SIPHONOSTOMATOIDA Burmeister, 1835

Zagami, Giacomo, Costanzo, Giuseppe & Brugnano, Cinzia, 2014, Two new species of asterocherid (Copepoda: Siphonostomatoida) associated with a sponge from the Strait of Messina, Italy: taxonomic and ecological traits, Journal of Natural History 48 (37 - 38), pp. 2255-2272 : 2257-2265

publication ID	https://doi.org/ 10.1080/00222933.2014.917209
persistent identifier	https://treatment.plazi.org/id/567F878C-FFB3-FF95-18C4-79D31D67AB7A
treatment provided by	Felipe
scientific name	SIPHONOSTOMATOIDA Burmeister, 1835
status

Treatment

Order SIPHONOSTOMATOIDA Burmeister, 1835 Family ASTEROCHERIDAE Giesbrecht, 1899

Genus Psilomyzon Stock, 1965 Psilomyzon laetitiae sp. nov.

( Figures 2–6 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 )

Material examined

Holotype female plus 2 female and 2 male paratypes are stored in the collections of the Zoological Museum Cambria ( ZMC), Department of Biological and Environmental Sciences , University of Messina , Italy, Reg. Nos 2013.6529 (holotype female), 2013.5630–33 (paratypes). Dissected female and male paratypes were mounted on 10 and 5 slides, respectively, Reg. Nos 2013.5634 (female) and 2013.5635 (male). In addition, in the collections of the Natural History Museum, London, are stored 1 female and 1 male paratypes Reg. Nos 2013.86 and 2013.87, respectively. A further 9 paratype females and 2 males (intact) are stored in the authors’ collection.

Description

Adult female. Holotype 0.78 mm long. Mean body length 0.70 mm, with range of 0.64–0.78 mm (based on 10 specimens).

Body ( Figure 2A View Figure 2 ) cyclopiform, symmetrical, with dorso-ventrally flattened prosome; anterior margin of cephalothorax rounded and with broad posterolateral expansions, in dorsal view. Prosome elongate, about 1.4 times longer than wide; comprising cephalothorax, incorporating first pedigerous somite, and 3 free pedigerous somites, all with well-developed rounded epimeral margin. Urosome 4-segmented, comprising 5th pedigerous somite; genital double-somite representing fused genital and first abdominal somites, swollen in anterior half, with 2 lateral rounded processes, and 2 free abdominal somites. Genital double somite about 1.5 times wider than long, bearing bipartite genital aperture, paired gonopores (oviduct openings) located on dorsolateral surface, and paired copulatory pores located on ventrolateral surface. Caudal rami short and concave on inner margin, bearing 6 setae, 2 on dorsal surface and 4 on distal margin, innermost setae plumose.

Antennules ( Figure 2B, C View Figure 2 ) 20-segmented; segmental fusion model as follows: 1 (I), 2 (II), 3 (III), 4 (IV), 5 (V), 6 (VI), 7 (VII), 8 (VIII), 9 (IX–XII), 10 (XIII), 11 (XIV), 12(XV), 13 (XVI), 14 (XVII), 15 (XVIII), 16 (XIX), 17 (XX), 18 (XXI–XXII), 19 (XXIII–XXIV), 20 (XXV–XXVIII). Segments 1–8 each bearing 2 setae; segment 9 with 8 setae; segments 10–17 each with 2 setae; segment 18 with 1 seta plus aesthetasc on distal margin; segment 19 with 2 setae; segment 20 with 9 setae.

Antenna biramous ( Figure 3A View Figure 3 ), with coxa and basis separate, both unarmed. Exopod 1-segmented, reduced to small lobe, with 1 subapical and 1 apical setae. Endopod 3-segmented, first segment elongate, naked; second segment short with 1 tiny seta; third segment with 1 small medial seta and 1 distal plumose seta, plus large claw on distal margin.

Oral cone short and rounded, produced into distal siphon, pyriform with bulbous tip reaching insertions of maxillipeds.

Mandible ( Figure 3B View Figure 3 ) with stylet-like gnathobase and slender 1-segmented palp. Stylet with enlarged apex. Palp ornamented with disto-medial setules bearing, distally, 1 short naked seta and 1 long curved seta, plumose on medial side.

