Class PYCNOGONIDA Latreille, 1810

<I>Anoplodactylus evansi</I> on the bryozoan <I>Orthoscuticella</I> sp., from NSW

Anoplodactylus evansi on the bryozoan Orthoscuticella sp., from NSW

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Class PYCNOGONIDA Latreille, 1810

Sea Spiders

Compiler and date details

May 2022 - Lauren Timms - Museums Victoria. Complete revision of included taxa

15 December 2008 - updated and upgraded by Claudia Arango, Queensland Museum, Brisbane

2002 - Bonnie A. Bain, School of Biological Sciences, Monash University, Melbourne, Victoria

Podosomata Leach, 1815.

Introduction

Pycnogonids or sea spiders are a group of intriguing marine arthropods of bizarre morphology and usually cryptic habits. They inhabit all marine habitats in the world from tropics to polar seas.

Sea spiders are extremely important in the context of arthropod evolution because of their peculiar characteristics and because their phylogenetic affinities poorly understood. They resemble spiders and other arachnids in aspects of their morphology and anatomy, but they also have unique features that separate them from other arthropod groups (see review in Dunlop & Arango 2005). Their relevance is highlighted in the recent flow of data in different fields, such as neuroanatomy (Maxmen et al. 2005), gene expression (Jager et al. 2006), genomics (Podsiadlowsky & Braband 2006; Park et al. 2007), morphology and ultrastructure (Fahrenbach & Arango 2007), all contributing to a better knowledge of the evolutionary history of an such intriguing group of animals.

More than 1300 extant species are known worldwide. Traditionally, pycnogonid species are divided among nine or 10 families (Stock 1994; Child 1998; Arango & Wheeler 2007), but cladistic analysis of morphological and molecular data (Arango 2002, 2003) reveal evidence of non-monophyly and possible parallelism in the reduction and loss of appendages. Arango & Wheeler (2007) produced a comprehensive phylogeny for the group based on morphology and sequences from six genes indicating that neither Ammotheidae and Callipallenidae, the two most diverse families, are monophyletic and require further analysis and taxonomic revision. Also, Nakamura et al. (2007) provided evidence supporting phylogenetic affinities among families and genera, particularly the Rhynchothoracidae. These studies have led to new interpretations of the evolutionary history of these ancient animals and should be the basis when proposing a taxonomic classification.

The fossil record of sea spiders has grown sparingly in the last few years. The oldest fossil recognised is larval material, dating back to the Upper Cambrian from Hunsruck Shale (Waloszek & Dunlop 2002). The records from the Devonian are significant in that some taxa exhibit character states quite different from those of extant species (e.g. Bergstrom et al. 1980; Poschmann & Dunlop 2006). However, a Silurian fossil, Haliestes dasos, has characters easily comparable with those of extant forms (Siveter et al. 2004). Similarly, Jurassic fossils recently discovered are tentatively assigned to three living families (Charbonnier et al. 2007). At least Devonian and Silurian fossils, included in the phylogeny (Arango & Wheeler 2007), seem to be related to extant forms, instead of forming one group of early, extinct taxa, as traditionally recognised with the Order Palaeopantopoda Broili, 1930 found in traditional classifications and general literature (see e.g. Bamber 2007). Further phylogenetic analyses including all extinct taxa might resolve the uncertain affinities of pycnogonid fossils, for the moment, the division into Orders Pantopoda Gerstaecker, 1863 and Paleopantopoda as proposed by Hedgpeth (1947) and sustained in subsequent literature (e.g. King 1973; Bamber 2007) cannot be validated from a phylogenetic point of view (see Arango & Wheeler 2007).

Sea spiders can be found from the shore down to depths of 6800m. They are part of cryptic communities on the benthos, or associated with a diversity of hosts from diverse habitats. Sea spiders generally feed on sessile fauna (e.g. cnidarians, bryozoans), sometimes on slow-moving organisms (e.g. molluscs, polychaetes) and juvenile instars can be parasitic (Arnaud & Bamber 1987; Miyazaki 2002a). A great diversity of life history traits are exhibited by Pycnogonida, many still to be described. General, comprehensive accounts of pycnogonid biology can be found in Helfer & Schlottke (1935), King (1973) and Arnaud & Bamber (1987).

