Leucon ( Crymoleucon ) rossi , a new species ( Crustacea : Cumacea : Leuconidae ) from the shelf waters of the Ross Sea ( Antarctica ) , with a key to the genus Leucon south of 60 ° S

A new leuconid cumacean, Leucon (Crymoleucon) rossi n. sp., is described from depths of 84 to 458 m in the Ross Sea off the Victoria Land coast. Leucon rossi n. sp. is distinguished from other members of the genus by a combination of characters including 1) a blunt, horizontally directed pseudorostrum without a distinctly protruding siphon; 2) strongly developed antennal notch; 3) entire dorsomedian margin of carapace appearing serrate, armed with 14 to 19 anteriorly curved spines in female (up to 21 in subadult males); 4) a small, but distinct, spine behind the frontal lobe; and 5) the uropodal peduncle slightly shorter than the exopod. After Leucon antarcticus Zimmer, 1907, L. rossi was the second most frequently occurring cumacean in the samples collected off Victoria Land. Statistical analyses showed significant differences in the proportion of carapace length and height of adult (all incubating in the present study) and immature females compared to immature males; no adult males were available for study.


INTRODUCTION
Leucon antarcticus, Zimmer 1907 is the only species of the genus Leucon Krøyer, 1846 currently reported from the Ross Sea (Jones, 1971). During the Victoria Land Transect Project onboard the Italian research vessel Italica in 2004, five additional species of the genus were obtained: Leucon assimilis Sars, 1887;Leucon intermedius Mühlenhardt-Siegel, 1996;Leucon parasiphonatus Mühlenhardt-Siegel, 1994; Leucon cf. sagitta Zimmer, 1907; and a new species, Leucon sp. A (Rehm et al., 2007), the description of which is the subject of this report.

MATERIAL AND METHODS
The material of Leucon rossi was collected during the 19th expedition of RV Italica to the Ross Sea. From February 9 to 22, 2004, 13 of 19 samples containing specimens of the species were collected in depths of 84 to 458 off the coast of Victoria Land with a modified Rauschert dredge (compare Rehm et al., 2006). Type material was collected at Terra Nova Bay and Cape Russell. For detailed data on the stations and further information on the species diversity and distribution see Rehm et al. (2007) in which Leucon sp. A refers to L. rossi.
Drawings were created from digital photographs using a digital drawing tablet as described by Coleman (2003Coleman ( , 2006. Measurements of body dimensions were statistically compared using the Mann-Whitney Rank Sum Test. Body length is measured from the tip of the pseudorostrum to the tip of the pleotelson. Carapace length is measured from the tip of the pseudorostrum to the posterior margin of the carapace, whereas carapace height is measured from the ventralmost to the dorsalmost margins. Length of articles is measured according to Mühlenhardt-Siegel (2005) and given as relative length of peduncle articles 1 to 3 of antenna 1 compared to total peduncle length (RLP). As the basis of appendages is part of the protopodite it is treated separately and compared with the endopodite (ischium to dactylus not including terminal seta) in the ratio B/R and given for maxillipeds and pereiopods. RLA refers to the relative length of each article of the endopodite (from ischium to dactylus) excluding terminal seta.
Type material has been deposited in the Zoological Museum Hamburg (ZMH) and in the Senckenberg Museum, Frankfurt (SMF). Additional paratypes have been deposited in the Museum of the University of Southern Mississippi Gulf Coast Research Laboratory (GCRL).
Description. Adult (incubating) female. Carapace (Fig. 1B), without setae, ridges or tubercles, but with single spine directly behind fron-  second free thoracic segments overlapped by the antero-lateral margin of the following segments. Pleon approximately as long as cephalothorax; pleonite 6 shorter than uropod peduncle. For dimensions of body see Table 1.
Antenna 1 (RLP 25/32/43) (Fig. 1A). Peduncle geniculate between basal and second article. First article with group of 3 sensory setae close to distal margin next to these sensory setae, and plumose seta on distal third of article, proximal half of article with several hair-like setae; second article with simple seta and sensory seta close to distal margin, group of 3 sensory setae on tubercle close to distal margin; third article with seta near distal end and two sensory setae at distal margin. Accessory flagellum uniarticulate slightly longer than article 1 of main flagellum, with 3 strong terminal setae each bearing a flagellum and sensory seta. Main flagellum with 3 articles; article 2 bearing seta and single aesthetasc; terminal article (article 3) about one fourth length of articles 2 and 1, with an aesthetasc, 2 simple setae, and 2 long terminal setae.
Antenna 2 (Fig. 1D). Diminutive; first peduncle article slightly shorter than wide, with 2 plumose setae; second article slightly shorter than wide, about one third of size of article 3, with simple seta; article 3 cylindrical, nearly equal in size of peduncle; terminal 3 sensory setae.
Mandibles ( Fig. 1F-H). Left mandible, lacinia mobilis and simple seta between molar and incisor process. Right mandible with single stout seta bearing denticles on inner margin distally and 2 simple setae between incisor und molar processes.
Maxilla 2 (Fig. 1C). Distal margin of protopod with row of plumose setae and long simple setae at outer distal edge, inner margin with hair-like setae. Outer lobe of endite with 4 stout setae, outer most plumose; inner lobe of endite with 4 stout setae. Maxilliped 1 (B/R 0.5; RLA -/25/33/26/17) ( Fig.  2A). Endite of basis with plumose setae at inner margin and distal end; 2 retinacula present; ischium not present; strongly developed plumose seta between basis and merus (probably inserting at basis) directed proximally turning 180° at about half length of endite of basis, slightly exceeding endite; merus with 2 plumose seta on inner edge of distal margin; numerous simple setae at and close to inner margin of carpus and propodus; carpus with large plumose seta on distal outer margin; propodus with 2 large plumose setae, one at distal end and one on outer margin; dactylus with single plumose seta distally.
Pereiopod 3 (B/R 1.7; RLA 17/19/37/20/7) (Fig.  4A). Basis, 2 simple and plumose annulated setae at distal third, simple seta at edge of distal margin; ischium, small simple seta close to distal margin, 2 strong annulated setae at edge of distal margin, one reaching tip of dactylus; merus, simple seta close to distal margin and strong annulated seta at edge of distal margin; carpus, small simple seta in proximal and distal parts, small simple seta, strong and annulated seta and blunt tooth-like structure at distal margin; propodus cylindrical with strong annulated seta at distal margin; dactylus, terminal small seta and strong annulated seta; exopod with spine at distal margin of basal article.
Uropod (Fig. 4C). Length of peduncle and endopod equal; inner margin of peduncle with 8-9 stout cuspidate setae, outer margin with 3-4 hair-like setae, simple seta at distal margin. Exopod 0.9 times length of endopod. Endopod two segmented, inner margin of basal article of endopod with 9 stout cuspidate setae, between distal 6-7 minute stout spines each, outer distal edge with simple seta. Inner margin of distal article with 4 stout cuspidate setae and 4 stout minute spines alternating; 2 terminal setae; outer margin of distal article with 2 small simple setae. Inner margin of exopod with 2 simple setae and longer seta, outer margin with 5 setae, terminal 2 long annulated setae and shorter seta.
Premature males. Second antenna incompletely developed and lacking articulation and setae, reaching posteriorly to free thoracic segments. Carapace (Fig. 4D) with dorsomedian margin bearing more denticles than in females (up to 21); denticles of ventral margin of pseudorostrum in premature males more distinct. Exopods developing on pereiopods 1 to 4 and on maxilliped 3; the spine at the distal margin of the basal article in females is followed by 1 or 2 additional spines in the premature males. Pleopod buds present on abdominal segments 1 and 2, with 3 or 4 minute setae distally. Zimmer, 1907 in the general shape of the carapace, which is slightly stouter in L. rossi. Both species have an uninterrupted row of dorsomedian denticles from the eyelobe to the posterior margin. They can be distinguished by the lateral spines on the carapace. A single spine only is situated dorsally directly behind the frontal lobe in L. rossi, whereas L. antarcticus is armed with a spine on the border of the frontal lobe, close to this spine is another spine on the rostral lobe. A third spine is located below the serrated ridge in a similar position as in L. rossi. In addition, 2 or more spines are located in the gastric region.

