A Comparison of Morphological Characters Within the Genus Rhomboarctus (Tardigrada: Heterotardigrada) with the Description of Two New Species

Zool. Anz. 242 (2003): 83–96 © by Urban & Fischer Verlag http://www.urbanfischer.de/journals/zoolanz

A Comparison of Morphological Characters Within the Genus Rhomboarctus (Tardigrada: Heterotardigrada) with the Description of Two New Species Jesper Guldberg HANSEN1, Maria GALLO D’ADDABBO2 and Susanna DE ZIO GRIMALDI2 1 2

Zoological Museum, University of Copenhagen, Copenhagen, Denmark Zoology Department, University of Bari, Bari, Italy

Abstract. Two new species of marine arthrotardigrades: Rhomboarctus aslaki sp. n. and Rhomboarctus duplicicaudatus sp. n. (Tardigrada: Halechiniscidae) belonging to the genus Rhomboarctus Renaud-Mornant, 1984 are described from the North Atlantic Sea and the Mediterranean Sea, respectively. New features for the formerly monotypic genus are presented and a complete description of the buccal apparatus is given for the first time. Key words. Arthrotardigrada, Faroe Bank, Mediterranean Sea morphology.

1. INTRODUCTION The knowledge of diversity, ecology and geographical distribution of marine tardigrades has been greatly improved during the last two decades, mainly by virtue of comprehensive studies carried out in the Mediterranean Sea, DE ZIO GRIMALDI & GALLO D’ADDABBO (2001) and of the Faroe Bank in the North Atlantic, HANSEN et al. (2001). Regarding marine tardigrades, these two areas are undoubtedly the most extensively studied in recent time, with more than 95 species recorded altogether. In 1984, Renaud-Mornant described the new species Rhomboarctus thomassini Renaud-Mornant, 1984, a Styraconyxinae closely related to the genus Raiarctus Renaud-Mornant, 1981, found in organogenic sand at 770 m depth in the Mozambique Channel. Since then, new species of the genus Rhomboarctus have been collected both in the Mediterranean Sea and in the North Atlantic. The Mediterranean specimens were found in 1985 in a sample of sediment taken between 20 m depth and 40 m depth in the Orosei Gulf (Sardinia, Tyrrhenian Sea) and recorded as Rhomboarctus thomassini. In the North Atlantic Sea, two specimens of Rhomboarctus were found in a sample of sediment collected at 260 m depth at the Faroe Bank, in 1990. In the period 1992–2001, more than 20 additional specimens have been collected at several localities at the Faroe Bank. A recent examination of both the Mediter-

ranean specimens and the North Atlantic specimens by the authors of this paper led to the recognition of important taxonomic elements that justify the establishment of two new species of Rhomboarctus.

2. MATERIAL AND METHODS 2.1. The North Atlantic Sea The new species from the Faroe Bank was encountered during expeditions in 1990, 1992, 1998 and 2001. BIOFAR Station 627 was collected from the German R/V Valdivia (1990), BIOFAR Stations 785 and 786 were collected from the Faroese Coast Guard vessel Olavur Halgi (1992) and BIOFAR Stations 2011 and 2019 were collected from R/V Magnus Heinason of the Faroese Fishery Investigations (1998 and 2001). Details on the sampling-localities are provided in NØRREVANG et al. (1994). To release the tardigrades from the sediment, a number of sub-samples at each station were fresh-water shocked, decanted through a 32 µm mesh net and fixed in 4% buffered formaldehyde for later sorting. A total of 22 specimens were mounted on micro slides in glycerol and studied using phase-contrast and Nomarskitechnique. Two specimens were prepared for scanning electron microscopy (SEM; Figs. 5 and 6). They were dehydrated in a graded series of ethanol and acetone prior to critical point drying. The dehydrated specimens were then mounted on aluminium stubs, coated with gold and observed in a JEOL JSM-840 scanning electron microscope. The typematerial is deposited in the collections of the Zoological Museum, Copenhagen, Denmark. 0044-5231/03/242/01-083 $ 15.00/0

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2.2. The Mediterranean Sea Samples were collected in September 1985 from Orosei Gulf (Sardinia). From each sample a sub-sample was fresh-water shocked, decanted through a 42 µm mesh net and fixed in 7% buffered formaldehyde. The animals were stained with Rose Bengal, mounted on micro slides using Hoyer’s fluid and examined by phase contrast and interference contrast microscopy. The type-material is deposited in the Department of Zoology, University of Bari, Italy.

