Aphelochaeta hormosa Blake 2018, new species

Aphelochaeta hormosa new species

Figures 11–12

Material examined. East Antarctica, Budd Coast, Wilke’s Station, coll. Virginia Institute of Marine Science, 03 Dec 1961, 66.354°S, 110.468°E to 66.354°S, 110.471°E, dredged, 238 m, holotype (USNM 1490710) and 2 paratypes (USNM 1490711).

Description. Holotype complete, broken into three parts, 16.4 mm long, 0.45 mm wide across expanded thoracic region, 0.145 mm across middle segments, with 97 setigerous segments. Holotype with anterior or thoracic segments expanded both dorsally and ventrally (Figs. 11A, 12A), more or less oval in cross section with narrow crowded segments; abdominal region with moniliform segments each about 1.5 times long as wide (Figs. 11B, 12B); posterior segments becoming narrow, wider than long, producing weakly expanded posterior end rounded dorsally, flattened ventrally with low ridge, then tapering to simple rounded pygidial lobe (Figs. 11C, 12C). Body tan in color; holotype with black pigment spots on dorso-lateral margins of peristomium (Fig. 11A).

Prostomium conical, tapering to narrow rounded tip (Figs. 11A, 12A); eyespots absent, nuchal organs narrow lateral slits; dorsal crest not apparent. Peristomium broad, with three annular rings best seen laterally on holotype (Figs. 11A, 12A), only weakly observed on paratypes; relatively smooth dorsal surface not cut with annular rings, ventrally forming lips around of mouth. Pair of dorsal tentacles arising medially in notch on posterior margin of peristomium (Fig. 11A); first pair of branchiae arising lateral and slightly posterior to dorsal tentacles (Fig. 11A); subsequent branchiae on setiger 1 and following segments dorsal to notosetae, continuing through expanded thoracic segments, not observed on abdominal segments.

Parapodia best developed in thoracic segments, inflated but weakly developed, consisting of low setal tori bearing fascicles of 8–10 thin, smooth capillaries in single row; parapodia of abdominal segments not evident, with 3–4 capillaries arising directly from body wall (Fig. 11B); notosetae longer than neurosetae, but long natatory-like setae not observed.

Methyl Green stain. Most of body with prominent subdermal glands that stain darkly, producing distinctive pattern; dorsal and lateral areas of pre-setiger area stain intensely on both prostomium and peristomium (Fig. 12A). Each segment with heavy stained band extending laterally, ventrally, and up on opposite side of each thoracic segment, encircling body from middle thoracic segments, strongest on parapodia (Fig. 12A); parapodial staining present on post-thoracic segments until moniliform segments begin, then generally reduced to staining intestine (Fig. 12 B–C).

Etymology. The epithet hormosa is derived from the Greek, hormos, for a chain or necklace, referring to the bead-like nature of the body segments of this species.

Remarks. Aphelochaeta hormosa n. sp. is similar to the type species, A. monilaris, from the eastern Pacific (Hartman 1960; Blake 1996) in having moniliform abdominal segments and a distinctive MG staining pattern. However, in A. monilaris, while MG stains the tip of the prostomium, it only weakly stains the peristomium; MG does produce broad stripes on the anterior margins of thoracic segments but these do not encircle the body as in A. hormosa n. sp. The two species further differ in that A. monilaris has a prominently expanded posterior end, whereas the posterior segments of A. hormosa n. sp. are only weakly expanded.

Among Antarctic species of Aphelochaeta, A. hormosa n. sp. is most similar to A. aubreyi n. sp. in having an expanded thoracic region, black spots on the posterior-lateral margin of the peristomium, and moniliform segments. However, A. hormosa n. sp. has a distinctive MG staining pattern that is lacking in A. aubreyi n. sp. and the prominent lateral peristomial grooves present in A. hormosa n. sp. are only weakly developed in A. aubreyi n. sp. In addition, the posterior segments of A. aubreyi n. sp. are enlarged dorsally and flattened ventrally, whereas, in A. hormosa n. sp. the posterior segments are not enlarged dorsally.

Despite these differences, there is no question that A. aubreyi n. sp. and A. hormosa n. sp. are closely related. Except for the prominent and distinctive MG staining pattern of A. hormosa n. sp. other differences could be attributed to variability. However, despite having samples from throughout Antarctica, A. aubreyi n. sp. appears to be limited to sites around the Antarctic Peninsula and Weddell Sea, whereas A. hormosa n. sp. has been collected only from the opposite side of Antarctica off Wilkes Land.

Distribution. East Antarctica, Budd Coast near Wilkes Station, 238 m.