Asterocheres siphonatus Giesbrecht 1897

Asterocheres siphonatus Giesbrecht, 1897

(Figs. 6–9)

Ascomyzon lilljeborgi Thorell, 1859, Sars, 1915 Artrotogus boeckii Brady, 1880

Asterocheres thorelli Bresciani and Lützen, 1962

Material examined. (a) One female (ZMO-F7645, 1 slide) collected in Norway by G.O. Sars. (b) One female (ZMO-F7646, 1 slide) collected in Norway by G.O. Sars. (c) 97 females (ZMO-F21603, in alcohol) collected in association with Corella parallelograma (Müller) in Norway by G.O. Sars. (d) 11 females (BEIM (COP-548) associated with the ascidian Synoicum argus (Milne Edwards) from Las Lajas, Algeciras Bay (southern Spain) at 5 m depth in 1991.

Description. Adult female. Body (Fig. 6 A) cyclopiform, with moderately broad prosome and cyclindrical urosome. Mean body length 910 µm (880–960 µm) and maximum width 510 µm (450–540 µm), based on 5 specimens. Ratio of length to width of prosome 1.7:1. Ratio of length of prosome to that of urosome 2.45:1. Prosome comprising cephalothorax fully incorporating first pedigerous somite and 3 free pedigerous somites.

Urosome 4-segmented comprising leg 5-bearing somite, genital double-somite and 2 free abdominal somites. Dorsal and ventral surfaces of free abdominal somites and genital double-somite ornamented with large, flattened epicuticular scales arranged in overlapping rows (Fig. 6 B, C). Posteroventral margins of abdominal somites ornamented with hyaline frills with serrated margins (Fig. 6 C). Integumental pores and sensilla present on urosomal somites. Leg 5-bearing somite wider than long with some epicuticular scales on dorsal surface (Fig 6 B, 9A). Genital double-somite slightly wider than long (150x145 µm), bearing paired genital apertures bipartite, each comprising lateroventral copulatory pore and dorsolateral gonopore; lateral margin with row of long spinules (about 8 spinules) in distal third (posterior to genital apertures) (Fig. 6 B, C). Genital area armed with two plumose seta (Fig. 6 B).

Caudal rami (Fig. 6 B,C) as long as wide (30x30 µm), covered by overlapping epicuticular scales; armed with 6 setae. Seta I absent; setae II and VII smooth, slightly offset onto dorsal surface; setae III, IV, V and VI plumose.

Antennule (Fig. 6 D) 21-segmented, about 470 µm long. Segmental homologies (expressed segment given first followed by ancestral segments in brackets) and setation pattern as follows: 1(I)-2, 2(II)-2, 3(III)-2, 4(IV)-2, 5(V)- 2, 6(VI)-2, 7(VII)-2, 8(VIII)-2, 9(IX-XII)-7, 10(XIII)-2, 11(XIV)-2, 12(XV)-2, 13(XVI)-2, 14(XVII)-2, 15(XVIII)-2, 16(XIX)-2, 17(XX)-2, 18(XXI)-2+ae, 19(XXII)-2, 20(XXIII-XXIV)-4, 21(XXV-XXVIII)-7. Segment 10 (XIII) reduced, partly overlapped by distal expansion of compound segment 9 (IX-XII). Three first segments with one seta with a circlet of cuticular denticles at each tip (Fig. 9 C).

Antenna (Fig. 6 E) biramous, 215 µm long including terminal claw. Small unarmed coxa with tuft of spinules on inner margin. Elongated unarmed basis ornamented with fine spinule rows. Exopod one-segmented, slender, bearing one medial and one terminal naked setae. Endopod three-segmented; proximal segment elongated, unarmed but ornamented with rows of spinules; middle segment small, protruded distally on medial side but articulating with third segment on lateral side and armed with smooth, distal seta; distal segment with rows of fine setules and spinules laterally, and armed with one smooth and one barbed setae, and distal claw, 55 µm long, with minute spinules on lateral margin.

Oral cone very long and slender, 620 µm long, reaching almost to posterior margin of intercoxal sclerite of leg 4.

Mandible (Fig. 7 A) comprising stylet-like gnathobase and slender one-segmented palp. Stylet located in oral cone, very long and slender but expanded at the apex as illustrated. Palp slender, one-segmented with spinules arranged like a fan in middle third and some spinules on lateral margin; armed with 2 equal apical setae, one of them with spinules.

