Amathia fimbria, Hirose & Gordon, 2020

Hirose, M. & Gordon, D. P., 2020, New seriated Amathia species in Japan, with a redescription of A. acervata Lamouroux, 1824 (Bryozoa: Ctenostomata), Zootaxa 4742 (2), pp. 311-331 : 320-322

Amathia fimbria n. sp.

( Figs 4 , 5 )

Material examined. Holotype: NSMT-Bry R55 , single bushy colony on polychaete tube, Najima , Sagami Bay, Honshu, no depth data, 10 June 1934 . Paratypes: NSMT-Bry R235 , colony on hydroid, Udorishima , Kasajima , Sagami Bay, Honshu, 9 m, 26 July 1964 ; NSMT-Bry R375 , two bushy colonies on hydroid, off Miihama , Suzaki , Izu, Honshu, 10–20 m, 22 June 1983 ; NSMT-Bry R381 , bushy colonies on hydroid stem, Ooura , Suzaki , Izu, Honshu, 10 m, 19 June 1987 . Other material examined: NSMT-Bry R 179, dried specimen with small colony, South Hirashima, Mitsuiso, off Nagaoka, Kaneda Bay, Honshu, 14 m, 15 July 1962.

Etymology. The specific name derives from the Latin fimbria which means fringe or tassel, referring to the compact ball-shaped colony morphology resembling Kikutoji which is a kind of tassel for Japanese traditional clothes Kimono.

Description. Colony ( Fig. 4A, B ) erect, bushy and densely branching; initially tufted, comprising stolons attached to a substratum, from which erect stems arise at intervals. As colonies grow, cross connections can occur so that the lower half of a colony tends to appear as a tangle, with branching in all planes. Colonies anchored by rhizoid-like stolons adherent to the substratum, becoming thick-walled with age. Tips of branches can also adhere to the substratum, continuing as adherent stolons, producing, at intervals, 5 opposing pairs of non-connate autozooids, each autozooid at an oblique angle to the stolon and somewhat flattened against the substratum, the whole cluster appearing overall like a pinna ( Fig. 4E ). The largest colony up to 60 mm high and 70 mm across, anchored by a short (<10 mm) stem, c. 2 mm thick, of several stolons/rhizoids bundled together. The most thickened rhizoidal stolons are curved and tangled and contain yellowish contents.

Branching of erect stems always trichotomous ( Figs 4C , 5 A–C). Depending on length, stolon segments more or less straight or gently curving, slightly sinuous in the distal part where surrounded by an autozooid spiral; mean stolon length 1.90± 0.43 mm (range 1.26–2.39 mm, n = 10), the angles between branches at trichotomies c. 40–90°. Mean stolon width 0.14± 0.03 mm (range 0.10–0.19 mm, n = 10).

Autozooid clusters ( Fig. 4D ) disposed in clockwise spirals on both of the lateral stolons or anticlockwise on one of them (some segments lacking a cluster), comprising c. 9–15 ‘pairs’ per cluster, each cluster describing at least one complete 360° circuit of the stolon from its commencement to its completion, or up to 1.8 turns; mean cluster length 1.20± 0.12 mm (range 1.00– 1.38 mm, n = 11); each cluster has a mean linear distance of 1.16± 0.20 mm (range 0.81–1.44 mm, n = 10) on the stolon and terminates at the next branch node; zooid cluster occupying 48–72% of stolon length. Mean inception distance of proximalmost zooid in the cluster on lateral stolon from bifurcation 0.49± 0.06 mm (range 0.41–0.56 mm, n = 4). Autozooids tilted distad c. 60° from the perpendicular, with a mean length of 0.40± 0.08 mm (range 0.33–0.56 mm, n = 11) in alcohol-preserved retracted specimens, connate; zooid width (as measured in lateral view of zooid cluster) averaging 0.13± 0.01 mm (range 0.10–0.15 mm, n = 11); 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. In reflected light, autozooids are opaque and pale (off-white) in appearance. Polypide with 8 tentacles.

Remarks. This species is similar to the holotype of A. acervata and A. brevisilva n. sp. described above; it differs in having trichotomous branching of the stems; it also has occasional pinnate branching of adherent parts of stolons. In this latter character, Amathia fimbria n. sp. is similar to Amathia reptopinnata n. sp. described below, however it differs from the latter species in having a shorter internode stem and shorter autozooid cluster with less spirality.

The present species also resembles Amathia rudis Kubanin, 1992 in modes of branching and some other features. The present species, however, differs from Kubanin’s (1992) description of A. rudis in having shorter and narrower stolons and a slightly higher value of the proportion of the autozooid cluster on the stolon. The present species also differs from A. rudis in having pinnate autozooid clusters adhering to the substratum branch tips.

The material in the Showa Collection occurs on a polychaete tube, hydroid stems and an undetermined species of Sargassum .

Distribution. Southern Sagami Bay region, the Pacific coast of Honshu, Japan, 9–20 m deep.

Figures

FIGURE 4. Amathia fimbria n. sp. A. Colony of holotype on polychaete tube (NSMT Bry R55). B. Colony of paratype on hydroid colony (NSMT Bry R375). C. Zooid clusters on trichotomous branches (NSMT Bry R375). D. Zooid cluster (NSMT Bry R375). E. Adherent stolon and autozooids (NSMT Bry R381).

FIGURE 5. Amathia fimbria n. sp. A. Zooid clusters and trichotomy (NSMT Bry R375). B. Early stage of trichotomy above the zooid cluster (NSMT Bry R375). C. Continuous trichotomy (NSMT Bry R55).