Acromegalomma omenae, Rebello & Nogueira & Carrerette, 2025

Acromegalomma omenae sp. nov.

Figures 6–8 View FIGURE 6 View FIGURE 7 View FIGURE 8 ; 17 View FIGURE 17 (E–H); 18 (F–J); Table 2 View TABLE 2

Material examined. Holotype: MNUFRJ-P57 , in soft-sediment (sand), 22.770ºS 41.886ºW, 1–3 m deep, February/1999, Praia da Ferradura , Búzios, Rio de Janeiro, Brazil GoogleMaps . Paratypes: MNUFRJ-P57 , 7 specimens, in soft-sediment (sand), 22.770ºS 41.886ºW, 1–3 m deep, February 1999, Praia da Ferradura , Búzios, Rio de Janeiro, Brazil GoogleMaps .

Diagnosis. Spherical eyes on most radioles; collar dorsal margins fused to faecal groove; collar dorsal lappets and dorsal pockets both present; anterior peristomial ring exposed dorsal- and laterally; caruncle present; collar ventral lappets rounded; first ventral shield entire, inferior thoracic notochaetae Type A sensu Capa & Murray (2009).

Measurements. Holotype body 19.7 (7.8–22) mm long. Radiolar crown 3.6 (2.5–3.5) mm long.

Thorax 4.1 (2.8–4.5) mm long, 1 (0.3–0.5) mm wide.

Description. General aspects and color patterns. Elongated body, thorax 7–9x longer than wide. Body whitish to light yellowish. Branchial crown shorter than thorax, 1/4 of total body length, also whitish. One pair of large, prominent dorsal eyes on dorsalmost pair of radioles; tiny subdistal compound spherical eyes present on most radioles, one per radiole, except on radioles of two ventralmost pairs.

Radiolar crown. Crown shorter than thorax, semicircular radiolar lobes, with 14 (11–14) pairs of radioles. Subdistal compound eyes on most radioles, except for two ventralmost pairs ( Figs 6C View FIGURE 6 ; 7A View FIGURE 7 ; 19B View FIGURE 19 ). Eyes on dorsalmost pair of radioles large, spherical ( Figs 6B–C View FIGURE 6 ; 17G–H View FIGURE 17 ), other eyes also spherical, progressively shortening dorsoventrally ( Figs 6B–C View FIGURE 6 ; 17G–H View FIGURE 17 ). Tips of radioles of dorsalmost pair short, radiolar tips progressively longer ventralwards ( Fig. 6D View FIGURE 6 ). Dorsal lips erect, triangular, about ¼ of crown length, with radiolar appendages (mid-rib), one pair of pinnular appendages present ( Fig. 7E View FIGURE 7 ). Ventral lips about half length of dorsal lips, broadly rounded. Basal ventral flanges absent.

Peristomium. Anterior peristomial ring exposed dorsally and laterally ( Figs 6E–F View FIGURE 6 ; 7E View FIGURE 7 ; 17F View FIGURE 17 ); caruncle present, short, triangular, half-length of second thoracic chaetiger ( Figs 6E View FIGURE 6 ; 7E View FIGURE 7 ), with rough surface, due to irregularly sinuous crests. Collar dorsal margins fused to faecal groove, dorsal lappets triangular, V-shaped dorsal pockets ( Figs 6E View FIGURE 6 ; 7E View FIGURE 7 ; 17F View FIGURE 17 ); collar ventral lappets as long as first ventral shield, triangular, not overlapping, mid-ventral incision reaching anterior margin of first ventral shield ( Figs 6D View FIGURE 6 ; 17E View FIGURE 17 ), ventral sacs and ventral parallel lamellae both present. Collar lateral margins oblique, not covering origin of radioles ( Fig. 6F View FIGURE 6 ; 7E View FIGURE 7 ).

Thorax. Chaetiger 1: notochaetae all elongated, narrowly hooded ( Figs 8A View FIGURE 8 ; 18F View FIGURE 18 ), those of superior row longer; first ventral shield with rounded anterior margin and short medial incision ( Figs 6D View FIGURE 6 ; 7A–C View FIGURE 7 ). Chaetigers 2–8: all thoracic tori about same length, not reaching corresponding shields, extending for 3/4 of distance between notopodia and ventral shields margins. Notochaetae of superior group elongated, narrowly hooded, 6–8 per fascicle, inferior notochaetae broadly hooded, with narrow tip, Type A sensu Capa & Murray (2009), 10–12 chaetae per fascicle ( Figs 8B–C View FIGURE 8 ; 18G View FIGURE 18 ). Uncini with main fang surmounted by numerous rows of minute teeth, covering half of main fang, handles 1.5x length of main fang, 20–28 uncini per torus; companion chaetae with teardrop-shaped membranes, handles longer than those of the uncini ( Figs 8D View FIGURE 8 ; 18H View FIGURE 18 ). Interramal eyespots absent throughout.