Maxillule (Fig. 7 B) bilobed; praecoxal gnathobase 1.7 times longer than palp. Praecoxal endite (70x30 µm) ornamented with tufts of setules at base and distally and a row of spinules on lateral margin; armed with 5 distal setae different in length, one of them very short and naked. Palp (40x10 µm) with spinules on lateral margin; armed with 2 subterminal and 2 terminal barbed setae.

Maxilla (Fig. 7 C) 2-segmented but with partial transverse suture on syncoxa (proximal segment) possibly marking plane of praecoxa-coxa fusion; coxal portion unarmed. Basis claw-like armed with small seta in distal third. Claw margins smooth.

Maxilliped (Fig. 7 D) 5-segmented, comprising short syncoxa, long basis and three-segmented endopod. Syncoxa with inner seta distally. Basis elongated with short seta on lateral inner margin in middle third. First endopodal segment short, bearing 3 naked setae; second endopodal segment armed with naked seta; third endopodal segment bearing recurved terminal claw plus additional subapical seta. Claw 62 µm long, with minute spinules on lateral margin.

Swimming legs 1–4 (Fig. 8 A–D) biramous, with three-segmented rami. Intercoxal sclerite present in legs 1–4, ornamented with rows of spinules in legs 1 and 2. Spine and seta formula as follows:

Coxa Basis Exopod segments Endopod segments Leg 1 0-1 1-1 I-1; I-1;III,4 0-1;0-2;1,2,3

Leg 2 0-1 1-0 I-1; I-1;III,I,4 0-1;0-2;1,2,3

Leg 3 0-1 1-0 I-1; I-1;III,I,4 0-1;0-2;1,1+I,3 Leg 4 0-0 1-0 I-1; I-1;III,I,4 0-1;0-2;1,1+I,2

Coxae ornamented with spinule rows laterally, as illustrated. Inner coxal seta pinnate in legs 1–3 and absent in leg 4. Basis ornamented with spinule rows laterally; outer seta naked in all legs; longer than first exopodal segment in legs 1 and 2 and shorter than first exopodal segment in legs 3 and 4. Surface of legs 1–4 ornamented with flattened epicuticular scales arranged in irregular pattern (Fig. 8 A–D). Lateral margins of exopodal segments with spinular rows; those of endopodal segments with rows of setules. Outer spines of exopodal segments bilaterally serrated in legs 1 and 4 and serrated only in external side in legs 2 and 3.

Fifth leg (Fig. 6 B) with protopod incorporated into somite; outer seta displaced laterally, with spinule row at base. Free segment (75x 33 µm) elongated oval, 2.3 times longer than wide; ornamented with spinules and epicuticular scales and armed with one subterminal and 2 terminal naked setae (Fig. 9 B).

Sixth leg (Fig. 6 B) represented by paired opercular plates closing off gonopores on genital double somite; armed with two plumose setae.

Colour of living specimens reddish.

Adult male: scarcely described by Thorell (1859) and Sars (1915).

Remarks. Thorell described this species as the type species of his new genus Ascomyzon, A. lilljeborgi, in 1859. The specific name lilljeborgi, however, had already been preoccupied by Asterocheres lilljeborgi Boeck, 1859. This was pointed out by Brady (1880) although he also considered Artotrogus Boeck 1859 as synonymous to these genera. While these three genera were described at the same year, there was no doubt about the priority of Boeck’s names since Thorell cited Boeck’s work in his monograph. Between Boeck’s two names, Brady favoured Artotrogus, considering it “less objectionable than the term Asterocheres ”. Therefore, he proposed the name of Artotrogus boeckii for Thorell’s species and Artotrogus lilljeborgii for Boeck’s. Brady’s suggestion was emended by Giesbrecht (1897) when he pointed out the certain synonymity of Asterocheres and Ascomyzon (and also Cyclopicera Brady, 1872), the validity of the genus Artotrogus and the difference between the species described by Thorell as Ascomyzon lilljeborgi and that described by Brady as Artotrogus boeckii. Hence, Giesbrecht proposed the name of Asterocheres siphonatus for Thorell’s species (Ascomyzon lilljeborgi) since the specific name of lilljeborgi was preoccupied by Asterocheres lilljeborgi Boeck, 1859, and considered Brady’s species as Asterocheres boecki (Brady, 1880).

