A new species of Sabellacheres M. Sars, 1862 (Copepoda: Gastrodelphyidae) from a deep-water benthic polychaete in Antarctic waters, with a key to the species of the genus
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Specimens of a deep-living sabellid polychaete of the genus Perkinsiana Knight-Jones dredged in Antarctic waters were found to be parasitised by an undescribed species of the cyclopoid copepod genus Sabellacheres M. Sars, 1862. Specimens of both sexes were studied using light microscopy and SEM, and compared with its congeners. The new species, Sabellacheres antarcticus n. sp., can be distinguished from its congeners by the shape and proportions of the body and brood-pouch, its 4-segmented antennae, the shape and length of the distal process of the second antennular segment, the position of leg 3, and the structure of the male maxilliped. This is only the second record of a species of this genus from the southern hemisphere. The new species was found on a single host species, as is typical for most species of Sabellacheres. A key for the identification of both sexes of the species of Sabellacheres is included.
Copepods belonging to the family Gastrodelphyidae List, 1889 are all symbionts of benthic polychaetes of the family Sabellidae (Dudley, 1964; Boxshall & Halsey, 2004). The Gastrodelphyidae currently includes eight species contained in two valid genera: Gastrodelphys Graeffe, 1883 and Sabellacheres M. Sars, 1862 (Boxshall & Halsey, 2004). Following the detailed revision by Dudley (1964), which is the most recent for the group, four species have been recorded from North American waters and all other reports are also from the northern hemisphere, including European (Mediterranean) and Russian seas (Markewitsch, 1940; Dudley, 1964), with the exception of a single species reported from off Madagascar (Carton, 1971). Specimens of a new sabellid polychaete of the genus Perkinsiana Knight-Jones, which is in the process of being described (Tovar-Hernández et al., submitted), were collected by benthic dredge in Antarctic waters during December, 1961. Some of these specimens were found to host parasitic copepods, which were removed for study. These specimens represent an undescribed species of Sabellacheres M. Sars, 1862, which is described here and compared with its known congeners. Also included is a key for the identification of species of Sabellacheres.
Materials and methods
Two specimens, a male and a female, were prepared for SEM (scanning electron microscopy), which included dehydration through graded ethanol solutions (80, 96, 100%), drying and coating following standard procedures (Silva-Briano et al., 2007). Observations were performed using a JEOL LV-5900 microscope at the Universidad Autónoma de Aguascalientes, Mexico. Drawings were prepared with the aid of a drawing tube mounted on a Nikon E-200 compound microscope.
Type-specimens were deposited in the National Museum of Natural History, Smithsonian Institution, Washington, DC (USNM) and in the Collection of Zooplankton of El Colegio de la Frontera Sur at Chetumal, Mexico (ECO-CHZ). The updated names and synonymies of the host sabellid polychaetes were obtained from Fauchald (2010).
Family Gastrodelphyidae List, 1889
GenusSabellacheresM. Sars, 1862
Sabellacheres antarcticus n. sp.
Type-host: The benthic polychaete Perkinsiana sp. (Tovar-Hernández et al., submitted) (Sabellidae) collected by the United States Antarctic Program (USARP) from a site off Wilkes Station, Sta 66 (66. 35361111°S; 110. 4677778°E), Antarctic Ocean, depth 237 m, December 3, 1961.
Type-locality: Site off Wilkes Station (66.35361111 S; 110.4677778 E), Antarctic Ocean.
Type-material: Holotype: adult female from the type-host and locality; partly dissected; slide sealed with Entellan® (USNM-1180549). Allotype: adult male from the same host, date of collection and locality; specimen partly dissected; slide sealed with Entellan® (USNM-1180551). Paratypes: two ovigerous females and one adult male from the same host, date of collection and locality; undissected, ethanol-preserved; vial (USNM-1180550). Paratypes: one adult male and one adult female from the same host, date and locality; ethanol-preserved, undissected; vials (ECO-CHZ-07563, ECO-CHZ-07564).
Additional material: Specimens prepared for SEM, one adult female and one adult male, at Microscopy Laboratory, Universidad Autónoma de Aguascalientes, Mexico.