Close to 210 sea spider species are recorded from Australian seas. In the early years, species were described mainly from south and south-eastern Australia (Hoek 1881; Clark 1963; Stock 1973) and a few from North Queensland (Haswell 1884; Flynn 1929). The first report from Western Australia was by Child (1975) and more recently Bamber (2005) and Staples (2005, 2007) have contributed to the knowledge of the south-western Australian fauna. For Queensland, a report from Lizard Island (Child 1990) and later on a comprehensive work on the sea spiders from the Great Barrier Reef (Arango 2003) and other publications on taxonomy of species from Queensland (Lee & Arango 2003; Arango & Maxmen 2006; Arango & Krapp 2007) have contributed to the knowledge of tropical Australian sea spiders, a group less well known and usually neglected in comparison with the southern fauna.

Antarctica and the Southern Ocean are probably the most diverse region in terms of species of sea spiders (Clarke & Johnston 2003; Munilla & Soler Membrives 2008). Current studies are attempting to unravel diversification and distribution patterns of Antarctic sea spiders (Mahon et al. 2008) and the possible radiation of species to Australasia (current AAD project grant led by the author).

Acknowledgements

Updating and revision of this checklist was part of a research project funded by the Australian Biological Resources Study (ABRS). I thank the Australian Museum and Queensland Museum (QM) for making their facilities available for the development of the project; Ward Wheeler at the American Museum of Natural History for ongoing collaboration and sequencing of material; Ayax Ruiz for collection of material from Heron Island; the Australian Antarctic Division for making material available for this and subsequent projects; Gary Cranitch from QM for photography; Gavin Dally from the Museum and Art Gallery of the Northern Territory for loan of material and organisation of a scientific visit to the collection; Museum Victoria for loan of material; Thierry Laperousaz from the South Australian Museum for loan of material and making records available; Penny Berents and Stephen Keable for loans of material and many years of support; Barbara Done from the Museum of Tropical Queensland for loans of material; Robert Raven from QM for his help and support through different stages of the project; Julianne Waldock and Mark Harvey from the Western Australian Museum for loan of material and discussions on material from Western Australia; Karen Gowlett-Holmes for her assistance and sharing of knowledge during fieldwork in Tasmania; Peter Davie from QM for facilitating access to material from the Moreton Bay Workshop 2005; Mick Harris from Shellharbour Dive Centre and Iain Gray from Sydney, NSW for sharing their amazing photographs of sea spiders from the coast of NSW; Franz Krapp, Roger Bamber, Tomas Munilla, for ongoing collaboration and discussion on Australian species as well as other pycnogonid specialists around the world for very helpful feedback, discussion and collaboration over many years; and Guillermo Diaz-Pulido from the Center for Marine Studies, University of Queensland, for his support and help in many instances of the project, particularly fieldwork.

Limital Area

Distribution data in the Directory is by political and geographic region descriptors and serves as a guide to the distribution of a taxon. For details of a taxon's distribution, the reader should consult the cited references (if any) at genus and species levels.

Australia is defined as including Lord Howe Is., Norfolk Is., Cocos (Keeling) Ils, Christmas Is., Ashmore and Cartier Ils, Macquarie Is., Australian Antarctic Territory, Heard and McDonald Ils, and the waters associated with these land areas of Australian political responsibility. Political areas include the adjacent waters.

Terrestrial geographical terms are based on the drainage systems of continental Australia, while marine terms are self explanatory except as follows: the boundary between the coastal and oceanic zones is the 200 m contour; the Arafura Sea extends from Cape York to 124 DEG E; and the boundary between the Tasman and Coral Seas is considered to be the latitude of Fraser Island, also regarded as the southern terminus of the Great Barrier Reef.

Distribution records, if any, outside of these areas are listed as extralimital. The distribution descriptors for each species are collated to genus level. Users are advised that extralimital distribution for some taxa may not be complete.

Diagnosis

Body segmented, abdomen reduced to a small protuberance with anus at tip. Mouth at the end of an external, prominent proboscis, sometimes longer than the body, such structure not present in other arthropods. Usually body consists of four segments, each with a pair of legs, but some deep-sea species have five or six body segments making them 10 or 12-legged. Cephalic region with three pairs of appendages, chelifores usually chelate and palps flanking the proboscis and ovigers located ventrally. These appendages show different degrees of reduction depending of the lineage. In Pycnogonidae and Endeidae females, all three appendages are totally absent in adult specimens. An ocular tubercle with 4 simple eyes is located dorsally on the cephalon, some deep-sea species lack an ocular tubercle, others lack the eyes.