Aplidium oculatum Beniaminson 1974

Aplidium oculatum (Beniaminson, 1974)

(Figure 1)

Amaroucium oculatum Beniaminson, 1974: 318.

Aplidium pliciferum: Sanamyan, 1998: 121 (part, specimens with massive colonies and white spots on thoraxes); Sanamyan, Sanamyan, 2010: 241.

Material examined. Kamchatka, Avacha Bay, Starichkov Island, 7 m, 15.10.2006, one colony (Figure 1 A,B); 7m, 28.07.2004, one colony (Figure 1 C), and many other colonies from Avacha Bay.

Description. The species forms large thick cushions not divided to lobes. The largest available colony is about 15x 10 cm in extent and 6 cm high. The test is always clear, without sand on surface or in internal layers, gelatinous, moderately soft and muddy translucent. Zooids are not visible through the test on living specimens. Colour varies from yellowish-orange to reddish.

Zooids are in typically small, crowded circular or short oval systems around common cloacal openings. Sometimes systems are somewhat depressed and separated by low ridges of the test, a condition seen in many Aplidium spp. (Figure 1 B). In many other specimens, and in almost all preserved colonies there are no borders of elevated test between the systems and the surface is almost smooth (Figure 1 C). Zooids are up to 30 mm long. Always simple atrial languet arises from the upper rim of the atrial aperture. Branchial sac has 12 or 13 rows of about 13 stigmata. Each row of stigmata is marked by prominent yellow spot on the thoracic wall on each side of the endostyle. A similar single pigment spot is at the upper end of the endostyle. In preserved specimens the colour of the spots fades to white, but spots are still recognizable in the specimens kept for more than 10 years in formaline. Stomach has 22–25 not quite regular folds. Postabdomen very long, contains several large ova just below the gut loop and serially arranged testis follicles along the whole length. Zooids in some colonies have embryos in atrial cavity, but no fully developed larva are found.

Remarks. Original description of A. oculatum based on two colonies from Paramushir Island (off south end of Kamchatka) is too superficial, but as it appears from the original figure the zooids have generally simple atrial languet, numerous stomach folds and 11 rows of stigmata marked by pigment spots near the endostyle, as in the present specimens (Beniaminson, 1974, Figire 1). Sanamyan (1998) synonymized A. oculatum with A. pliciferum (Redikorzev, 1927), although A. pliciferum was originally described from much warmer waters (in original description the locality listed as "Japanisches Meer, Missaki", see Redikorzev, 1927) and hardly could be identical with the species from Kamchatka. We reexamined the holotype of Amaroucium pliciferum (Zoological Institute, St. Petersbourg, ZIN957). Zooids are rather fragile and difficult to extract, but otherwise still in a perfect condition. They correspond fully to original description and figure, including fully expanded thorax with a simple atrial languet and two unusually long rectal caeca figured by Redikorzev (1927, Figure 10). Colony contains no larvae, and no pigment spots along the endostyle. We do not think A. oculatum and A. pliciferum are conspecific and treat here A. oculatum as a valid species.

Despite large number of specimens collected we failed to find fully developed larva and its structure is not known. A larva described for this species by Sanamyan (1998, Figure 7 A) comes from a colony composed of several cushion-like lobes, a feature not characteristic for A. oculatum, so it may belong to another species. Aplidium oculatum is fairly common on the hard bottom on diver accessible depths in the vicinity of Petropavlovsk-Kamchatsky.