Site: Distributed over the tentacular crown of the polychaete hosts.
Antennule (Fig. 3) of 5 segments, with 2 proximal segments flattened, approximately twice as wide as distal 3 segments. Second segment roughly triangular, bearing heavily chitinised spiniform process at antero-distal angle. Setal formula as follows (s = seta, ae = aesthetasc): 1(3s, 2ae), 2(14s, spiniform process), 3(5s), 4(3s, 1ae), 5(7s, 1ae). Outer setae of second segment articulated, slender. Distal aesthetasc as long as longest distal seta.
Antenna (Fig. 6) of 4 segments. First segment (coxobasis) bearing single distal seta. Second segment short, with 2 comb-like rows of spinules enclosing field of pilose surface ornamentation (Fig. 15). Third segment with short medial seta. Terminal segment with 2 bicuspidate hooks of different sizes.
Mandible (Fig. 4) very small, concealed by labral processes, with basal process armed with short spiniform element; distal part of mandible elongate and flexed, with row of inner minute spinules.
I-0; I-1; I,I,4
I-0; I-1; I,I+2,4
0-1; 0-2; I,2,2
I-0; I-1; I,I+1,4
0-1; 0-2; 0,1,2
Leg 4 uniramous, reduced to cylindrical lobe with single terminal seta (Fig. 12). Fifth legs absent.
Maxillipeds present only in males, 4-segmented (Figs. 23, 24). Basal segment narrow, unarmed. Second segment longest, twice as long as wide, naked. Third segment about half length of second, bearing single short seta on medial margin. Fourth segment represented by long sickle-shaped process with short medial seta, finely serrate along inner margin.
Legs 1–4 as in female. Fifth legs absent.
The specimens share with other members of Sabellacheres the following characters: post-oral plate smooth, lacking distal teeth or processes, and swimming legs 1–3 biramous, with 3-segmented rami (Dudley, 1964; Boxshall & Halsey, 2004). The new species differs from its known congeners in several characters. The female has a relatively short modified fourth pedigerous somite and the fourth legs are set very close to the third legs, whereas in S. aenigmatopygus Carton, 1971, S. gracilis M. Sars, 1862, S. drachi Laubier, 1968 and S. illgi Dudley, 1964, the fourth pedigerous somite is highly modified and more elongate. Consequently, the fourth legs are widely separated from both the third swimming legs and the urosome in these last four species (Dudley, 1964; Carton, 1971). The new species has a brood-pouch which is about as long as the cephalosome plus first three pedigerous somites combined (Fig. 13), which differs from the proportions in S. illgi, S. drachi and S. aenigmatopygus (brood-pouch at least twice as long as cephalosome plus first three pedigerous somites combined) and S. gracilis (pouch about half the length of cephalosome plus first three pedigerous somites combined) (Green, 1961; Dudley, 1964; Carton, 1971).
The new species carries relatively few eggs (6–14) in its brood-pouch and these are aligned in well-defined rows in all specimens examined. In the other species of Sabellacheres, the number of eggs generally appears to be higher (more than 17 eggs in all cases, as discerned from the available illustrations), except in S. drachi which has about ten (Laubier, 1968). However, in all of the other species the eggs are irregularly arranged within the pouch and do not form definite rows, as in the new species.
The antero-distal chitinised process on the second antennular segment differs in form between species; in the new species, this process is short, barely reaching the middle of the third segment and its acute tip is slightly hooked. In the other species, this process is clearly stronger, distally blunt (as in S. gracilis) or acute (as in S. illgi, S. drachi and S. aenigmatopygus), reaching the middle of the fourth segment (S. gracilis, S. illgi and S. drachi) or even beyond (S. aenigmatopygus).
The males of the new species differ from those of its congeners in the development of the maxilliped. Only in S. antarcticus n. sp. is this appendage slender, relatively weakly developed, with an elongate terminal claw-like process which is about twice as long as the second segment of the maxilliped. In other species, the maxilliped has robust segments and a relatively short distal claw, which is about as long as the second segment (in S. gracilis) or slightly shorter (as in S. illgi). In S. aenigmatopygus the terminal claw is slightly longer than the second segment. The male of S. drachi is unknown.
