Asterocheres minutus ( Claus, 1889 )

Conradi, Mercedes & Bandera, Eugenia, 2011, Asterocherids (Copepoda: Siphonostomatoida) associated with marine invertebrates in the Strait of Gibraltar, Zootaxa 2925, pp. 1-18 : 8-10

Asterocheres minutus ( Claus, 1889)

( Fig. 5 )

Echinocheres minutus Claus, 1889

Asterocheres minutus sensu Giesbrecht, 1987

Material examined. 2 females ( BEIM (COP-562) associated with the sea urchin Paracentrotus lividus (Lamarck) from Tarifa Island (southern Spain) at 12 m depth in 1991.

Description. Adult female: Body ( Fig. 5 A) cyclopiform, slender with cephalothorax oval and cylindrical urosome. Mean body length 470 µm (460–480 µm) and maximum width 265 µm (260–270 µm), based on 2 specimens. Ratio of length to width of prosome 1.29:1. Ratio of length of prosome to that of urosome 2.2:1. Prosome comprising cephalothorax fully incorporating first pedigerous somite and 3 free pedigerous somites.

Antenna ( Fig. 5 C) biramous, 125 µm long including terminal claw. Coxa and basis unarmed. Basis ornamented with spinule row medially. Exopod 1-segmented, slightly longer than wide; with one smooth subterminal seta and two pinnate terminal setae. Endopod 3-segmented; proximal segment elongated with row of spinules laterally; middle segment protruded distally on medial side but articulating with distal segment proximally on lateral side, bearing one distal seta; distal segment with smooth distal seta and distal claw (20 µm long) ornamented with minute spinules on lateral margin.

Oral cone very short, about 90 µm long, reaching to the insertion of maxilliped ( Fig. 5 B), with membranous lateral flanges.

Mandible ( Fig. 5 D) comprising stylet-like gnathobase and slender 1-segmented palp. Palp ornamented with rows of setules laterally and armed with two unequal distal densely plumose setae, the longer ornamented with three spinules apically.

Maxillule ( Fig. 5 E) bilobed; inner lobe (25x15 µm) as long as outer lobe (25x5 µm). Inner lobe armed with five distal setae, one setulose very long seta, one shorter smooth seta, two median setae ornamented with setules on distal part and one short smooth seta. Outer lobe armed with three terminal and one subterminal smooth setae.

Maxilla ( Fig. 5 F) 2-segmented but with partial transverse suture on syncoxa possibly marking plane of praecoxa-coxa fusion; praecoxal part bearing flaccid aesthetasc-like element medially, representing tubular extension of external opening of maxillary gland; coxal part unarmed. Basis claw-like recurved, distally armed with one naked, small seta at mid length and minute spinules in distal portion.

Remaining appendages as described by Bocquet et al. (1963).

Adult male: As described by Bocquet et al. (1963).

Remarks. Asterocheres minutus was poorly described and illustrated as Echinocheres minutus by Claus in 1889 and as Asterocheres minutus by Giesbrecht in 1899. Later on, it was described and illustrated by Bocquet et al. (1963) who also made a comparative study of A. minutus and A. echinicola (= A. violaceous , see Bandera & Conradi 2009b). The population of this species found in Tarifa Island (Southern Spain) shows some discrepancies from the previous descriptions. (1) The antennary exopod has not two but three elements (Bocquet et al. (1963) missed one lateral seta); (2) The armature of the third segment of the antennal endopod consists of one apical seta, claw and lateral row of setules, and not two setae and one claw as illustrated by Bocquet et al. (1963); (3) The palp of the mandible possess two distal setae as illustrated by Bocquet et al. (1963) but the longer one is thicker and has three spinules apically in addition to the setules of the distal part. (4) The inner lobe of the maxillule bears 5 distal setae but the length and ornamentation differ from those described by Bocquet et al. 1963 (5) The maxilla has a flaccid element medially, representing a tubular extension of the external opening of the maxillary gland on the proximal part of the syncoxa, and the claw-like basis is armed with a small seta and ornamented with setules on the distal part which were not illustrated or mentioned by previous descriptions.

