Amathia reptopinnata Hirose & Gordon 2020, n. sp.

Amathia reptopinnata n. sp.

(Figs 6–8)

? Amathia distans: Mawatari, 1948: p. 11; 1963: p. 6.

Material examined. Holotype: NSMT-Te1213, colony on leaves of eelgrass (Zostera sp.), Lake Komuke, Hokkaido, 44°15’58.36” N, 143°30’53.70” E, ~ 1 m, 10 August 2007. Paratypes: NSMT-Te893, 894, colonies on leaves of eelgrass (Zostera sp.), same collection data as for holotype, Lake Komuke, Hokkaido, 44°15’58.36” N, 143°30’53.70” E, ~ 1 m, 10 August 2007; NSMT-Te895–897, colonies on leaves of eelgrass (Zostera sp.), Lake Saroma, Hokkaido, 44°11’7.00” N, 143°45’48.60” E, ~ 1 m, 6 October 2010; NSMT-Te898–900, some branches of colonies, entrance to Kamo Bay, Oki Islands, Shimane Prefecture, Honshu, 36°10’23.77” N, 133°16’50.04” E– 36°10’28.08” N, 133°16’48.72” E, 13–16 m, 26 June 2012; NSMT-Te901, colonies on oyster shell, collected at Matsushima Bay, Miyagi Prefecture, 2 m in depth, 11 November 2013.

Etymology. The specific name derives from the Latin reptilis (creeping) and pinnatus (pinnate), referring to the pinnately arranged, prostrate zooids.

Description. Colony (Fig. 6A) erect, tufted, fairly densely branching, up to 50 mm high, comprising stolons attached to a substratum, from which erect stems arise at intervals. As colonies grow, occasional cross connections can occur (Fig. 7A, B). Colonies anchored by rhizoidal stolons adherent to the substratum, becoming thick-walled with age (Fig. 7C). Tips of branches can also adhere to the substratum, continuing as adherent stolons with short, planar grappling-hook-like processes at intervals along their length or with blind side branches also with a planar, grappling-hook morphology (Fig. 7C, E). Adherent stolons also producing, at intervals, up to 11 opposing pairs of non-connate autozooids, each autozooid at an oblique angle to the stolon and flattened against the substratum, the whole cluster appearing overall like a pinna (Fig. 7D, F).

Erect growths are initiated as one main stem with side branches, and branching is always dichotomous (Fig. 6B, C). Stolon segments more or less straight or gently curving, with a mean length of 2.60± 0.62 mm (range 1.62–3.99 mm, n = 86), the angle between branches at dichotomies 45–80°. Stolon with white or light-brown pigments (Fig. 6C). Mean stolon width 0.18± 0.04 mm (range 0.12–0.26 mm, n = 89).

Autozooid clusters (Fig. 8) disposed in clockwise spirals on most stolon segments (some segments with anticlockwise spirals and others lacking a cluster) in Matsushima, whereas mostly anticlockwise in Oki, and clockwise or anticlockwise depending on the colony in Hokkaido. Autozooid clusters comprising 12–23 ‘pairs’ per cluster, each cluster describing at least one complete 360° circuit of the stolon from its commencement to its completion, or up to 2.3 turns; mean cluster length 1.45± 0.21 mm (range 1.01–1.98 mm, n = 99); each cluster has a mean linear distance of 1.21± 0.43 mm (range 0.57–2.23 mm, n = 79) on the stolon and terminates at the next branch node or at the next stolon septum (there can be three stolon segments between branch nodes); zooid cluster occupying 39–76% of stolon length. Autozooids tilted distad c. 60° from the perpendicular, with a mean length of 0.45± 0.05 mm (range 0.36–0.53 mm, n = 48), connate; zooid width (as measured in lateral view of zooid cluster) averaging 0.14± 0.02 mm (range 0.11–0.19 mm, n = 47); owing to the zooid tilt, the distal end of each cluster generally leans past the point of termination of the zooid insertions at each branch node. Outer walls of zooids slightly thicker than interior walls, evidenced by darker cuticularisation, and the outer distal rim of the cuticle can be slightly flared. Tentacle number 8.

Descending rhizoids evident in proximal parts of larger, older colonies, each appearing to issue from a point immediately proximal to an autozooid cluster.

Remarks. This species is similar to the previous species but is slightly more robust, darker in colour, and with dichotomous branching instead of being mostly trichotomous. The mean and maximum lengths of stolons and autozooid clusters are also larger than the previous species.

The present species resembles Amathia brasiliensis Busk, 1886, Amathia vidovici Heller, 1867, and Amathia aegyptiana d'Hondt, 1983 in that neighboring autozooids are connate for half their length. Amathia reptopinnata n. sp. differs, however, from A. brasiliensis, A. vidovici, and A. distans in having a greater maximum number of zooid pairs per autozooid cluster. Amathia reptopinnata n. sp. differs from A. aegyptiana in having a slightly narrower stolon, and pinnately arranged autozooid clusters at attachment areas on the substratum. Amathia reptopinnata n. sp. resembles Amathia similis Gordon & Spencer Jones, 2013 in the dimensions of colony, stolon and autozooid, but differs by having both a larger proportion of occupancy of the autozooid cluster on the stolon and a pinnately arranged autozooid cluster on the substratum.

Although the present specimen from Matsushima is similar to A. rudis in stolon length, it differs by having a typically larger proportion of occupancy of the autozooid cluster on the stolon; A. reptopinnata n. sp. from Matsushima (and Oki and Hokkaido) also has a longer autozooid cluster and pinnately arranged prostrate zooids. The present species differs from A. acervata in having longer stolon segments and longer autozooid clusters.

Colonies grow on the leaves of eelgrass (Zostera sp.) in Lakes Saroma and Komuke (northern Hokkaido), on oyster shells in Matsushima Bay (northeastern Honshu) and on Sargassum sp. at Oki Islands (Japan Sea), at depths of 1–16 m depending on locality and substratum.

Distribution. Japan Sea off western Honshu, the Pacific coast of Tohoku region, and northern Hokkaido in the Okhotsk Sea.