Remarks. Leucon (Crymoleucon) rossi n. sp. resembles L. (Crymoleucon) antarcticus
In the redescription of L. antarcticus by Ledoyer (1993) only 1 spine on the frontal lobe is shown in the drawing (compare key); in the text the dentition is not mentioned. The pseudorostrum of L. rossi is blunt and directed straight forward, while in L. antarcticus it is pointed and turned slightly upward. The uropod peduncle is slightly shorter than the exopod and equal to the endopod in L. rossi. It differs to the peduncle of L. antarcticus, which is shorter than both rami. The first antenna of Leucon rossi is geniculate between articles 1 and 2. Following the key presented by Watling (1991) the species should either belong to the genus Bytholeucon Watling, 1991or Pseudoleucon Zimmer, 1903. However, in Leucon rossi two pleopods occur in premature males (Bytholeucon only 0 or 1), the uropod endopod is somewhat longer than the exopod (Pseudoleucon much smaller), and the pseudorostrum is extending straight forward (Pseudoleucon upturned). The remaining characters indicate that the species belongs to the genus Leucon, subgenus Crymoleucon since the accessory flagellum of antenna 1 is longer than the first article of the main flagellum. The character states "antenna 1 not or weekly geniculate" and "antenna 1 geniculate between peduncle article 1 and 2", are unfavourable features for dividing the genera Nippoleucon Watling, 1991 and Leucon from Bytholeucon and Pseudoleucon. The key itself contains contradicting information, since in the pictures provided, which were taken from the original descriptions, the angles between peduncles one and two of the first antenna of the species Bytholeucon hiscens Bishop, 1981 and Leucon (Alytoleucon) medius Bishop, 1982 are about 90°. The first antenna of Pseudoleucon japonicus Gamô, 1964 is geniculate as indicated in the text of the original description. However, in the drawing it is straight, and an articulation is only indicated between peduncles 1 and 2 (Gamô, 1964). This demonstrates that this character state of the first antenna can be variable within a species and therefore it should be treated carefully.
Bionomy. The species was found along a latitudinal transect off the Victoria Land coast in depths ranging from 84 to 458 m. Depth distribution was limited by the depth sampled. Specimens were found in high numbers (in total 1090 specimens, from 2 to as many as 378 were found in 12 samples from 13 stations) and in all areas sampled. Adult specimens were restricted to incubating females and occurred in low numbers only (3%). Premature (12%) and juvenile (24%) females were found more frequently than males of the same stages (8% and 14% respectively), while mancas (39%) dominated the samples. The species is a typical component of the cumacean fauna of the Victoria Land coast of the Ross Sea and it appears to be endemic to the Ross Sea.
The statistical comparison between premature males and premature females showed significant differences (p<0.001) in all variables measured (Table  1), except carapace height (p = 0.681). The index of carapace height vs. carapace length was also compared with the incubating females; premature males and incubating females show significant differences (p<0.001), whereas differences in premature and incubating females were not significant (p<0.072).
Artificial key to the females of the genus Leucon from Antarctic waters south 60ºS