a1 Museo Nazionale dell'Antartide (MNA), Università di Genova, Viale Benedetto XV n° 5, Genova I-16132, Italy
a2 Dipartimento di Biologia Animale e dell'Uomo, Università La Sapienza, Viale dell'Università 32, Roma I-00185, Italy
a3 Istituto di Scienze Marine (ISMAR), CNR, Via Gobetti 101, Bologna I-40129, Italy
a4 British Antarctic Survey, NERC, High Cross, Madingley Road, Cambridge CB3 OET, UK
a5 NIWA National Institute of Water & Atmospheric Research, Greta Point, Wellington, New Zealand
a6 Dipartimento per lo Studio del Territorio e delle sue Risorse (Dip.Te.Ris.), Università di Genova, C.so Europa 26, Genova I-16132, Italy
(Received January 16 2008)
(Accepted January 28 2008)
List of Figures and Tables
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 2000).
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 Antarctic gastropod Dickdellia labioflecta (Dell, 1990) (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. 2007) and lays its eggs on the pycnogonid's legs where embryos complete their life cycle (Hedgpeth 1964, Sirenko 2000, Lehmann et al. 2007). 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, 1996 provisionally included in the family Zerotulidae (Warén & Hain 1996). Although information is quite scant, to date, two pycnogonid host species are known for Dickdellia: Colossendeis megalonyx megalonyx Fry & Hedgpeth, 1969 (Lehmann et al. 2007) and Nymphon isabellae Turpaeva, 2000 (Sirenko 2000).
Dickdellia labioflecta had recently been considered to be endemic to the Weddell Sea (Linse et al. 2006), 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. 2002). 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).
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 2004; 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.
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. 2006 for molluscs and Rehm et al. 2007 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.
Partial funding by PNRA (proj. CARBONANT, to MT). Thanks to A. Remia. CAML contr. #11 and IGM scientific contr. #1585.