3. RESULTS 3.1. Taxonomic account Family Halechiniscidae Thulin, 1928 Diagnosis: Arthrotardigrades without sclerotized dorsal segmental plates. The complete set of cephalic appendages in halechiniscids is 11, but the secondary clavae may be indistinguishable (9) or a pair of tertiary clavae may be present (13). Each adult leg with four digits with claws. The claws simple or with accessory hooks, spurs or thin bristles. Peduncles can be present on all digits or only on external digits. Two cuticular seminal receptacles always present in adult females. Subfamily Styraconyxinae Kristensen & RenaudMornant, 1983 Diagnosis: Halechiniscidae with peduncles always present, either on all digits or only on the first and fourth ones; heart-shaped proximal pads sometimes present at the base of second and third digits. Crescent shaped claws with one, two or no accessory points according to the genera. Complete set of halechiniscid cephalic cirri always present. Secondary clavae sometimes not visible. Primary clavae and fourth leg papilla often similar. Cuticular plates absent. Genus Rhomboarctus Renaud-Mornant, 1984 Emended diagnosis: Styraconyxinae with convex frontal edge; indistinct or lenticular secondary clavae; dorsal cuticle punctuated; cuticular fins supported by medium size pillars, extending on both sides of the body, between primary clavae and the fourth pair of legs. More or less evident caudal appendage with cuticular fan with pillars. Peduncles present only on the external digits, proximal pads on the internal ones; all the claws with very thin or vestigial secondary hooks. Type species: Rhomboarctus thomassini Renaud-Mornant, 1984 Rhomboarctus aslaki sp. n. (Figs. 1–6 and Tab. 1) Diagnosis: Rhomboarctus with large lenticular secondary clavae; primary clava asymmetrical shaped;

small dorsal cephalic fin with pillars; small, but welldeveloped caudal appendage with the dorsal cuticle supported by extended pillars and a lateral surrounding cuticular fan supported by long pillars. Regular intersegmental membrane-shaped folds with medium size pillars in the dorsal cuticle. Flagellar portion of cephalic cirri and the first leg’s cirri terminate abruptly, with a distal cuticular ring surrounding a large pore; second and third cirri are spine-like, without recognizable segmentation. All four legs with a v-shaped strengthening bar inside tarsus. Female with two protruding ventrolateral genital tubes, on which ducts of the seminal receptacles open. Seminal receptacles consist of small vesicles with short and straight ducts. Type material: The holotype is an adult female (137 µm long) with well-developed seminal receptacles; it was collected by R. M. Kristensen on 1 April, 1992 from carbonate sand with basalt pebbles at 249 m depth on the edge of the Faroe Bank near the Faroe Islands at BIOFAR station 785 (61°17.78′ N; 08°32.25′ W). Paratypes include 22 females and 1 twoclawed larva; 2 females were collected at BIOFAR station 627 (61°17.66′ N; 08°32.25′ W) on 18 April 1990 at 260 m depth, the larva and 16 adult females were collected at the same BIOFAR station 785 as the holotype, 1 female was collected at BIOFAR station 786 (61°12.33′ N; 08°28.23′ W) on 1 April 1990 at 148 m depth, 1 female was collected at BIOFAR station 2011 (61°12.0′ N; 08°31 W) on 18 Sep 1998 at 200 m depth, and 2 females were collected at BIOFAR station 2019 (61°12.21′ N; 08°29.20′ W) on 1 July 2001 at 138.5 m depth. Type locality: The Faroe Bank (BIOFAR station 785: 61°17.78′ N; 08°32.25′ W), which is situated southwest of the Faroe Islands in the North Atlantic Sea, in fine shell sand between 138.5 m and 260 m depth. Etymology: The name of this species epithet refers to a very strong Nordic Viking who single-handedly saved the lives of his men when they were attacked by enemies at sea in the North Atlantic. It is also the name of a dear friend and fellow tardigradologist, Aslak Jørgensen. The new species is dedicated to him, for his invaluable help and support. Holotype: The holotypic female (Fig. 1, ventral and Fig. 2, dorsal) is 137 µm long excluding the caudal appendage, and has a maximum width of 52 µm, excluding the lateral fins. The latter, a ventro-lateral cuticular fin supported by a row of approximately 55 pillars, is present on each side of the body and extends between the primary clava and the fourth leg. The pillars measure 1 µm in the anterior end, and 7 µm in the

