Ichthyohesione Uchida, 2019

Hiro, Uchida, omi, Lopéz, Eduardo & Sato, Masanori, 2019, New Hesionidae (Annelida) from Japan: Unavailable Names Introduced by Uchida (2004) Revisited, with Reestablishment of their Availability, Species Diversity 24, pp. 69-95 : 88-91

Genus Ichthyohesione Uchida , gen. nov

Ichthyohesione Uchida, 2004: 82 (nomen nudum, without type species fixation); Ruta et al. 2006: 101.

Diagnosis. Body small, depressed, with few segments. Prostomium embedded into peristomium; with two minute antennae, without palps. Two pairs of tentacular cirri on first segment. Mouth opening at distal margin of prostomium. Proboscis sucker-shaped, with sucking face directed dorsally; without marginal papillae, but with densely ciliated inner surface. Parapodia biramous, with ventral lamellae; notochaetae simple, with serrated edge; neurochaetae compound falcigers.

Type species. Ichthyohesione gorgasiae sp. nov., fixed by original designation.

Gender. Feminine.

Etymology. The genus is named after the finding of its symbiotic relationship with a fish, the garden eel ( Gorgasia sp. , Congridae ).

Remarks. The presence of two pairs of tentacular cirri is a very uncommon condition in Hesionidae . Only a single genus, Orseis Ehlers, 1864 has been described as having the same number of tentacular cirri, although it possessed three antennae, non-articulated palps, and uniramous parapodia without notochaetae ( Ehlers 1864). However, this genus was described based on a single, juvenile specimen and subsequently synonymized with Oxydromus by Pleijel (1998). Two pairs of tentacular cirri also fits within the known intraspecific variability in Heteropodarke heteromorpha from Peru ( Hartmann-Schröder 1962a), but all the species of Heteropodarke bear articulated palps and three non-articulated antennae, together with the presence of marginal papillae in the proboscis.

Ichthyohesione gorgasiae Uchida , sp. nov. [New Japanese name: Uo-tsuki-otohime] ( Figs 17 , 18 )

Ichthyohesione gorgasiae Uchida, 2004: 82–85 , figs 21, 22 (unavailable specific name, without explicit fixation of name-bearing types).

Type material. Holotype: (NSMT-Pol H-763), found in the sea water in the vinyl plastic bag used for transporting a garden eel ( Gorgasia taiwanensis Shao, 1990 ), collected off Sabiura, Kushimoto , Wakayama Prefecture (33º28′51″N, 135°44′45″E), sandy bottom, 40 m depth, 17 November 1995.

Diagnosis. Holotype 10 mm long for 64 chaetigers, body dark brown. Prostomium embedded into peristomium, with two minute antennae ( Fig. 17A, B ). Proboscis expanded as sucker, with many minute cilia on its inner surface ( Fig. 17A, C, D ). First segment achaetous, with two pairs of tentacular cirri ( Fig. 17A–D ). Parapodial lobes of chaetiger 1 shorter than followers, lacking ventral cirri and bearing two elliptical dorsal projections ( Fig. 17A–C ). All following parapodia biramous, with ventral lamella on their bases ( Figs 17F , 18A, B ). Notopodia with spindle-shaped dorsal cirri ( Fig. 18A, B ) and a single acicula ( Fig. 18C ); notochaetae present from chaetiger 5 onwards, short, capillary, very finely spinulose ( Fig. 18E ). Neuropodia with postchaetal lobe mucronated ( Fig. 18D ); with about 20 compound chaetae decreasing in size ventrally ( Fig. 18F–H ). Pygidium simple, without anal cirri ( Fig. 17E ).

Etymology. The specific name is a noun in the genitive case of the fish genus Gorgasia , which is likely to host it.

Remarks. The detailed description of the type material was presented in Uchida (2004). Within Hesionidae , the presence of only two pairs of tentacular cirri is extremely rare and no other species showing this trait has the prostomium fused to peristomium and lacking median antenna.

Although common in other annelids (i.e., Hirudinea), external parasitism on vertebrates is an extremely rare condition in polychaetous annelids. Thus far, Ichtyotomus sanguinarius Eisig, 1906 (Ichthyotomidae) has been known as the only polychaete species symbiotic with a fish, and it is interesting that its host species also belongs to the family Congridae ( Eisig 1906; Culurgioni et al. 2006) even though the two polychaetes belong to different systematics groups. Both I. sanguinarius and the new species are external para- sites, with the prostomium truncated, besides inhabiting very distant geographical areas. The two species are clearly different from each other in the following morphological features: (1) I. saguinarius possesses chitinous stylet-shaped mandibles that are used as attachment devices, whereas the new species apparently attaches by means of an anterior sucker, probably aided by modified notopodia. In this way, the new species might behave as Calamyzas amphictenicola Arwidsson, 1932 ( Chrysopetalidae , Calamyzinae), which lives as an ectoparasite on ampharetid polychaetes in Swedish waters ( Arwidsson 1932; Aguado et al. 2013) attaching to them by means of an oral sucker; (2) the parapodia of I. sanguinarius are subbiramous with notoacicula but lacking notochaetae, whereas those of the new species are biramous; and (3) the compound neurochaetae of I. sanguinarius have highly modified blades being long and smooth or very short and dentate, in contrast to those of the new species with typical hesionid blades.

Figures

Fig. 17. Ichthyohesione gorgasiae gen. et sp. nov., holotype (NSMT-Pol H-763). A, dorsal view of the anterior body with proboscis everted. B, dorsal view of the anterior end with proboscis not everted (a rough sketch in living condition). C, enlarged dorsal view of the anterior end. D, enlarged ventral view of the anterior end (dark violet pigment inside the body shown as dotting). E, dorsal view of the posterior end. F, anterior view of chaetiger 21. Scale bar: 0.2mm for A, B, E, F; 0.1mm for C, D.

Fig. 18. Ichthyohesione gorgasiae gen. et sp. nov., holotype (NSMT-Pol H-763). A, posterior view of left parapodium 29. B, anterior view of right parapodium 28. C, tip of notopodial acicula. D, tip of neuropodial postchaetal lobe and tip of acicula. E, notochaeta. F, inferiormost neurochaeta. G, median neurochaeta. H, superiormost neurochaeta. Scale bar: 0.1mm for A, B; 0.02mm for C–H.