Acromegalomma punctum, Rebello & Nogueira & Carrerette, 2025

Acromegalomma punctum sp. nov.

Figures 9–13 View FIGURE 9 View FIGURE 10 View FIGURE 11 View FIGURE 12 View FIGURE 13 ; 17 View FIGURE 17 (I–L); 18 (K–O); 19C; Table 2 View TABLE 2

Material examined. Holotype. MNRJ P992 , complete specimen, continental shelf, 20.590ºS 39.916ºW, 142–146m deep, June to July, 2013, Espírito Santo Basin , Espírito Santo, Brazil GoogleMaps . Paratype. MNRJ P992 , incomplete specimen, continental shelf, 20.590ºS 39.916ºW, 142–146m deep, June to July 2013, Espírito Santo Basin , Espírito Santo, Brazil GoogleMaps .

Diagnosis. Eyes only on dorsalmost pair of radioles, spherical; collar dorsal margins not fused to faecal groove; collar dorsal lappets and dorsal pockets both absent; anterior peristomial ring totally exposed dorsally; caruncle present; collar ventral lappets triangular; first ventral shield divided transversely into two parts. Inferior thoracic notochaetae Type C sensu Giangrande et al. (2015).

Measurements. Holotype body 14.6 (4, incomplete) mm long. Radiolar crown 5.2 (5.6) mm long. Thorax 2.9 (3.9) mm long, 0.7 (0.7) mm wide.

Description. General aspects and color patterns. Elongated body, thorax 4 (6)x longer than wide. Body from white, opaque, to yellowish, changing along body, lighter on abdomen. Branchial crown longer than thorax, 1/3 of total body length, translucent, with 3–4 brown bands, each extending for around space of origin of 6 pairs of pinnules, basal band shorter. One pair of large, prominent dorsal eyes on dorsalmost pair of radioles. Thoracic ventral shields with darkened lateral edges extending to neuropodia, gradually shorter posteriorwards ( Fig. 9A–E View FIGURE 9 ).

Radiolar crown. Crown longer than thorax, semicircular radiolar lobes, with 10 (10) pairs of radioles. Subdistal compound eyes only on dorsalmost pair of radioles ( Figs 9B, D View FIGURE 9 ; 11B, D View FIGURE 11 ; 19C View FIGURE 19 ), eyes large, spherical. Tips of radioles of dorsalmost pair barely conspicuous ( Figs 9A, B, D View FIGURE 9 ; 11B, D View FIGURE 11 ; 17K View FIGURE 17 ), others radioles without compound eyes and with elongate tips ( Figs 11B View FIGURE 11 ; 17L View FIGURE 17 ). Dorsal lips erect, triangular, about 1/4 of crown length, with radiolar appendages (mid-rib) ( Fig. 11E View FIGURE 11 ), and one pair of pinnular appendages. Ventral lips about half length of dorsal lips, broadly rounded. Basal ventral flanges present ( Fig. 12A, B View FIGURE 12 ).

Peristomium. Anterior and posterior peristomial rings both totally exposed dorsally ( Figs 9E View FIGURE 9 ; 10A, C View FIGURE 10 ; 11C View FIGURE 11 ; 12C View FIGURE 12 ), caruncle long, triangular, twice the length of second thoracic chaetiger, with rough surface, due to irregularly sinuous crests ( Figs 11E View FIGURE 11 ). Collar dorsal margins not fused to faecal groove, dorsal lappets and dorsal pockets both absent ( Figs 9E View FIGURE 9 ; 10C View FIGURE 10 ; 11C View FIGURE 11 ; 12C View FIGURE 12 ; 17J 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 9C View FIGURE 9 ; 10A, D View FIGURE 10 ; 11A View FIGURE 11 ; 12A, B View FIGURE 12 ; 17I View FIGURE 17 ), ventral sacs and ventral parallel lamellae both present ( Fig. 11E View FIGURE 11 ); collar lateral margins oblique, not covering origin of radioles ( Figs 9A–C, E View FIGURE 9 ; 10A, C–D View FIGURE 10 ; 11A–C, E View FIGURE 11 ; 12A–C View FIGURE 12 ).