With the addition of Sabellacheres antarcticus n. sp., there are now five nominal species in the genus: S. aenigmatopygus from off Madagascar, S. drachi from off Spain, S. gracilis from Norwegian and Russian waters and the Northeastern Pacific, and S. illgi from the Northeastern Pacific. This is the second record of the genus from the southern hemisphere, after S. aenigmatopygus, and the first from Antarctic waters.
Adult females of species of the family Gastrodelphyidae have been recorded only in association with sabellid hosts. Two gastrodelphyid species are known to utilise three host species: Gastrodelphys dalesi (Green, 1961) occurs on two species of Eudistylia Bush (Dudley, 1964) and one of Megalomma Johansson (Gómez & Tovar-Hernández, 2008), and Sabellacheres illgi is found on Parasabella rugosa (Moore), Megalomma splendidum (Moore) and Pseudopotamilla occelata Moore (Dudley, 1964; Boxshall & Halsey, 2004). Gastrodelphys clausii Graeffe, 1883 is known only from Bispira volutacornis (Montagu). The other species of Sabellacheres are each known from only one polychaete host species: Myxicola infundibulum (Montagu) (S. gracilis), Potamilla reniformis (Brugière) (S. aenigmatopygus) and Potamilla torelli (Malmgren) (S. drachi) (Dudley, 1964; Laubier, 1968; Carton, 1971; Boxshall & Halsey, 2004). The new species, S. antarcticus, is recorded only from Perkinsiana sp., which inhabits sandy sediments on the continental shelf (Giangrande & Gambi, 1997; Tovar-Hernández et al., submitted).
The specimens of S. antarcticus were positioned in the oral area, along the tentacular crown, of the polychaete host. This microhabitat usage is in accord with the observations of other authors reporting gastrodelphyid copepods as symbionts (e.g. Nash & Keegan, 2006). Both male and female individuals of the new species were collected as free forms, most probably living as motile symbionts, a behaviour more common in species of Gastrodelphys (see Boxshall & Halsey, 2004).
Key to the known species of Sabellacheres
Third swimming legs not separated from legs 1 and 2……… 2
Third swimming legs widely separated from legs 1 and 2……… S. gracilis
Brood-pouch twice as long as cephalosome plus first three pedigerous somites combined; fourth pedigerous somite elongate; fourth legs widely separated from third legs and from urosome………3
Brood-pouch as long as, or slightly shorter than cephalosome plus first three pedigerous somites combined; fourth pedigerous somite short; fourth legs located close to third legs………S. antarcticus n. sp.
Antero-distal process of second antennular segment elongate, reaching distal end of fourth segment……… 4
Antero-distal process of second antennular segment reaching distal end of third segment………S. illgi
Anal somite shorter than preceding somite……… S. aenigmatopygus
Anal somite longer than preceding somite……… S. drachi
Maxillule with three terminal elements of equal length………2
Maxillule with middle terminal element longer than other two……..….S. gracilis
Maxilliped robust, with terminal claw as long as second segment of appendage or shorter……… 3
Maxilliped relatively weak, slender, with terminal claw twice as long as second segment of appendage………S. antarcticus n. sp.
Genital somite with slender median process extending beyond tip of sixth legs………S. aenigmatopygus
Genital somite with vestigial median process located between bases of sixth legs……… S. illgi
The copepod specimens examined were kindly made available to us by Brigitte Hilbig (Senckenberg Institute, Frankfurt), head of the Southern Ocean Annelid Project (SOAP), and James Blake (ENRS Marine and Coastal Center, Woods Hole). Mariana Tovar (Geomare, Mexico) and Beatríz Yáñez (ICMyL-UNAM, Mazatlán) encouraged this study; they sorted these specimens during their investigations on sabellid worms and are describing the new species of Perkinsiana, the host of our new species. Araceli Adabache and Marcelo Silva-Briano kindly processed the specimens for SEM and guided our examination of this material at the Universidad Autónoma de Aguascalientes, Mexico. We also thank Linda Ward for registering the type-specimens at the National Museum of Natural History, Smithsonian Institution, Washington, DC.
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