Two New Species of the Genus Rhomboarctus

Fig. 1. Rhomboarctus aslaki sp. n.: adult female, ventral view. Drawing by Stine B. Elle.

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Fig. 3. Rhomboarctus aslaki sp. n.: caudal appendage, dorsal view. Drawing by Stine B. Elle.

Fig. 2. Rhomboarctus aslaki sp. n.: adult female, dorsal view. Drawing by Stine B. Elle.

Fig. 4. DIC-micrographs (Nomarski technique) of Rhomboarctus aslaki sp. n. A. Close up of the head and bucco-pharyngeal apparatus before whole-mount preparation. B. Close up on the right fourth leg, p4-sense organ and the protruding genital duct. The van der Land´s body is seen inside the cirrophorus of p4. C. Area between fourth legs, ventral view showing female gonopore. D. Seminal receptacle, with vesicle, duct and protruding part of the duct. E. Posterior end of an adult female, dorsal view showing the p4-sense organ, caudal appendage and cirrus E. The sensillum is visible inside the papilla of p4. Note the hexagonal pattern of the dorsal cuticle. bc, buccal canal, cap, caudal appendage, cE, cirrus E, ec, external cirrus, go, gonopore, gp, protruding part of seminal duct, ic, internal cirrus, lc, lateral cirrus, le4, leg IV, p4, sense organs of leg IV, pb, pharyngeal bulb, pl, placoid, sd, seminal duct, se, sensillum, st, stylet, su, stylet support, sv, seminal vesicle, vb, van der Land´s body.

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Fig. 5. SEM of Rhomboarctus aslaki sp. n. A, C, E. Sense organs of leg 1-leg 3. B, D, F. Digits and claws of leg 1-leg 3. The tips of the primary hooks and secondary hooks are not fully retracted in the claw sheaths. ed, external digit, id, internal digit, le1–3, leg I-III, p1–3, sense organs of leg I-III, ph, primary hook, sh, secondary hook.

Two New Species of the Genus Rhomboarctus

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Fig. 6. SEM of Rhomboarctus aslaki sp. n. A. Ventral view. B. Right external cirrus, ventral view. C. Digits, claws and peduncles of leg 4. D. Sense organs of leg 4. E. Accordion-shaped basal part of cirrus E. cap, caudal appendage, cE, cirrus E, ec, external cirrus, ed, external digit, id, internal digit, lc, lateral cirrus, le4, leg IV, mo, mouth cone, p4, sense organs of leg IV, pc, primary clava, pe4, peduncle of leg IV, ph, primary hook.

2

95 35

100–137 43–57 10–14 3–5 18–24 12–15 17–24 7–12 10–16 14–19 23–32 6–11 8–10 21–25 6–10 8–12 9–14 3–4 3–4 9 9 8 8 8 8 7 9 9 9 9 9 8 9 9 9 9 9 9 13,9 4,3 1,5 0,8 1,9 1,1 2,2 1,4 1,7 1,7 3,0 1,6 0,7 1,5 1,2 1,3 1,5 0,3 0,3 117,8 48,1 11,0 3,1 18,7 12,0 15,6 10,2 13,8 16,6 28,9 9,3 8,2 23,1 7,9 9,4 10,8 3,9 3,1 137 52 14 3 24 15 24 12 16 19 32 9 10 24 9 12 14 4 3 Body length Body width Caudal appendage Heigth of ceph. fin Buccal tube Pharyngeal bulb Stylet Median cirrus Internal cirrus External cirrus Lateral cirrus Primary clava Secondary clava Cirrus E 1st leg sense organ 2nd leg sense organ 3rd leg sense organ 4th leg papilla 4th leg papilla spine