Abdomen. Segments: 71 (39–71). Neurochaetae broadly hooded, those of posterior rows longer, 10–12 chaetae per fascicle ( Figs 8E View FIGURE 8 ; 18I View FIGURE 18 ). Uncini with main fang surmounted by rows of minute teeth, covering half of main fang, and shorter handles than thoracic uncini ( Figs 8F View FIGURE 8 ; 18J View FIGURE 18 ), 14–16 uncini per torus.

Pygidium. Broadly rounded, pygidial eyespots absent.

Tubes. Mucous, with shell fragments and coarse sand embedded.

Methylene blue staining pattern. Thoracic ventral shields stain uniformly, first shield entire, M-shaped anterior margin, subsequent shields rectangular with concave lateral edges, some with anterior transverse fissure. Abdominal shields each divided into two at midlength, by faecal groove. Entire dorsum and lateral sides of body pale, unstained ( Figs 2 View FIGURE 2 ; 6D View FIGURE 6 ; 7A–E View FIGURE 7 ; 17E View FIGURE 17 ).

Etymology. The specific name is a tribute to Professor Dr. Elianne Omena, who collected and identified several specimens of Sabellidae throughout the Brazilian shore, including members of this species.

Habitat. 1–3 m deep, in soft-sediments.

Distribution. Only known from the type locality, Praia da Ferradura, Búzios, Rio de Janeiro, Southeastern Brazil, Southern Atlantic Ocean.

Remarks. Members of this species were previously identified as A. quadrioculatum ( Willey, 1905) by Omena & Creed (2004), but the original description of that species is very brief and no redescriptions are available. The comparison presented herein follows the information provided by Capa & Murray (2009) and Tovar-Hernández & Carrera-Parra (2011); according to these authors, the main differences between specimens of A. quadrioculatum and A. omenae sp. nov. are: members of our new species have (1) eyes on most radioles (in contrast to individuals of A. quadrioculatum , which have eyes only on the five dorsalmost pairs of radioles); (2) spherical eyes (spiraled in specimens of A. quadrioculatum ); (3) tips of dorsalmost radioles shorter, progressively longer ventralwards (uniformly short tips in members of A. quadrioculatum ); (4) inferior thoracic chaetae Type A sensu Capa & Murray (2009) (type B in members of A. quadrioculatum ); and (5) handles of thoracic uncini twice the length of main fang (shorter than the main fang in specimens of A. quadrioculatum ).

Considering the species of Acromegalomma which members have eyes on most radioles, collar fused to the fecal groove, with dorsal pockets and dorsal lappets, and thoracic inferior chaetae type A sensu Capa & Murray (2009), specimens of A. omenae sp. nov. resemble members of A. perkinsi ( Tovar-Hernández & Salazar-Vallejo 2006) , but members of the first species have the anterior peristomial ring margin exposed dorsally and laterally, differently from individuals of A. perkinsi . In addition, members of A. perkinsi possess rounded ventral lappets on collar, do not have a caruncle ( Tovar-Hernández & Salazar-Vallejo 2006), and their inferior thoracic notochaetae are of Type B sensu Capa & Murray (2009), while specimens of A. omenae sp. nov. have triangular ventral lappets, a small caruncle and inferior thoracic notochaetae of Type A sensu Capa & Murray (2009). Furthermore, specimens of A. omenae sp. nov. possess entire ventral shields, similar to those of A. perkinsi , but Brazilian specimens have an anterior transverse fissure on some thoracic shields, which is absent in members of A. perkinsi .

Specimens of A. omenae sp. nov. differ from representatives of A. josei sp. nov. described above by the (1) lateral margins of the collar not covering the origin of radioles (covering origin of radioles, in members of A. josei sp. nov.); (2) collar ventral lappets distally triangular (distally rounded in individuals of A. josei sp. nov.); and (3) by the length of thoracic tori on chaetigers 2–3, occupying half distance between notopodia and ventral shields lateral margins, in members of the latter species (3/4 of that distance, in specimens of A. omenae sp. nov.).

Finally, members of A. omenae sp. nov. differ from those of the other South Atlantic species in this genus, A. schwindtae , by having subdistal compound eyes on most radioles, differently from specimens of the latter species, which have eyes only on the dorsalmost pair. Furthermore, members of A. omenae sp. nov. have progressively longer radiolar tips ventralwards, what does not occur in individuals of A. schwindtae Tovar-Hernández, de León-González & Bybee, 2017 (dorsalmost radioles with long tips decreasing gradually ventralwards) ( Tovar-Hernández et al. 2017).