Sertularella affinicostata Calder & Faucci 2021, sp. nov.

Sertularella affinicostata, sp. nov.

Figs 7a–e, 8

Sertularella costata.— Calder et al., 2003: 1190, fig. 14a, b [not Sertularella costata Leloup, 1940].

Type locality. Ecuador: Galapagos Islands, Darwin Island, within SCUBA depth, on barnacle cluster (Calder et al. 2003, as Sertularella costata).

Etymology. The specific name is derived from the Latin adjective affinis (akin to) and the name costata, given its resemblance to Sertularella costata.

Voucher material. HOLOTYPE: Ecuador: Galapagos Islands, Isla Darwin, SCUBA depth, on barnacle cluster, 26 January 1999, one colony, 1.5 mm high, with gonothecae, coll. K. Collins, ROMIZ B3339.

PARATYPES: Ecuador: Galapagos Islands, Cousin Rock, 10 m, on sponge, 17 June 2001, one colony, 5.2 mm high, without gonothecae, coll. D. Calder, ROMIZ B3409.— Ecuador: Galapagos Islands, Marchena Island, Punta Espejo, ca. 8 m, 18 June 2001, four colonies, to 5 mm high, without gonothecae, coll. L. Garske, ROMIZ B3418.— French Frigate Shoals, on calcareous rubble, 13.ix.2002, one colony, 2 mm high, without gonothecae, coll. A. Faucci, ROMIZ B5486.

OTHER MATERIAL: Ecuador: Galapagos Islands, Wolf Island, 6 m, on algae, November 1992, two colonies, to 0.55 mm high, without gonothecae, coll. P. Humann, ROMIZ B3315.— Ecuador: Galapagos Islands, Wolf Island, on stolon of Aglaophenia sp., 25 January 1999, one colony, 1.1 mm high, without gonothecae, coll. J. Mallinson, ROMIZ B3318.

Description. Holotype colony unbranched, diminutive, with cauli reaching 1.5 mm high, arising from a hydrorhiza overgrowing cluster of small barnacles. Paratype colonies unbranched to occasionally with a branch or two, with cauli reaching 5.2 mm high, overgrowing a sponge. Basal stolons without internal pegs, diameter 59–83 µm; cauli mostly erect, caulus bearing a maximum of three hydrothecae in holotype, occasionally with a single hydrotheca borne on a short internode in parts of colony, paratypes with as many as 13 cauline hydrothecae (ROMIZ B3418). Hydrocaulus monosiphonic, nearly straight to irregular, length of first internode from insertion with hydrorhiza to base of first hydrotheca 0.14–0.35 mm, smooth to slightly wrinkled but without regular annulations, expanding gradually in circumference distally, hydrocaulus beyond first hydrotheca divided into typically short internodes, with first of these growing out directly from base of hydrotheca without nodes or annulations, more distal internodes separated by oblique nodes sloping alternately in opposite directions; internodes 322–423 µm long, 73–88 µm wide at nodes, 128–151 µm wide at base of hydrotheca, often with an annulation or slight swelling at proximal end, mostly smooth elsewhere, widening gradually from proximal end to base of hydrotheca, then tapering again to distal node, every internode bearing a hydrotheca; perisarc of moderate thickness. Hydrothecae sessile, alternate, typically biseriate but sometimes given off in multiple planes, quite deep, barrel-shaped, extending upwards and outwards, arranged alternately on opposite sides of hydrocaulus, adnate to internode for 1/3 or less of their length, with axis oblique to that of hydrocaulus; hydrothecal walls with about 10–14 sharp-edged horizontal ridges, widest near mid-point, tapering towards proximal and distal ends, narrowest at base, slightly constricted just below margin, then expanding to rim, length abcauline wall 338–377 µm, length adcauline wall adnate 72–99 µm, length adcauline wall free 279–310 µm, maximum diameter 189–218 µm, both adcauline and abcauline walls convex over much of their length, slightly concave at distal end; hydrothecal base 90–118 µm wide. Hydrothecal rim quadrangular, with four distinct, pointed, equally developed cusps separated by shallow embayments, no evidence of renovations in present material; hydrothecal orifice 150–183 µm wide; hydrothecal cavity enclosed by an operculum of four triangular valves; submarginal cusps three, one being abcauline and two latero-adcauline. Hydranths with an abcauline diverticulum.