136 47 13 5 23 14 20 10 15 18 31 10 10 25 8 10 12 4 3 110 46 14 4 – – – 11 14 17 30 11 10 23 8 10 11 4 3 122 50 11 – 20 12 17 10 13 18 29 9 9 23 10 10 10 4 3 100 44 13 4 20 14 20 10 13 14 25 6 8 21 6 8 10 4 3 122 47 – 3 21 13 – 11 14 16 29 10 9 24 8 9 10 4 4 100 43 10 – 18 14 19 7 10 14 23 8 – 22 7 8 10 3 3 109 47 11 4 20 12 19 10 14 16 30 10 9 21 7 8 9 4 3

2019– 1.003

124 57 13 5 22 14 21 11 15 17 31 11 9 25 8 10 11 4 3

785.051A 785.093A 785.107A 785.108A 785.119A 785.129A 2011– 4.004 BIOFAR Stations

2019– 1.005

Holotype Paratype Paratype Paratype Paratype Paratype Paratype Paratype Paratype Female Female Female Female Female Female Female Female Female Measurements in µm

Tab. 1. Morphometric summary of the specimens of Rhomboarctus aslaki sp. n. examined and measured in LM.

Mean

St.dev. n

Range

785.006A

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Paratype Larva

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posterior end. A dorsal cuticular fin, supported by a row of short pillars, extends between the cirrophores of both primary clavae and lateral cirri. The height of the pillars ranges from 1 µm at the extremities of the fin to 3 µm close to the middle. In the middle, the fin describes a downward curvature where the median cirrus arises. The procuticular caudal appendage (Fig. 3), which arises dorsally, is encircled by a lateral epicuticular fin. The latter is supported by 15 long pillars, ranging in length from 1 µm to 5 µm. The dorsal cuticle of the appendage is supported by at least 15 basal pillars, ranging in length from 0.5 µm to 3 µm. Both the lateral and dorsal pillars are shortest at the base of the appendage. Inside the caudal appendage, the procuticle forms a cone, upon which four lateral pillars and, at least, nine dorsal pillars, stands. The internal cone continues in a narrow cylinder with six ledges upon which six dorsal pillars and 11 lateral pillars stand. The entire dorsal cuticle has evident small pillars arranged in a hexagonal pattern. Inter-segmental epicuticular folds of the dorsal cuticle are present with transverse rows of both thicker and longer pillars. In the posterior end, a small fold is overlaps the base of the caudal appendage with a single row of short pillars. The cephalic region is convex, and the head is much broader (48 µm) than it is long (24 µm). A complete set of well-developed cephalic sense organs is present. The primary clava is asymmetrical in shape. In ventral view it appears like a golf club (9 µm × 4 µm), but in dorsal view it is more oval (9 µm × 2,5 µm). It has a small terminal pore and a van der Land’s body inside its base. Primary clava and lateral cirrus arise on the same cirrophore (7 µm), and a thin common membrane (extended margin of cirrophore) surrounds the base of primary clava and lateral cirrus. The lateral cirrus, which is anterior to the clava, consists of scapus (9 µm) and a tubular flagellum (23 µm). Dorsally, be-

