Plakina tanaga Lehnert, Stone & Heimler, 2005, n.sp.

Plakina tanaga n.sp.

Material: holotype: 6204­18­12, collected by HL in Little Tanaga Strait (51°52`14.6``N, 176°15`42``W) at 146 m depth. The holotype is deposited at the Senckenberg Museum, Frankfurt/Main, Germany under the registration number SMF 10323.

Description: The holotype is a fragment of a larger sponge, scraped off the surface of a vertical bedrock wall. In life, it is a beige or light brown encrusting sponge (Fig. 1). In ethanol it is still light brown with reddish tinges in the interior. It is common on vertical and overhanging sides of bedrock and boulders and covers relatively large areas (approx. up to 0.5 m 2) with a thickness of approx. 1–1.5 cm. It has a convoluted surface with deep grooves between strands with a microtuberculated surface, thus somewhat rough to the touch. Tubercles are 120–440 µm in height and 60–320 µm in diameter and closely spaced. It’s consistency is cheese­like, only slightly compressible. The interior is dense. No oscules were visible on the fragment collected but, video sequences obtained show circular oscules, 3–5 mm in diameter, that are flush with the surface (Fig. 1).

The ectosome is packed with lophocalthrops in a layer 20–250 µm in thickness. Spicule categories are diods, triods, calthrops and lophocalthrops. Diods are characteristically bent, often centrotylote or, with a reduced third ray in the form of a spine. Dimensions are 85– 97 x 2–4 µm (Fig. 2). Triods occur in two categories: smooth, thinner triods, measuring 24– 50 x 2–5 µm/ray and thicker triods, often with one short, slender spine near the base of each ray, 22– 38 x 7–9 µm (Fig. 3). Calthrops (Fig. 2) are rare and the rays have the same dimensions as those of the triods. Trilophose calthrops are abundant, the non­lophose ray is sometimes bifurcate. Lophose rays are tetrafurcate and, again microfurcate at the points (Fig. 4). Lophocalthrops are smaller than the triods and concentrated in the ectosome. Total length of lophocalthrops is 28–42 µm (Figs. 3 and 4).

Discussion: This species belongs to the Plakina trilopha ­ complex. Plakina trilopha is a Mediterranean species and differs from P. tanaga n.sp. in its white color and smooth surface. Additionally, spicule sizes differ considerably (cf. Tab. 1) and, the lophocalthrops of P. trilopha are mono­, di­, tri­ and tetralophose while P. tanaga n.sp. has trilophose lophocalthrops only.

The three species P. fragilis, P. microlobata and P. pacifica are known only from the Galapagos Islands. They are much thinner encrusting forms than P. tanaga n.sp. (0.3mm for P. fragilis, 1­3mm for P. microlobata) and do not share the convoluted surface pattern of P. tanaga n.sp.. P. microlobata also differs in having smooth triods and dilophose calthrops only. P. pacifica has monolophose calthrops. P. fragilis shares the trilophose calthrops in which the non­lophose ray tends to be bifurcate with P. tanaga n.sp. In addition to the differences in growth form, the lophocalthrops of P. fragilis have trifurcate rays rather than the tetrafurcate rays in P. tanaga n.sp. Plakina bioxea from California differs in the occurrence of two size categories of oxeas, one of them considerably larger than in all known species of Plakina. P. bioxea has small mono­, di­ and trilophose calthrops whereas P. tanaga n.sp. has trilophose calthrops only. Basal spicular tracts described for P. bioxea are not present in P. tanaga n.sp. P. tetralophoides was initially described as P. tetralopha by Tanita & Hoshino (1988) from Sagami Bay, Japan and is geographically the closest record of a congener to the species presented here. P. tetralophoides differs in being a “petaline” sponge, only 3mm in thickness and 15mm height with a smooth surface. In spiculation, P. tetralophoides differs in having smaller tetralophose calthrops. P. corticolopha differs in color, growth form and spiculation. This species has apart from trilophose calthrops, a large category of spined calthrops only. Diods and triods are lacking completely. P. monolopha sensu Tanita & Hoshino, 1989 is described with monolophose calthrops but, judging from their figure of the calthrops (Tanita & Hoshino, 1989: p. 12, text­fig. 1c) they are trilophose and therefore, this species also belongs to the Plakina trilopha species­complex. P. monolopha sensu Tanita & Hoshino, is an erect, conical sponge with a hollow central cavity and so differs considerably in growth form from all other species of Plakina. In spiculation, this species differs in having simple, smooth diods only, and, in the occurrence of smaller triods and smaller trilophose calthrops. Due to the unusual growth form of P. monolopha sensu Tanita & Hoshino, 1989 it seems probable that this species represents another undescribed species of Plakina.

Distribution: Known only from the type locality.

Etymology: Named after the locality ­ Little Tanaga Strait ­ where the holotype was found and collected.