiid diversity now stands at 68 species, including seven species of giant clams (Tridacninae). Subfamilial representation has changed little (except for the addition of Tridacninae previously in its own family, the Tridacnidae), and key genera in Cardiinae (e.g. Fulvia), Fraginae (e.g. Fragum) and Trachycardiinae (e.g. Acrosterigma and Vasticardium) still deliver much of the diversity. This updated biodiversity census has implications within and beyond Western Australia, with 16 new species records for the state and 14 new records for Australia, including four new species: Acrosterigma extremattenuatum sp. nov., Ctenocardia pilbaraensis sp. nov., Microcardium scabrosum sp. nov. and Pratulum occidentale sp. nov. Further taxonomic decisions include recognition of Vasticardium serricostatum (Melvill & Standen, 1899) and Lunulicardia tumorifera (Lamarck, 1819) as distinct species. Vasticardium swanae Maxwell, Congdon & Rymer, 2016 is considered a new synonym of Cardium variegatum G.B. Sowerby II, 1840; Vasticardium lomboke Vidal, 2003 is considered a new synonym of Cardium mindanense Reeve, 1844 and Acrosterigma abrolhense Vidal, 1999 is considered a new synonym of Cardium cygnorum Deshayes, 1855. All subspecies in the Vasticardium elongatum (Bruguière, 1789) complex are raised to species level.

New acquisitions arising from tropical survey work in the north have yielded all of the newly added species and all newly described species in the Western Australian Museum (WAM) cardiid collections. This is in contrast to the relatively stable biodiversity of southwestern Western Australia. In the north, diversity is highly structured with sixteen taxa found restricted to offshore islands and shoals (e.g. Vasticardium philippinense (Hedley, 1899), V. elongatum (Bruguière, 1789), Corculum cardissa (Linnaeus, 1758), Ctenocardia fornicata (G.B. Sowerby II, 1840), C. gustavi Vidal & Kirkendale, 2007), and a smaller cohort found exclusively inshore (e.g. A. extremattenuatum sp. nov., V. wilsoni (Voskuil & Onverwagt, 1991), C. pilbaraensis sp. nov.). Biodiversity comparisons are made with other tropical states in northern Australia, including the Northern Territory (NT) and Queensland (Qld), and considered together these records support recognition of a vast Dampierian Province. Regional biogegraphic comparisons are also made to examine whether cardiids in northern Australia are as diverse as other central Indo West Pacific (IWP) areas. To improve management of the commercially important Tridacninae (giant clams), we provide the first formal records of species diversity and distribution in this group for WA. While cardiids are relatively well characterised, persistent gaps remain that are not unique to this family. Our knowledge of small and/or cryptic species, as well as the deep water fauna broadly, lags behind our understanding of shallow water macromolluscs.