Spirobranchus minutus

Spirobranchus minutus (Rioja, 1941)

(Figs. 6, 8, 9)

Pomatoceros minutus Rioja, 1941.

Pomatoceros minutus. Bastida-Zavala & Salazar-Vallejo (2000); Bastida-Zavala (2008).

Serpulorbis catella Weisbord, 1962.

Spirobranchus minutus. Bastida-Zavala et al. (2017).

Examined material: 67 specimens. Alagoas state, Maceió Municipality: Garça Torta reef: 09°34′55.72″S, 35°39′22.56″W,MNRJP-002780(6specimens).Ipioca Reef: 09°30′54.08″S, 35°35′11.64″W, MNRJP-002781 (1 specimen). Marechal Deodoro Municipality: Frances sandstone reef: 09°46′16.02″S, 35°50′18.57″W, MNRJP-002782 (11 specimens). Rio de Janeiro state, Armação dos Búzios Municipality: Geribá Beach: 22°46′50.62″S, 41°54′14.59″W, MNRJP-002783 (8 specimens). Santa Catarina state, Florianopolis Municipality: Daniela Beach: 27°26′32.23″S, 48°31′24.37″W, MNRJP-002785 (32 specimens). Armação Beach: 27°45′00.45″S, 48°30′01.91″W, MNRJP-002786 (5 specimens). Pantano do Sul: 27°47′01.06″S, 48°30′24.69″W, MNRJP-002784 (4 specimens).

Diagnosis (after Bastida-Zavala et al., 2017): Tube white, with three longitudinal keels and rows of alveoli between keels; without peristomes. Opercular peduncle with thin distal wings, white. Operculum conical, with calcareous distal plate, with rounded tip. First notopodium with 2-3 limbate collar chaetae, but sometimes lacking collar chaetae altogether, with glandular spot in position of collar chaetae.

Description

Tube: Opaque, white, or pinkish. Triangular in cross-section, lumen circular. Three longitudinal keels along the tube, extending over the opening forming pointed tips. Two longitudinal rows of alveoli present between the keels (Fig. 6B). Tubes solitary.

Radiolar Crown: Live specimens with alternating bands of brown pigment and non-pigmented areas. Base of crown usually dark brown (Figs. 6C, 6F). Coloration faded after preservation (Fig. 6A). Crown composed of two lobes arranged in semicircles, each with 10 radioles, basally joined by inter-radiolar membrane for one third of their length. Radioles cylindric in cross-section, inner side with two rows of filiform pinnules of even length through radioles, with naked tips.Tips filiform and about as long as pinnules. Ocelli absent.

Mouth Parts: Two thin, smooth lips. Dorsal lips shorter than ventral lip, as pair of lateral digitiform palps, about half as long as pinnules. Ventral lips longer, rectangular, connecting both radiolar lobes at base of radiolar crown.

Peduncle: Smooth, triangular in cross-section, inserted slightly left to dorsal mid-line. Pair of smooth lateral wings at tip of peduncle, triangular, not extending beyond operculum (Figs. 6A, 6C). Constriction between peduncle and opercular base conspicuous. Dark brown pigmentation as lateral spots on peduncle, not extending into distal wings.

Operculum: Opercular ampulla round-shaped, distally flat, circular in top view, with flat to convex calcareous endplate, generally white or purplish, bilobed (Fig. 6D) with dorsal edge pointed (Fig. 6E).Talon absent.

Collar and Thoracic Membranes: Three-lobed collar. Dorso-lateral lobes twice as long as mid-ventral lobe, triangular, with entire edges, reaching near base of crown; continuous with thoracic membranes, forming a ventral apron covering 2 abdominal chaetigers.Mid-ventral lobe rectangular. Tonguelets present between dorso-lateral and ventral lobes, thin, leaf-shaped, often brown colored (Figs. 6C, 9E, 9F).