Although Sars (1915) accepted the synonymy of Asterocheres and Ascomyzon and the priority of the first, he favoured the name of Ascomyzon to Asterocheres because “ the species of this genus are by no means exclusively parasites of asterids, but are found to infest many other invertebrate animals ”. He returned Thorell`s species to the name Ascomyzon lilljeborgi, and considered Asterocheres siphonatus Giesbrecht, 1897 as a junior synonym of Ascomyzon lilljeborgi and changed the specific name of Boeck’s to Ascomyzon asterocheres. Sars’ erroneous suggestion was followed by Gurney (1927), Van Oorde-de lint et al. (1936), Bocquet (1952) and Lang (1949). Ten years later, Bresciani and Lützen (1962) re-established the priority of Asterocheres and proposed the specific name of thorelli for Thorell’s species without considering that this species had already been named by Giesbrecht. Since then, this species has been erroneously named as A. thorelli (Sars G.O., 1879) (Brun 1976; Barel & Kramers 1977; Humes 1986), until Gotto’s excellent monograph (1993) where, following the International Code of Zoological (article 60.3), he cited this species as A. siphonatus Giesbrecht, 1897. Except for Walter (2009), most authors identify this species as such these days.

This species is easily recognized by its very long siphon and this may be the reason why it has only been illustrated by Thorell in 1859 and Sars in 1915 under the name of Ascomyzon lilljeborgi. Our study of Asterocheres siphonatus has revealed some important differences with respect to these previous descriptions: (1) For example, this species is commonly described as possessing 19–20 segments in the antennules of females, but in fact the antennule has 21 segments, (2) The antennary exopod has 2 elements, a medial and a terminal setae instead of a single element, and the last segment of the antennary endopod bears a stouter claw and one seta more than those previously illustrated. (3) The mandibular stylet was omitted, and the palp has two terminal setae equal in length as illustrated by Thorell, and not unequal as drawn by Sars. (4) The inner lobe of the maxillule has five setae instead of four, and the outer lobe has four setae, 2 subterminal and 2 terminal, in contrast with the three setae described by Thorell and the 1 subterminal and 3 terminal illustrated by Sars. (6) The basis of the maxilliped has one seta and the first endopodal bears 3 setae which were overlooked in the previous descriptions. (7) The subterminal seta of the free segment of leg 5 and the two plumose setae of leg 6 were also unobserved. (8) The flattened epicuticular scales on the urosomite and legs 1–4 were omitted.

From now on, A. siphonatus belongs to the group of species with a 21-segmented antennule in the females and a 1-segmented mandibular palp which includes only four species (A. bacescui, A. madeirensis, A. echinicola and A. minutus) together with the additional A. intermedius which has an undetermined mandibular palp. The very short siphon and the equal length of both maxillular lobes of A. echinicola and A. minutus separate them from A. siphonatus. The siphons of A. bacescui and A. madeirensis reach the insertion of the maxillipeds and that of A. intermedius extends to the intercoxal plate of leg 1 while that of A. siphonatus reaches the intercoxal plate of leg 4 (Bandera et al. 2007; Bandera & Conradi 2009a; Marcus & Por 1960).

Host. Although this copepod was initially recorded in association with ascidians, it has also been recorded associated with starfishes, free or among dredged material. The host ascidian species are: Corella parallelograma (Müller) (as Ascidia parallelograma in Thorell 1859; Sars 1915) and Ascidia virginia Müller (as Phallusia virginia, Aurivillus 1882).

Our specimens were found associated with the polyclinidae ascidian Synoicum argus (Milne Edwards) which has an Atlantic-Mediterranean distribution and generally lives in photophilic communities (Naranjo 1996). However, in Algeciras Bay, this compound ascidian is very common in harbour areas and it has been considered as an indicator of areas which have been subjected to intense stress over a long period (Naranjo et al. 1996). Up to now, the only fauna reported to be associated with S. argus has been the cyclopoid copepod Doroixys uncinata Kershner, 1879 (López-González et al. 1997).

Distribution. Sweden (Thorell 1859; Aurivillus 1882), Norway (Sars 1918); France (Van oorde-de Lint et al. 1936; Bocquet 1952); Suez Channel (Gurney 1927); Spain (present record).