Maxillule ( Figure 3C View Figure 3 ) bilobed; inner lobe larger than outer. Inner and outer lobes armed with 4 and 3 setae, respectively.

Maxilla ( Figure 3D View Figure 3 ) 3-segmented, comprising syncoxa with long flaccid element, possibly aesthetasc or tube pore, and recurved distal claw-like basis ornamented with 3 spinules along margin.

Maxilliped ( Figure 3E View Figure 3 ) 5-segmented, comprising syncoxa, basis and 3-segmented endopod. First endopodal segment naked; second endopodal segment with 1 spinule and third endopodal segment with long apical claw plus 1 distal seta.

Swimming legs 1–4 ( Figure 4A–D View Figure 4 ) biramous, with 3-segmented rami, with exception of leg 4, bearing 2-segmented endopod. Spine and setal formulae of swimming legs as in Table 1.

Leg 1 ( Figure 4A View Figure 4 ) with basal medial seta and setiform terminal exopodal element. First and second exopodal segments of leg 2 ( Figure 4B View Figure 4 ) with minute serrations on lateral margins; second endopodal segment with 2 acute outer processes.

Leg 3 ( Figure 4C View Figure 4 ) second endopod segment with 2 acute processes on outer margin. Leg 4 ( Figure 4D View Figure 4 ), bearing minute (hardly visible) exopodal setae; first endopod segment with 1 minute spiniform process; second segment characterized by inner bifid spinous projection and 2 distal minute spinous processes; lateral margin of endopodal segments each ornamented with row of setules.

Leg 5 ( Figure 2A View Figure 2 ) with elongate free segment, 3 times longer than wide, bearing 1 sub-distal outer seta and 2 distal setae. Outer seta located on surface of fifth pedigerous somite.

Leg 6 ( Figure 2A View Figure 2 ) represented by paired opercular plates closing off gonopores on genital somite, each bearing very small seta.

Adult male. Paratype 0.64 mm long. Mean body length 0.64 mm, with range of 0.62–0.68 mm (based on five specimens).

Body ( Figure 5A View Figure 5 ) cyclopiform, similar in body shape to female, but without lateral processes on genital somite. Prosome broad, about 1.3 times longer than wide; comprising cephalothorax fully incorporating first pedigerous somite and 3 free pedigerous somites, characterized by well-developed epimeral margins. Urosome 5-segmented, comprising fifth pedigerous somite, genital somite and 3 free abdominal somites. Genital somite about 1.3 times wider than long, with convex lateral margins, bearing paired genital apertures located on ventral surface near posterior margin. Paired ovoid spermatophores visible inside somite. Anal somite 1.4 times wider than long. Caudal rami as for female. Appendages as for female, except antennules, legs 4 and 6.

Antennule ( Figure 6A, B View Figure 6 ) 17-segmented, geniculate on both sides, segmental fusion model as follow: 1 (I), 2 (II), 3 (III), 4 (IV), 5 (V), 6 (VI), 7 (VII), 8(VIII), 9 (IX–XII), 10 (XIII), 11 (XIV), 12 (XV–XVI), 13 (XVII), 14 (XVIII), 15 (XIX–XX), 16 (XXI–XXIII), 17 (XXIV–XXVIII). Geniculation located between segments 15 and 16. Segments 1–8 each with 2 setae; segment 9 with 8 setae; segments 10 and 11 each with 2 setae; segment 12 with 4 setae; segment 13–15 each with 2 setae; segment 16 with 2 setae plus 1 aesthetasc; segment 17 with 10 setae. Chaetotaxis of leg 4 exopod reduced to I-1; I-1; II,I,2; first endopod segment naked, second bearing inner projection with 3 long distal setiform processes and 1 outer bifid process. Inner projection variable intra-specifically in having variable number of 2 or 3 setiform elements ( Figure 6D View Figure 6 ).

Leg 6 ( Figure 5A View Figure 5 ) represented by paired opercular plates closing off genital apertures, each armed with 2 thin setae.

Etymology

The species name is dedicated to one of the authors’ (G. Zagami) wife, Letizia, in gratitude for her support and grace under all circumstances, allowing him to do this surprising job.

