Rhamphostomella townsendi Osburn 1952

Rhamphostomella townsendi Osburn, 1952

(Fig. 21)

Rhamphostomella townsendi Osburn, 1952, p. 430, pl. 51, figs 2, 3.

Material examined. Holotype: USNM 11032, record name Che981–2, colony encrusting sponge, US Fish Commission Collection, RV Albatross, Stn 5695, 26 April 1911, Santa Rosa Island, SW of Channel Islands, Southern California, Pacific Ocean, 33°33.0ʹ N, 120°17.0ʹ W, depth 977 m.

ZIRAS 1/50117, colony encrusting branch of erect bryozoan Microporina okadai, PIBOC Collection, RV Akademik Oparin, 14th Expedition, Stn 5, 2 August 1991, southeast off Medny Island, Commander Islands, Bering Sea, 54°12.0ʹ N, 168°37.3ʹ E, depth 569 m, Sigsbee trawl, collector A.V. Smirnov. ZIRAS 2/50118, one colony encrusting sponge, PIBOC Collection, RV Akademik Oparin, 14th Expedition, Stn 5, 2 August 1991, southeast off Medny Island, Commander Islands, Bering Sea, 54°12.0ʹ N, 168°37.3ʹ E, depth 569 m, Sigsbee trawl, collector A.V. Smirnov. Specimen AL–WP–0019–0022, M.H. Dick Collection, one colony, Alaska Fisheries Science Center and National Marine Fisheries Service Collection, FV Sea Storm, Haul 190, Stn 114–21, 23 July 2004, coastal waters of Amchitka Island, Rat Islands, western Aleutian Islands, Bering Sea, 51°51.6ʹ N, 178°27.8ʹ E, depth 224– 235 m, collector M.H. Dick.

Measurements. ZIRAS 1/50117, Medny Island, Commander Islands, Bering Sea (Fig. 21C, F, H, I, K). ZL, 0.63–0.98 (0.76 ± 0.07) (n = 20). ZW, 0.45–0.65 (0.53 ± 0.06) (n = 20). ZD, 0.35–0.38 (n = 2). OrL, 0.18–0.22 (0.20 ± 0.02) (n = 10). OrW, 0.20–0.25 (0.22 ± 0.02) (n = 10). OeL, 0.22–0.30 (0.26 ± 0.02) (n = 10). OeW, 0.35–0.40 (0.37 ± 0.02) (n = 10). Av(s)L, 0.12–0.17 (0.14 ± 0.02) (n = 10). P(m)N, 12–19 (16) (n = 10). P(oe)N, 24–41 (34) (n = 6).

Description. Colonies encrusting, multiserial, unilaminar (Fig. 21A), small, irregular in form, attaining about 5 mm in maximal dimension, light-brown to beige when dry. Zooids of medium size, rhombic, hexagonal to oval, sometimes tapering proximally (Fig. 21D, G, J), arranged in quincunx, demarcated by fine interzooidal sutures recognisable in all parts of colony.

Frontal shield umbonuloid (Fig. 21A, D–K), thin, strongly convex, tessellated, with oval and circular areolae along margins (Fig. 21D–H). Interareolar ridges lacking or indistinct in young zooids (Fig. 21D, J), short ridges appearing in zooids and areolae becoming angular in older parts of colony (Fig. 21F–H); reticulate pattern of frontal shield becoming coarser with age. Umbonuloid component occupying about 60% of length of frontal shield (61% in one measured zooid), with fine parallel lineation and accretionary banding (Fig. 21I, K). Ring scar indistinct (Fig. 21K), with uneven boundary between exterior-wall microstructure and extra-umbonuloid calcification.

Primary orifice (Fig. 21A, B) slightly submerged, transversely oval or trapezoidal; distal and lateral margins formed by upper part of distal transverse wall. Distal margin of orifice more or less straight; proximal margin bisinuate, with median bifurcate lyrula and pair of triangular lateral processes with rounded tip (Fig. 21B). Condyles absent. Pair of articulated oral spines sometimes present in distolateral corners of orifice in young, non-ovicellate zooids (Fig. 21B, D, J); oral spines absent in ovicellate zooids (Fig. 21A, H).