Two New Species of the Genus Rhomboarctus

tween the internal and external cirri, two large and flat lenticular secondary clavae (10 µm × 5 µm) are present. The internal and external cirri arise from a short pedestal (indistinct cirrophore) and consist of a scapus, tubular part and a cut off flagellum. The internal cirri are inserted dorso-laterally, close to the frontal edge. Their scapus measures 7 µm and the two-parted flagellar portion 10 µm. The external cirri are positioned ventro-laterally (scapus: 7 µm; flagellum: 12 µm). The medial cirrus consists of an evident cirrophore (1 µm), scapus (6 µm) and a tubular flagellum (5 µm), and is inserted dorsally, 5 µm from the frontal edge. Each scapus of all the cephalic cirri, except for the medial cirrus, is swollen in the distal end, making a strong collar around the telescopically inserted flagellar portion. The flagellar portion of the cephalic cirri and of first leg’s sense organs are not sharply pointed, but terminate abruptly with a distal cuticular ring, surrounding a large pore. The first leg sense organ, P1, consists of an evident cirrophore (0.5 µm), scapus (2.5 µm), and a tubular flagellum (6 µm). The P 2 and P3 are simpler, without any recognizable segmentation, resembling the shape of a curved spine. The sense organ on the fourth leg is a spherical papilla (4 µm) with a strong cirrophore (3 µm) and a thin terminal tube (3 µm), which has a strong basal collar. The terminal part of its internal sensory organ (the sensory cilium) is visible inside the papilla. The sensory structure passes through the van der Land’s body and reaches the terminal tube. The cirrus E has a strong cirrophore (3 µm), a weakly accordion-pleated scapus (3 µm), and a terminal flagellum (18 µm). In the cephalic region, three amoebocytes are embedded in the brain (for more information, consult JØRGENSEN et al. (2001)). These cells, variable in number from 3–5, were observed in all the examined specimens. Tarsus on all four legs is simple and cylindrical, with a slight expansion at the attachment of the digits. A distinct V-shaped strengthening bar is present inside the tarsus. Peduncles are only present inside the external digits, and are of the same type as Styraconyx nanoqsunguak Kristensen & Higgins, 1984, not reaching the claw bases. They have an enlarged medial part with two small lateral points, and two sharp ends. On the first three pairs of legs, the peduncles are 3 µm long, whereas on the fourth pair of legs they are longer (5 µm). The internal digits have thin sheath-like proximal pads, which are not heart-shaped. Digits on the fourth pair of legs are longer than the digits on the first three pairs of legs. All the claws have vestigial secondary hooks, which are stronger on the claws of the external digits than on the claws of the internal digits. The claws are slightly sheathed, and the tip of the primary hook is free, even when the claw is retracted.

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The mouth opening is sub-terminal on a distinct mouth cone. The terminal part of the mouth cone has a large outer ring, which can be protruded. The mouth opening is surrounded by six buccal lamellae. The pharyngeal apparatus (Fig. 4A) is complete, consisting of buccal canal (24 µm), pharyngeal bulb (15 µm), stylets (24 µm) and large stylet supports. The buccal tube has two lateral asymmetrical apophyses and a larger ventral one. At the posterior end of the tube, three dropletshaped apophyses are inserted between the placoids. The stylet supports are quite robust, each having two strong apophyses; a proximal ball-shaped one reaching the buccal tube, and a strong medial one. A large muscle-complex (9 µm), almost circular in shape, is associated with the stylet supports anterior to the pharyngeal bulb. The reproductive system of the holotypic female consists of a single ovary bearing two small oocytes and one very large ovum. The ovary is 60 µm long and reaches the P2. The gonopore is circular and consists of a rosette with six cells. It is situated on a small elevation, 3 µm anterior to the anus. The genital ducts of the seminal receptacles protrude from the body ventro-laterally, like small tubes, each with one opening to the exterior. The genital duct of the seminal receptacles is short and straight, ending in a small dorso-lateral vesicle. The ducts lie almost parallel to the longitudinal direction of the animal, with the vesicles in an anterior position to the gonopore, and the opening of the genital tube in a posterior position to the gonopore. Larva: One two-clawed larva has been found. The larva is 95 µm long, excluding the caudal appendage, and has a maximum width of 35 µm, excluding the lateral fins. The pillars in the lateral fin are 4 µm long, whereas in the dorsal cephalic fin the pillars are 2 µm long. The larva has two digits on each leg. Peduncles are present, but not fully developed. A v-shaped strengthening bar is visible only in the tarsus of the fourth pair of legs. The claws, sensory organs and caudal appendage are as in the adults. Neither anus nor gonopore is present. Rhomboarctus duplicicaudatus sp. n. (Figs. 7–9 and Tab. 2) Diagnosis: Rhomboarctus with large lenticular secondary clavae; small dorsal cephalic fin with pillars; small caudal appendage with two overlapping cuticular fans with pillars. Cephalic and coxal cirri with a cut off flagellar portion and a terminal pore. In the females, seminal receptacles are small vesicles with sinuous ducts, which open ventrally on the sides of the body. Type material: Holotype: adult male 102 µm long and 36 µm wide; allotype: adult female 110 µm long and