Remarks

The genus Psilomyzon consists of only one known species, P. pauciseta Stock, 1965 . This species was recorded from the sponge Ircinia (Sarcotragus) muscarum (Schidt) , collected in Mediterranean Sea coastal waters (Cape Creus, northeastern Spain), at a depth of 30– 42 m. Psilomyzon genus is a typical siphonostomatoid belonging to the Asterocheridae family. Many asterocherid genera show a reduction in the segmentation and setation of the swimming legs. Huys and Boxshall (1991) considered the setal formula of Asterocheres reginae Boxshall and Huys, 1994 to be the most primitive in Siphonostomatoida . In Psilomyzon genus the leg chaetotaxis is very modified.

In the family Asterocheridae several genera, such as Cletopontius Thompson and Scott, 1903 ; Discopontius Nicholls, 1944 ; Tuphacheres Stock, 1965 ; Peltomyzon Stock, 1975 ; Cystomyzon Stock, 1981 ; Oedomyzon Stock, 1981 ; Meandromyzon Stock, 1989 ; Siphonopontius Malt, 1991 ; and Kolocheres Johnsson, 1998 , are characterized by reductions in both segmentation and setation of the swimming legs. In Psilomyzon Stock, 1965 ; Laperocheres Ivanenko, 1998 ; Sinopontius Boxshall, 1990 ; and Inermocheres Boxshall, 1990 , the legs do not present any reduction in segmentation, showing only a reduced chaetotaxis.

The new species shares a 2-segmented leg 4 endopod with Siphonopontius , Discopontius , Peltomyzon and Meandromyzon .

However, Psilomyzon laetitiae sp. nov. is closely related to P. pauciseta . Female and male body shape of both species are very similar to each other; both species share 20 and 17 segmented antennule, in the female and male, respectively, with aesthetasc on antepenultimate and penultimate segments in the female and male, respectively.

The new species can be easily distinguished from P. pauciseta by the 2-segmented leg 4 endopod, with the second endopod segment carrying an inner projection, possibly homologous to the third segment of the leg 4 endopod of P. pauciseta . Besides, the new species is characterized by sexual dimorphism in the second segment of the leg 4 endopod, which is not present in P. pauciseta . By far the most striking character of the new species is the structure of the sexually dimorphic second segment of the leg 4 endopod. Sexual dimorphism in the asterocherids was registered in Laperocheres genus, on the third exopodal segment of leg 2 ( Ivanenko 1998).

Detailed comparison between P. pauciseta and P. laetitiae shows a number of significant differences including mostly mandibular palp and swimming leg chaetotaxis (the characters of P. pauciseta are in parenthesis).

Mandibular palp unsegmented (2-segmented). Maxillule with outer lobe bearing 3 setae (4 setae); maxilla with distal claw bearing 3 spinules (2 group of setules); maxilliped with syncoxa, basis and first endopodal segment naked, second and third endopodal segment with 1 spinule and 1 seta, respectively (each segment with 1 minute spinule). The new species shows reductions in the chaetotaxis of all swimming legs and in the endopodal segmentation of leg 4. In P. laetitiae the first exopod segment of legs 2 to 4 has 1 seta (unarmed); the second and third exopodal segment of leg 4 have 1 and 2 setae (0 and 1 seta); leg 4 endopod 2-segmented (3-segmented); in the female leg 4 second endopod with an inner bispinous projection and 2 distal minute spinous processes, in the male inner projection with 3 long distal setiform processes and 1 outer bifid process (naked).

In P. laetitiae the reduction of the endopod of the fourth swimming leg has proceeded further than in P. pauciseta . In P. laetitiae the tiny spinous processes on the first and second endopodal segments of the female and the setiform elements of the male, and the inner projection on the second endopodal segment, may represent vestigial elements of the armature and of the third endopodal segment, respectively. Furthermore, in the new species the bifid outer process on male leg 4 second endopodal segment and the distal tiny processes on female leg 4 second endopod seem to be serially homologous of the similar process on the same segment in leg 1–3.

The differences in mouth appendages and swimming legs provide significant evidence to justify the establishment of the new species.