Thorax: Six or seven chaetigers, depending on presence of collar chaetae. Collar chaetae limbate or absent. After collar, notopodial lobes conical, emerging between thoracic membranes and neuropodia. Thoracic chetae limbate, longer than collar chaetae, in one fascicle (Fig. 8H). Neuropodial lobes rectangular, all bearing tori curved downwards, gradually smaller through thorax. Thoracic uncini saw-to-rasp-shaped, with 8-10 rows of curved teeth in profile, general formula = p:4:3:2:2:2:1:1:2:1, peg flat and squared. Posterior teeth larger than anterior ones (Fig. 8I). Parapodial lobes not approaching each other posteriorly, thus not forming a ventral triangular depression. Glandular clusters as scattered spots in neuropodia, 6 pairs of trapezoidal glandular thoracic shields, leaving longitudinal mid-ventral gap (Fig. 6F).

Abdomen: Largest analyzed specimen with 18 abdominal segments. Abdomen in entire animals usually three times longer than thorax, with chaetigers progressively more densely packed posteriorly region. Notopodial uncini smaller than thoracic ones, rasp-shaped, with 10-12 rows of curved teeth in profile, general formula = p:7:6:5:5:4:4:4:4:5:4:3:2, anterior peg squared (Fig. 8J). Neuropodial chaetae true trumpet-shaped, abruptly bent, with 2 distal rows of denticles separated by groove and lateral serrated filiform projection (Fig. 8K). Glandular clusters present (Fig. 6G) basally to tori. Pygidium bilobed, with terminal anus.

Remarks: The flattened and ventrally elongated opercular plate of members of S. minutus may render the distinction between these animals and members of both the congeneric S. americanus (Day, 1973) and S. triqueter (Linnaeus, 1758) problematic. Spirobranchus americanus was described from animals from the Beaufort region of North Carolina (USA) (Day, 1973). The absence of a bilobed distal end on the ventral opercular face and the single-keeled tube of those animals differentiate them from specimens of S. minutus. In the original description of S. americanus, Day (1973) reports and illustrates opercular variations similar to those observed in members of S.minutus, although his illustrations demonstrate that calcareous reinforcement occurs in the most basal portion of the opercular ampulla and forms a diagonal line in transverse view of the operculum. Bastida-Zavala et al. (2017) also report that individuals of S. americanus have more rows of alveoli on the sides of their tubes than among S. minutus. Spirobranchus triqueter was originally described and reported from the European Atlantic coast (Linnaeus, 1758). The operculum of those animals has three characteristic pointed projections distally and their tubes are single-keeled, unlike the triple-keeled tube of members of S. minutus, which also lack such opercular projections. Çinar (2013) does not mention S. minutus as an introduced species, but there are multiple reports of this species beyond the Pacific Ocean (Zibrowius, 1970; Morgado, 1980; Souza, 1989; Rocha, 1993, 1995; Ananias, 2017). The specimens we analyzed for the present study are morphologically identical to others reported from Brazil (Zibrowius, 1970; Ananias, 2017) and further afield (Rioja, 1941; Bastida-Zavala & Salazar-Vallejo, 2000; Bastida-Zavala, 2008, 2009).

The absence of calcareous projections in the operculum, radiolar photoreceptor structures and special collar chaetae render members of S. minutus immediately distinguishable from those of S. giganteus and S. tetraceros. Individuals of both S. minutus and S. lirianeae sp. nov. share the absence of opercular projections, but among members of S. minutus the operculum is slightly convex and bilobed, photoreceptor structures are absent, and their tubes are triple-keeled, with rows of alveoli. By having rasp-shaped uncini and simple (rather than lobed) tonguelets, individuals of S. minutus can be differentiated from specimens of the other species of the genus recorded in Brazil.

Type-locality: Acapulco, Guerrero, Mexico (Pacific Ocean).

Habitat: Intertidal: on rocky shores, beneath rocks.

Distribution: Considered as amphiamerican by Bastida-Zavala et al. (2017). Pacific Ocean: Baja California, Mexico to Peru; Atlantic Ocean: Gulf of Mexico, Caribbean Sea and Brazil, in the states of Paraíba (Ananias, 2017) and São Paulo (Zibrowius, 1970; Morgado, 1980; Souza, 1989; Rocha, 1993, 1995). In this study, members of this species were found among material from the states of Alagoas, Rio de Janeiro and Santa Catarina,extending the distribution of these animals from off the northeastern through the southern Brazilian coast, until Santa Catarina.