Secondary orifice (Fig. 21D–H) trapezoidal to irregularly oval and triangular, entirely formed by low tubular peristome; distal curvature formed by vertical extension of distal zooidal wall (Fig. 21D). Laterally and proximally restricted by two lappets of slightly elevated peristome, one incorporating cystid of suboral avicularium on left or right side; lappets low in non-ovicellate zooids, angular in ovicellate zooids. In ovicellate zooids, lappets defining U- to V-shaped, slightly asymmetrical pseudosinus (Fig. 21E–H) but not fusing with proximolateral corners of ooecium; extensions of lateral walls of contiguous distolateral zooids connecting proximolateral corners of ooecia with corresponding peristomial lappet, conferring irregular outline to secondary orifice (Fig. 21E–H).

Cystid of suboral avicularium relatively small, with tessellated surface and one or two minute communication pores, asymmetrically situated proximal to orifice (Fig. 21B–D, F–H). Frontal surface (rostral/postmandibular areas) of avicularium gently concave, normally crossing zooidal midline, facing obliquely proximally to proximolaterally. Rostrum elongate triangular, pointed, extending slightly across and overhanging proximal margin of orifice, gently curved, directed obliquely medially to laterally and upwards (Fig. 21B–D, F–H). Mandible elongate triangular, with rounded or pointed tip (Fig. 21C, E, G). Palate and palatal foramen triangular, with rounded distal end; opesia semicircular. Crossbar complete.

No adventitious avicularia.

Ovicells hyperstomial (Fig. 21E–H). Ooecium formed by distal autozooid; ooecial fold developing at colony periphery concurrently with frontal shield of distal zooid. Ooecium with straight proximal margin and smooth ectooecium with numerous circular and oval pseudopores. Fully formed ovicells remaining prominent, with ooecia not embedded in secondary calcification.

Zooids interconnected by two mural pore chambers in each distolateral wall (Fig. 21L) and multiporous septula in basal half of transverse walls.

Basal wall of zooids fully calcified (Fig. 21A, J, L), smooth. Sparse white spots (presumably less-calcified areas) visible in semitransparent basal wall by light microscopy. Basal surface with sporadic small, tubular protuberances about 0.07 mm in diameter and up to 0.26 mm long. Boundaries between zooids visible as fine meandering sutures.

Ancestrula and early astogeny not observed.

Remarks. R. townsendi resembles R. curvirostrata in possessing: 1) a tessellated frontal shield; 2) a primary orifice with a bisinuate proximal margin, median bifurcate lyrula and pair of triangular lateral processes; 3) an asymmetrically set suboral avicularium with a triangular rostrum that partly overhangs the proximal orificial margin; and 4) an ooecium with uniformly scattered pseudopores. At the same time, R. townsendi clearly differs from R. curvirostrata in that 1) the suboral avicularium is rather short and overhangs the proximal orificial margin in R. townsendi but is strongly elongate, extending across and overhanging the entire orifice in R. curvirostrata; 2) the marginal areolae are separated by short, sometimes reduced ridges in R. townsendi, but by moderately to strongly elongate ridges in R. curvirostrata; 3) fully-formed ovicells remain prominent in R. townsendi, but ooecia become embedded in the frontal shield of the distal zooid by secondary calcification in R. curvirostrata; and 4) all major measurements are generally smaller in R. townsendi than in R. curvirostrata.

While the holotype specimen (USNM 11032) from the Santa Rosa Island, California, is clearly conspecific with our specimens from the Commander and western Aleutian Islands, the holotype appears to lack oral spines altogether. Furthermore, there appears to be intercolony variation in tessellation, with some specimens (e.g. Fig. 21A, F–H) more coarsely tesselated than others (e.g. Fig. 21D, E).

Ecology. Rhamphostomella townsendi is known from depths of 224–977 m on sandy and silty bottoms, encrusting glass sponges and the erect bryozoan Microporina okadai.

Distribution. Rhamphostomella townsendi is a little-known species originally described from southwest of the Channel Islands, California, USA. It is now also known from two northwestern Pacific localities, including the Pacific slope of Medny Island, Commander Islands, and the Beringian slope of Amchitka Island, Rat Islands, western Aleutian Islands. Thus R. townsendi appears to be an Eastern Pacifc boreal, sublittoral to upper bathyal species, extending to the edge of the subtropics.