Amathia brevisilva, 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 : 317-320

Amathia brevisilva n. sp.

( Fig. 3 )

? Amathia distans: Okada & Mawatari, 1938: p. 446 ; Mawatari, 1952: p. 262.

? Bowerbankia gracilis: Mawatari, 1952: p. 262 .

Material examined. Holotype: NSMT-Te1209, single colony on pebble, Manazuru, Sagami Bay, 35° 9'23.78"N, 139° 8'38.56"E, intertidal (~ 1 m), 7 August 2017 . Paratypes: NSMT-Te1210, Te1211, some branches, same collection data as for holotype, Manazuru, Sagami Bay, 35° 9'23.78"N, 139° 8'38.56"E, intertidal (~ 1 m), 7 August 2017 ; SMBL-30 and NSMT-Te1212, colony on hydroid stem, single specimen with several labels in Seto Marine Biological Station, Kyoto University and a small fragments in NSMT, collected at Kitahama and/or off Tonda, Wakayama Prefecture, no depth data, collected by beach seine, 15–21 April 1928 and/or 13 August 1929 and/or March 1944, possibly the specimens reported in the study by Okada and Mawatari (1938) and/or Mawatari (1952).

Etymology. The specific name derives from the Latin brevis (small/low) and silva (bush, forest, or mass), referring to the small bushy colony morphology growing on the surface of the substrate.

Description. Colony ( Fig. 3A, B ) erect, small, sparsely branching; narrow 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 narrow rhizoid-like stolons adherent to the substratum; lacking any autozooids on the substratum. The largest colony up to 40 mm high and 55 mm across, anchored by a short (<3 mm) stem of stolons/rhizoids. In the intertidal habitat, the largest colony up to 15 mm high and 18 mm across, anchored by a short (<10 mm) rhizoidal stem. Adherent stolons producing, at intervals, up to five non-connate autozooids, each autozooid at an oblique angle to the stolon and randomly erect from the substratum, the whole cluster appearing tufted and not pinnate.

Branching of erect stems dichotomous ( Fig. 3C, D ), at angles of 20–60°. Depending on the length, stolon segments more or less straight or very gently curving, slightly sinuous in the distal part where surrounded by an autozooid spiral; mean stolon length 1.77± 0.46 mm (range 0.81–3.36 mm, n = 48), the angle between branches at dichotomies 20–60°. Mean stolon width 0.13± 0.02 mm (range 0.08–0.21 mm, n = 77). Stolon with white or refringent light-yellow granules ( Fig. 3F ).

Autozooid clusters ( Fig. 3E ) disposed either clockwise on both of the dichotomous stolons or anticlockwise on one of them, comprising c. 6–12 ‘pairs’ per cluster, each cluster describing up to 1.5 turns; mean cluster length 0.93± 0.20 mm (range 0.58–1.59 mm, n = 82); each cluster has a mean linear distance of 0.74± 0.31 mm (range 0.28–1.61 mm, n = 78) on the stolon and terminates at the next branch node or at the next stolon septum; zooid cluster occupying 34–86% of stolon length. Autozooids tilted distad c. 60° from the perpendicular, with a mean length of 0.45± 0.07 mm (range 0.33–0.64 mm, n = 34) in alcohol-preserved retracted specimens, connate; zooid width (as measured in lateral view of zooid cluster) averaging 0.14± 0.03 mm (range 0.09–0.17 mm, n = 34); 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 sparkle in life but are opaque and pale (off-white) in fixed specimens. Outer walls of zooids not significantly thicker than interior walls. Polypide with 8 tentacles.

Remarks. This species is very similar to the holotype of A. acervata described above in the short stolon length, width and zooid-cluster length; it differs in having fewer autozooid pairs per cluster. In having a short stolon length, Amathia brevisilva n. sp. is similar to Amathia fimbria n. sp. described below, but differs from the latter species in having dichotomous branches.

The present species also resembles Amathia rudis Kubanin, 1992 in the spirality of the autozooid cluster and some other features. The holotype colony of A. rudis was collected from the southwestern part of Peter the Great Gulf in the Japan Sea, in the Posyet area at a depth of 2 m on blades of Zostera ; paratype material was found a little further to the east in Novgorod Bay at 3 m depth on a pier. Among the most characteristic features noted by Kubanin was the branching, comprising variable dichotomy, trichotomy and even a rare tetrachotomy. He described the colony as creeping, but attached mainly in the area of the ancestrula and rarely elsewhere. The form of the spiral of autozooidal clusters on stolon segments was not described but his figures seem to indicate both anticlockwise and clockwise spirality. The polypide had eight tentacles. The present species, however, differs from Kubanin’s (1992) description of A. rudis in having narrower and shorter stolons and slightly shorter autozooid clusters.

This species had been reported as Amathia gracilis and/or Amathia imbricata in various previous reports ( Mawatari 1948, 1952, 1962); however, it differs from A. gracilis in erect branching colony morphology and from A. imbricata in less density of zooids along the stolon.

This is very likely the species that Okada & Mawatari (1938) and Mawatari (1952) reported as Amathia distans , Bowerbankia [i.e. Amathia ] imbricata , and Bowerbankia [i.e. Amathia ] gracilis from various localities in Wakayama Prefecture, including Kii Peninsula.

Distribution. Sagami Bay and Kii Peninsula, the Pacific coast of Honshu, Japan, from the intertidal to the shallow nearshore subtidal, on pebbles and hydroid stems.

Figures

FIGURE 3. Amathia brevisilva n. sp. A, C, D. Specimen collected from Wakayama, in the collection of the Seto Marine Biological Station. B, D, F. Specimen collected from Manazuru, Sagami Bay. A. Entire specimen (SMBL-30). B. Entire specimen on pebble surface from Manazuru (NSMT 1209). C. Zooid clusters on dichotomous branches of colony from Wakayama (SMBL-30). D. Zooid clusters on dichotomous branches of colony from Wakayama (NSMT 1212). E. Zooid cluster of colony from Wakayama (SMBL-30). F. Zooid cluster consists of autozooids with yellow pigments (NSMT 1210).