Gonophores fixed sporosacs. Gonothecae of holotype colony male, containing spermatocytes, borne on a short, smooth pedicel arising from hydrorhiza or from base of hydrocaulus adjacent to a hydrotheca, nearly circular in cross-section, elongate-oval in lateral view, with rounded base and distal taper to a narrow neck, 0.8–1.0 mm long from base to orifice, maximum diameter 0.45–0.60 mm; margin entire, 0.15–0.18 mm wide, orifice round or nearly so. Gonothecal walls with seven to eight transverse, rounded ridges, those at distal end prominently developed, those at proximal end flattened, less pronounced; perisarc quite thick.

Remarks. Of the many described species of Sertularella Gray, 1848, these hydroids resemble several in a group bearing numerous sharp-edged horizontal ridges around walls of the hydrothecae. Amongst them, it appears closest to S. costata Leloup, 1940, originally described from Sagami Bay, Japan. The hydroid examined here is nevertheless readily distinguished from that species in having fewer ridges, with ca. 10–14 instead of ca. 20 (Leloup, 1940; Hirohito 1983, 1995). Its hydrothecae also differ in being barrel-shaped rather than distinctly tapered distally, and a smooth part just below the rim is more elongated. Moreover, the basal-most internode of the hydrocaulus is typically short rather than forming a long, slender, smooth peduncle as in S. costata, and cauline internodes are shorter and thicker. Given these differences, the hydroid is considered distinct from S. costata, as described by Leloup (1940) and Hirohito (1983, 1995), and from other known species of Sertularella. It has therefore been described here as new, under the binomen S. affinicostata. Of note, specimens from French Frigate Shoals are morphologically indistinguishable from colonies identified as S. costata from the Galapagos Islands by Calder et al. (2003). Those specimens, like the colony examined here, differ from S. costata in the characters noted above, and are taken here to have been misidentified. All are taken to be referable to the same species, S. affinicostata.

In being sterile, the single colony from French Frigate Shoals has been designated as a paratype (ROMIZ B5486). Selected as the holotype (ROMIZ B3339) is a fertile colony of the species from Darwin Island, Galapagos Islands, erroneously assigned to S. costata by Calder et al. (2003). Additional paratypes include previously unrecorded specimens from the Galapagos (ROMIZ B3409, ROMIZ B3418) in collections at the ROM. Cauli of these colonies are noteworthy in being taller (to 5.2 mm vs. 2 mm high), and they bear more hydrothecae (as many as 13 vs. 5) than those from either Darwin Island (ROMIZ B3339) or French Frigate Shoals (ROMIZ B5486). Other specimens from the Galapagos Islands (ROMIZ B3315, ROMIZ B3318), assigned to S. costata by Calder et al. (2003) but here considered conspecific with S. affinicostata, are too miniscule to be considered adequate type material.

Similar hydrothecal ornamentation appears on the hydrothecal walls of several other species of Sertularella, including S. mirabilis Jäderholm, 1896 and S. sinensis Jäderholm, 1896. Hydroids of those species differ in morphology from S. affinicostata, with colonies of S. mirabilis forming three-dimensional lattices and resembling sponges, and those of S. sinensis being bushy (Jâderholm 1896; Vervoort 1993; Hirohito 1995; Schuchert 2015). Stems of both species diverge from those of the hydroids described here in being repeatedly branched, and their colonies are larger (to 5 cm or more high). Sertularella crenulata Nutting 1905, a species originally described from Hawaii, differs in having finer and more numerous hydrothecal ridges (>30). Its colonies are also significantly larger (to 7.5 cm high) and more robust, with basally polysiphonic cauli (Nutting 1905, Vervoort 1993). Two tropical western Pacific species, S. paucicostata Vervoort, 1993 and S. pseudocostata Vervoort, 1993, are immediately distinguishable from S. affinicostata in lacking intrathecal cusps. Their hydrothecae are also considerably larger, with a total depth × maximum diameter of 715–825 µm × 305–390 µm in S. paucicostata, and 1105–1300 µm × 540–585 µm in S. pseudocostata. Others of the genus with similar hydrothecal ornamentation include S. patagonica (d’Orbigny, 1842), with 6–8 transverse ridges, S. peculiaris Leloup, 1935, with about five ridges, S. helenae Vervoort, 1993, with 8–9 ridges encircling cylindrical hydrothecae, and S. fraseri Galea, 2010, with 5–6 ridges.

Sertularella affinicostata is unusual in being a species from the Hawaiian Archipelago that is currently known elsewhere only from the eastern Pacific rather than the Indo-west Pacific region. Both poorly known and easily overlooked, it undoubtedly occurs elsewhere in the tropical Pacific Ocean.

Reported Distribution. Hawaiian archipelago. First record.

Elsewhere. Galapagos Islands, Wolf and Darwin islands (Calder et al. 2003, as Sertularella costata).