This species belongs to a group of Asterocheres species characterized by possessing a 21-segmented antennule in females and a 1-segmented mandibular palp. This group is composed of only three species: A. bacescui (Marcus, 1965) , A. madeirensis Bandera et al., 2007 , and A. echinicola (Norman, 1868) . As commented above, we also have to include A. intermedius , since there is no available information about its mandibular palp. Asterocheres minutus can be separated from A. bacescui , A. intermedius and A. madeirensis by the length of the siphon which is shorter than those of the other three species ( Bandera et al. 2007; Marcus & Por 1960, Bandera & Conradi 2009a). Furthermore, the inner lobe of the maxillule is longer than the outer lobe in A. bacescui and A. madeirensis , while the two lobes of A. minutus are more or less of equal length.

Asterocheres minutus is most similar to A. echinicola , and these two species provide a classic example of sibling species from regular echinoids on the western European coastlines (Bocquet & Stock 1963; Bocquet et al. 1963; Gotto 1979; Bandera & Conradi 2009b). The two copepods overlap in their distribution in the Mediterranean and may be found together on the same sea-urchin without displaying any territorial preference. Bocquet et al (1963) considered A. minutus to be derived from A. echinicola and believed that the present situation can be interpreted as a consequence of allopatric speciation. In this case, therefore, a single ancestral species of Asterocheres is envisaged, which parasitized sea-urchins over a wide geographical range, but became divided into “western” (Atlantic) and “eastern” (Mediterranean) components by a land barrier. The Atlantic population remained relatively unchanged due to a stable oceanic environment. The Mediterranean group, however, trapped in a relatively small sea subject to considerable fluctuations throughout its history, accumulated sufficient mutations to transform it into the species A. minutus . By the time the Strait of Gibraltar had opened to re-establish the communication, specific separation was complete. The new sea link allowed the euryplastic A. echinicola to recolonize the Mediterranean, but did not permit range-extension westward by A. minutus , a species by now stenoplastically adaptated to the conditions peculiar to an island sea (Bocquet & Stock 1963). Therefore, the most likely cladistic model of the origin of these two Asterocheres species is the budding hypothesis described by Queiroz (1998) since one of the Asterocheres species is the origin of the other, and both species (original and new) coexist in the time, naturally isolated on their respective hosts. Such demonstrations of the important role played by geographical isolation in the speciation of parasitic copepods makes information on the existence of geographic races or subspecies very desirable but such information is very scanty.

Paired species of copepods associated with echinoid hosts have also been recorded in the genera Paramolgus , Plesiomolgus and Metaxymolgus ( Humes 1975) . It may be presumed that their evolution has followed a similar course to that suggested for Asterocheres . Other examples of speciation which occur in the Strait of Gibraltar are the twin species Astericola clausi Rosoll, 1889 and A. asterinae ( Bocquet, 1952) , lichomolgid symbionts of asteroids, and Doridicola botulosus (Stock & Kleeton, 1963) and D. comai Conradi et al., 2004 rynchomolgid symbionts of gorgonaceans which are likely to be derived from a common ancestor ( Bocquet et al. 1970; Conradi et al. 1993, 2004).

Host. This tiny copepod is restricted to Echinoidea and it has been recorded in association with three species Paracentrotus lividus , Psammechinus microtuberculatus (Blainville) and Sphaerechinus granularis (Lamarck) (Bocquet et al. 1963; Claus 1889; Giesbrecht 1987; present record).

Distribution. Mediterranean endemic: France (Bocquet et al. 1963), Italy ( Claus 1889; Giesbrecht 1897), and Spain (present record).

Figures

FIGURE 5. Asterocheres minutus (Claus, 1889), female. A, dorsal view. B, cephalic appendages C, antenna. D, mandible, E, maxillule. F, maxilla.