The Antarctic gastropod Dickdellia labioflecta (Dell, Reference Dell1990) (originally described as Laevilittorina (Corneolittorina) labioflecta) is an obligate parasite of pycnogonids, which exploits their body fluids through the cuticular gland holes (Lehmann et al. Reference Lehmann, Gailer, Melzer and Schwabe2007) and lays its eggs on the pycnogonid's legs where embryos complete their life cycle (Hedgpeth Reference Hedgpeth1964, Sirenko Reference Sirenko2000, Lehmann et al. Reference Lehmann, Gailer, Melzer and Schwabe2007). The ecology of D. labioflecta appears to be unique, as no other examples of such a specialized parasitic behaviour on pycnogonids are known. This life-style and the related anatomical specializations (gut and digestive gland morphology), prompted the erection of a new genus, Dickdellia Warén & Hain, Reference Warén and Hain1996 provisionally included in the family Zerotulidae (Warén & Hain Reference Warén and Hain1996). Although information is quite scant, to date, two pycnogonid host species are known for Dickdellia: Colossendeis megalonyx megalonyx Fry & Hedgpeth, 1969 (Lehmann et al. Reference Lehmann, Gailer, Melzer and Schwabe2007) and Nymphon isabellae Turpaeva, 2000 (Sirenko Reference Sirenko2000).

Dickdellia labioflecta had recently been considered to be endemic to the Weddell Sea (Linse et al. Reference Linse, Griffiths, Barnes and Clarke2006), probably due to its type locality (off Elephant Island, South Shetland Islands: Eltanin station 410). However, the paratypes (Eltanin stations 1870 and 1871, both from Cape Adare, and Eltanin station 1907 [figured at http://acsmith.si.edu/, cat. No. 886103] from McMurdo Sound) were collected in the Ross Sea.

We report here new findings from the Ross Sea, originating both from recent cruises and from the re-examination of historical collections, which enlarge the host spectrum and the bathymetric distribution of this peculiar parasitic gastropod.

The first recent finding in the Ross Sea occurred in the Mawson Bank (CARBONANT cruise, RV Italica, sta. CARB34, dredge, start 73°14.56′S, 175°38.35′E - 389 m, end 73°13.59′S, 175°36.84′E - 384 m: Remia et al. Reference Remia, Oliverio, Hart and Ramorino2002). A specimen of the pycnogonid Colossendeis lilliei Calman, 1915 was found with a D. labioflecta firmly attached to its trunk (Fig. 1). This is the first record of C. lilliei as a host species for D. labioflecta. A second smaller living specimen of D. labioflecta (not shown) was subsequently isolated from the sediment of the same sample (National Antarctic Museum collection, Section of Genoa, no. MNA1355, C. lilliei carrying the gastropod, and MNA1356, the small D. labioflecta specimen).

Fig. 1. The specimen of D. labioflecta on the pycnogonid C. lilliei. a. Top view. b. Lateral view: note the gastropod shell has its edge still firmly attached to the host by a sticky mucus sheet (see also Sirenko Reference Sirenko2000).

The second recent record in the Ross Sea consists of eggs capsules of D. labioflecta on a Colossendeis notialis Child, 1995 specimen, collected by the New Zealand BioRoss Expedition in 2004 (Mitchell & Clark Reference Mitchell and Clark2004; RV Tangaroa, sta. TAN0402/19, 71°44′06″S, 171°43′59″E, 429–454 m: NIWA Invertebrate Collection (NIC), Wellington, no. NIWA 28477). A third Ross Sea record of D. labioflecta is based on a specimen of Colossendeis megalonix Hoek, 1881 (NIC collection, no. NIWA 6653), carrying eggs of D. labioflecta at an advanced stage of development (pre-hatching larvae with a completely formed shell). This historical sample was collected in the 1960s station number (E0193C) coordinates 71°18′00″S, 170°02′00″E (26 January 1965), depth 64 m. The latter record considerably enlarges, towards shallower depths, the bathymetric distribution of D. labioflecta and confirms its occurrence on the pycnogonid Colossendeis megalonix in the Ross Sea also.

The new findings enlarge the host spectrum for D. labioflecta to further two pycnogonid species, and confirm a circumpolar distribution of D. labioflecta (Fig. 2), based on both living material (and including at least paratypes from Eltanin sta. 1907) and the gastropod eggs on the pyconogonids.

Fig. 2. Occurrence of D. labioflecta around the Antarctic continent. Symbols as follows: asterisk = type locality, filled circle = present records plus sign = paratypes from the Ross Sea, stars = other paratypes and recent records from the Weddell Sea.

The circumpolar distribution of a gastropod with a very specialized, parasitic behaviour, lends further support to the more general concept of faunal homogeneity of the Ross and Weddell seas. It also emerged in other recent contributions based on extensive collections from the Ross Sea (e.g. Schiaparelli et al. Reference Schiaparelli, Lörz and Cattaneo-Vietti2006 for molluscs and Rehm et al. Reference Rehm, Thatje, Mühlenhardt-Siegel and Brandt2007 for peracarids), suggesting caution in comparisons of raw numbers of species from these two basins as, due to a very different sampling effort, severe biases could be introduced in the evaluation of α-diversity.