Systematic Parasitology

, Volume 83, Issue 1, pp 65–75 | Cite as

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

Article

Abstract

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.

Introduction

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).

Order Cyclopoida

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.

Description

Female (Figs. 117). Average total length measured from frontal margin of dorsal cephalic shield to posterior margin of brood-pouch 1.52 ± 0.16 mm (n = 5). Body (Figs. 1, 8) robust, widest at posterior margin of dorsal cephalic shield; anterior margin rounded. Cephalosome representing c.30% of total body length. Rostrum conspicuous, distally bifurcate (Fig. 2). Cephalosome with large mid-ventral, tongue-shaped post-oral plate protruding posteriorly; plate unarmed, separating cephalic appendages from first swimming legs (Figs. 8, 13, 14). Brood-pouch arising from dorsal surface of fourth pedigerous somite, extending posteriorly over urosome, well beyond tips of caudal rami; urosome lying in mid-ventral groove along underside of brood-pouch (Figs. 8, 13), not visible in dorsal view. Brood-pouch (Fig. 13) with average length 0.72 ± 0.075 mm (n = 5). Urosome consisting of 5 somites: fifth pedigerous somite, subquadrate genital double-somite (not projecting ventrally), 2 free abdominal somites and short anal somite carrying subrectangular caudal rami. Caudal rami (Fig. 13) subrectangular, longer than anal somite; each ramus with single lateral seta, slender dorsal seta and 2 terminal setae; innermost seta robust, branched proximally, with short basal inner process; seta about as long as egg-pouch, twice as long as urosome.
Figs. 1–7

Sabellacheres antarcticus n. sp., ovigerous female. 1. habitus, dorsal view; 2. rostrum, ventral view; 3. antennule; 4. mandible; 5. maxillule; 6. antenna; 7. maxilla

Figs. 8–12

Sabellacheres antarcticus n. sp., ovigerous female. 8. habitus, lateral view (paratype specimen); 9. first leg; 10. second leg; 11. third leg; 12. fourth leg

Figs. 13–17

Sabellacheres antarcticus n. sp., ovigerous female, SEM-prepared specimen. 13. habitus, ventral view; 14. anterior part of cephalosome, ventral view; 15. distal segment of antenna, pilose cuticular field arrowed; 16. maxillae (mx); 17. maxillule (mxl)

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.

Maxillule (Figs. 5, 17) represented by short lobe with 3 short terminal elements of about equal size; 1 naked and 2 pinnate elements.

Maxilla (Figs. 7, 16) reduced, represented by lobe with flattened ventral surface, heavily sclerotised.

Maxilliped absent.

Legs 1 to 3 each with broad and narrow intercoxal sclerite; coxa and basis short; biramous with both rami 3-segmented, most endopodal segments ornamented with setules along outer margin (Figs. 911). Spine and setal formula of swimming legs 1–3 as follows:
 

Coxa

Basis

Exopod

Endopod

Leg 1

0-0

1-0

I-0; I-1; I,I,4

0-1;0-1; I,1,3

Leg 2

0-0

1-0

I-0; I-1; I,I+2,4

0-1; 0-2; I,2,2

Leg 3

0-0

1-0

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.

Male (Figs. 1832). Total length of allotype 1.15 mm; average length of males 1.15 mm ± 0.03 mm (n = 3). Body (Figs. 18, 19, 25) robust, often flexed; prosome widest at level of pedigerous somites 2–3. Urosome not clearly delimited from prosome, 5-segmented, comprising narrow fifth pedigerous somite, robust genital somite and 3 free abdominal somites. Genital somite expanded dorsally, with very large paired genital openings; mid-ventral part of genital somite protuberant, forming subtriangular ventral process (Figs. 20, 21, 29, 30). Genital somite with sixth legs forming large, bilobate, genital opercular plates, closing off paired genital apertures (Figs. 29, 31). Inner lobe of sixth leg bearing single spiniform element with distal spinulation; outer lobe with single apical seta. Caudal rami as in female; innermost seta proximally branched (Fig. 32), c.3.5 times as long as urosome.
Figs. 18–24

Sabellacheres antarcticus n. sp., adult male allotype. 18. part of cephalothorax and urosome, dorsal view; 19. same, lateral view; 20. urosome, lateral view; 21. genital area, ventral view; 22. antenna; 23. right maxilliped; 24. left maxilliped

Figs. 25–28

Sabellacheres antarcticus n. sp., adult male, SEM-prepared specimen. 25. habitus, ventral view showing antennules (A1) and antennae (A2); 26. cephalic part, showing antennules and antennae, semi-lateral view; 27. distal segment of antenna, ventral view; 28. maxillule

Figs. 29–32

Sabellacheres antarcticus n. sp., adult male, SEM-prepared specimen. 29. genital area and sixth legs, ventral view; 30. urosome and caudal rami, ventral view; 31. fourth and sixth legs, ventral view; 32. detail of branched proximal part of inner caudal seta

Antennules and antennae as in female (Figs. 22, 27). Mouthparts including mandibles, maxillules (Fig. 28) and maxillae as in female.

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.

Remarks

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.

Discussion

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

Females
  1. 1A

    Third swimming legs not separated from legs 1 and 2……… 2

     
  2. B

    Third swimming legs widely separated from legs 1 and 2……… S. gracilis

     
  3. 2A

    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

     
  4. B

    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.

     
  5. 3A

    Antero-distal process of second antennular segment elongate, reaching distal end of fourth segment……… 4

     
  6. B

    Antero-distal process of second antennular segment reaching distal end of third segment………S. illgi

     
  7. 4A

    Anal somite shorter than preceding somite……… S. aenigmatopygus

     
  8. B

    Anal somite longer than preceding somite……… S. drachi

     
Males
  1. 1A

    Maxillule with three terminal elements of equal length………2

     
  2. B

    Maxillule with middle terminal element longer than other two……..….S. gracilis

     
  3. 2A

    Maxilliped robust, with terminal claw as long as second segment of appendage or shorter……… 3

     
  4. B

    Maxilliped relatively weak, slender, with terminal claw twice as long as second segment of appendage………S. antarcticus n. sp.

     
  5. 3A

    Genital somite with slender median process extending beyond tip of sixth legs………S. aenigmatopygus

     
  6. B

    Genital somite with vestigial median process located between bases of sixth legs……… S. illgi

     

Notes

Acknowledgements

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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Copyright information

© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  • Eduardo Suárez-Morales
    • 1
  • Geoffrey A. Boxshall
    • 2
  1. 1.El Colegio de la Frontera Sur (ECOSUR), Unidad ChetumalChetumalMexico
  2. 2.Department of ZoologyNatural History MuseumLondonUK

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