Thorax. Chaetiger 1: notochaetae all elongated narrowly hooded, those of superior row longer ( Figs 13A View FIGURE 13 ; 18K View FIGURE 18 ); first ventral shield with straight anterior margin and short, medial incision ( Figs 9C View FIGURE 9 ; 11A, E View FIGURE 11 ; 12A, B View FIGURE 12 ; 17I View FIGURE 17 ). Chaetigers 2–8: all thoracic tori about same length, not reaching shields, extending for 3/4 of distance between notopodia and ventral shields lateral margins ( Fig. 10A, B, D View FIGURE 10 ). Notochaetae of superior group of elongated, narrowly hooded, 10– 12 per fascicle; inferior notochaetae broadly hooded, with thick and short, distally rounded shaft, ending by broad mucronate hood, Type C sensu Giangrande et al. (2015), 10–12 chaetae per fascicle ( Figs 13B, C View FIGURE 13 ; 18L View FIGURE 18 ). Uncini with main fang surmounted by 6–7 rows of minute teeth, covering half of main fang, handles 1.5x length of main fang, 15–18 uncini per torus; companion chaetae with teardrop-shaped membranes, and longer handles than those of uncini ( Figs 13D View FIGURE 13 ; 18M View FIGURE 18 ). Interramal eyespots absent throughout.

Abdomen. Segments: 61 (paratype incomplete). Neurochaetae narrowly hooded, those of posterior rows longer, 8–10 chaetae per fascicle ( Figs 13E View FIGURE 13 ; 18N View FIGURE 18 ). Uncini with main fang surmounted by 5–6 rows of minute teeth, covering half of main fang, and shorter handles than those of thoracic uncini ( Figs 13F View FIGURE 13 ; 18O View FIGURE 18 ), 8–10 uncini per torus.

Pygidium. Broadly rounded, pygidial eyespots absent.

Tubes. Unknown.

Methylene blue staining pattern. Thoracic ventral shields stain uniformly, first shield divided transversely into two parts, posterior one larger, with medial ciliated patch; subsequent shields entire, each with medial ciliated patch only visible after staining. All shields rectangular with concave lateral edges. 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 ; 10 View FIGURE 10 ; 12 View FIGURE 12 ).

Etymology. The specific name is an adjective derived from the patches of cilia, as ‘dots’ at midlength of each thoracic ventral shield.

Habitat. 142–146 m deep, in soft-sediments.

Distribution. Only known from the type locality, 20.590ºS 39.916ºW, Espírito Santo Basin, Southeastern Brazil, Southern Atlantic Ocean.

Remarks. Specimens of only two other currently known species of Acromegalomma , A. gesae ( Knight-Jones, 1997) and A. pigmentum ( Reish, 1963) , share with members of A. punctum sp. nov. the presence of eyes only on the dorsalmost pair of radioles, collar not fused to the faecal groove, and inferior thoracic notochaetae Type C sensu Giangrande et al. (2015). However, in members of both A. gesae and A. pigmentum , the tori of chaetigers 2–3 occupy half of the distance between the notopodia and the corresponding ventral shield margins, whereas in representatives of A. punctum sp. nov., they extend over three-quarters of that distance. The pygidium also differs among these species, rounded in members of A. punctum sp. nov., triangular in specimens of both the other species. Furthermore, members of all these three species are unique within members of Acromegalomma in presenting radiolar basal ventral flanges, but members of both A. pigmentum and A. punctum sp. nov. have the first ventral shield divided transversely into two parts, while specimens of A. gesae have it entire.

A distinctive feature observed in specimens of A. punctum sp. nov. is the presence of ciliated patches at midlength of thoracic ventral shields. Those cilia are likely to have sensory function and also help to generate water currents inside the tubes ( Tovar-Hernández & Dean 2014). This characteristic has been poorly investigated because it requires scanning electron microscopy (SEM) or specific staining techniques, such as methylene blue or methyl green, to be visualized. Similar structures have been documented in species of Pseudobranchiomma Jones, 1962 , Laonome Malmgren, 1866 , Amphicorina Claparède, 1864 , Paradialychone Tovar-Hernández, 2008 , and Dialychone Claparède, 1868 . Tovar-Hernández & Dean (2014) suggested that those patches vary in shape, types of cilia, and size, making them useful for species delimitation. In members of A. punctum sp. nov., these ciliary patches could represent an important diagnostic feature, helpful for species delimitation within the genus, but further research is still necessary to fully understand the functional roles of those cilia and check if they are also present in members of other species of the genus.

The redescription of A. pigmentum by Tovar-Hernández & Carrera-Parra (2011) is quite elucidative, but does not mention the "patches of cilia", present in members of A. punctum sp. nov. However, it is possible to observe in their Figure 22 (B and F) (Tovar-Hernández & Carrera-Parra, 2011) markings very similar to the patches of cilia described herein for A. punctum sp. nov. Furthermore, members of A. punctum sp. nov. differ from those of A. pigmentum by having handles of thoracic uncini 1.5x the length of main fang, while specimens of A. pigmentum have them twice as long as main fang. A re-analysis of the type material of A. pigmentum is necessary to confirm if members of that species also have the "patches of cilia", as observed in material of A. punctum sp. nov.