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

Plakina atka n.sp.

Material: holotype: 6220­3B­1, collected by Dave Carlile south of Atka Island (51°55`1.4``N, 175° 17`35.5``W) at 118 m depth. The holotype is deposited at the Senckenberg Museum, Frankfurt/Main, Germany under the registration number SMF 10324.

Description: In life it is a pink to reddish­brown coloured sponge (Fig. 5). The sponge is light brown in ethanol. It is similar in growth form to Plakina tanaga n. sp. but with a slightly less convoluted surface and a different color. Single strands of the sponge have a smooth surface which is not microtuberculate like Plakina tanaga n.sp. The material consists of a single specimen encrusting a cobble. Thickness of encrustations is 0.3–0.8 cm. Consistency is more soft than P. tanaga and elastic. The specimen has an ectosomal dense spicule crust, averaging 100 µm in thickness. The choanosome is somewhat less densely packed with spicules. Spicules are vaguely arranged in tracts of varying orientation and with many spicules in between. Spiculation consists of diods, triods, calthrops and trilophose calthrops. Diods occur in two categories. Thin, smooth diods are typical plakinid diods, slightly bent and thickest in the center. Their dimensions are 70– 108 x 3–6 µm (Fig. 6). The thick, spined category of diods, is possibly derived from the the spined category of triods and measures 82– 95 x 8–10 µm (Fig. 7). Triods also occur in two distinct categories: relatively rare thin, smooth triods (28–33 µm/ray) and, much more abundant, thick, spined triods (23– 40 x 3–6 µm/ray). These have a row of large spines on each ray, close to the center of the spicule. (Fig. 6). Calthrops are rare and were observed with a reduced fourth ray only (Fig. 8). Tetralophose calthrops (Fig. 9) have tetrafurcate, occasionally pentafurcate rays, with microfurcate ends (Fig. 10) and measure 18–23 µm in total length.

Discussion: P. a t k a n.sp., also a member of the trilopha –species­complex, differs from other Pacific species of Plakina in the conspicuous convoluted surface, shared only with P. tanaga n.sp. P. microlobata and P. pacifica have dilophose and monolophose calthrops respectively, so conspecifity can be excluded. P. fragilis, P. bioxea, and P. tetralophoides are also members of the trilopha species­complex and should be discussed further. P. fragilis differs in color and in being an extremely thin encrustation (0.3 mm). It does not have the conspicuously spined category of triods and is also lacking the tetralophose calthrops of P. a t k a n.sp. P. bioxea differs in having two categories of diods, the large category much longer than in P. a t k a n.sp. Lophose calthrops occur in P. bioxea as mono­, di­ and trilophose calthrops, in contrast to only tetralophose calthrops in P. a t k a n.sp. P. tetralophoides shares the spined triods and tetralophose calthrops of P. a t k a n.sp. and appears to be the most closely related species to P. a t k a n.sp but differs in the species habit. Furthermore, P. a t k a n.sp. has, in addition to the spined triods present in both species, the usual smooth diods and triods. As argued above, P. monolopha sensu Tanita & Hoshino, 1989 is likely to belong to the trilopha species­complex and therefore should be discussed here. P. monolopha sensu Tanita & Hoshino differs in growth form, in having trilophose calthrops and in lacking the spined triods of P. a t k a n.sp. Remarkably, both new species described here have both smooth triods and a second category with spines at the base of the rays; a character shared only with P. tetralophoides. Based on these observations we conclude that P. tetralophoides is the most closely related species to P. a t k a n.sp, perhaps not surprising since it is the geographically the closest record of another species of Plakina.

Plakina atka differs from P. tanaga in growth form, color, surface structure and spiculation. In the pink colored Plakina atka single strands of the sponge have a less distinctive outline and are more widely spaced than in the beige colored P. tanaga (cf Figs. 1 & 5). P. a t k a is softer than P. tanaga. The spined spicule categories of P. a t k a are considerably more strongly spined than the ones in P. tanaga (cf Figs. 3, 7 & 8). The lophocalthrops of P. a t k a is a tetralophose calthrops of roughly half the size (tab. 1) of the trilophose calthrops of P. tanaga (cf Figs. 4 & 9).

Distribution: Known only from the type­locality.

Etymology: Named after Atka Island close to where the holotype was found.