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Fig. 7. Rhomboarctus duplicicaudatus sp. n.: Adult male, dorsal view. Drawing by Alberto Troccoli.

Two New Species of the Genus Rhomboarctus

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Fig. 8. Rhomboarctus duplicicaudatus sp. n. A. cephalic region with secondary clavae (sc) and dorsal cephalic fin (df). B. Female caudal region with caudal appendage (cap) and seminal receptacle and duct (sr). C. Caudal region with fourth leg sense organ (p4). D. Caudal region with cirrus E (cE). E. First stage larva. F. First stage larva: peduncles are visible on the two fourth leg digits.

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Fig. 9. Rhomboarctus duplicicaudatus sp. n. Adult female, caudal region, ventral view. Drawing by Alberto Troccoli.

46 µm wide; paratypes, two adult females, and three first stage larvae. Type locality: Orosei Gulf (Sardinia) 40° 20′ 30′′ N, 9° 40′ 15′′ E, in Tyrrhenian Sea, in sediment of a Posidonia meadow between 20 m and 40 m depth. Etymology: The name of this species refers to the appearance of its caudal appendage. Holotype: Adult male (Fig. 1) 102 µm long and 36 µm wide, excluding the caudal appendage and the lateral fins. The lateral fin, supported by a row of pillars 7 µm high, is present on each side of the body and extends between the primary clava and the fourth leg. The pillars are only very short (2 µm) at the two extremities of the fin. The cephalic region is convex; all the cirri arise from short pedestals and consist of scapus, tubular part and a cut off flagellum. The medial cirrus (10 µm) is 6 µm from the frontal edge; the internal dorsal cirri (12 µm) are almost at the same level as the medial cirrus, whereas the external ventral cirri (15µm) are very close to the frontal edge. Between the internal and external cirri, two large lenticular secondary clavae (12 × 5 µm) are present (Fig. 8 A). Primary clavae and lateral cirri arise on the same cirrophore (3.3 µm). The primary club shaped clava (8 × 3µm) has a small terminal pore and a van der Land’s body inside its base. The lateral cirrus, which is dorsal to the clava, is 27 µm long.

A dorsal cephalic fin, supported by a row of short pillars, extends dorsally between the cirrophores of both primary clavae and lateral cirri (Fig. 8A). The height of the pillars ranges between 1 µm, at the extremities of the fin, and 3 µm in its middle. They are 1.5 µm apart. There is also a very small caudal appendage with a terminal long narrow fin consisting of two overlapping fans supported by few pillars. The dorsal fan, with six pillars, is short (4 µm), whereas the ventral fan, with four pillars, is longer (10 µm) (Figs. 8B and 9). Two of the four digits on each leg are external and shorter and the other two are medial and longer. The external digits have peduncles that do not reach the claw bases. The peduncles are of the Styraconyx type and look like those of S. nanoqsunguak, with an enlarged medial part with two small lateral points, and two sharp ends. On the first three pairs of legs, they are 3 µm long, whereas on the fourth they are longer (5 µm). The two medial digits have a heart-shaped basal pad. All the claws have vestigial secondary hooks and are sheathed. The first leg sense organ, P1, consists of cirrophore (2 µm), scapus, (3 µm) and flagellum (6 µm); the P 2 and P3 are more simple, they have a scapus 2 µm long and a flagellar part which is, respectively, 7 and 8 µm long. The fourth leg sense organ is a papilla (8 × 5 µm) with a thin terminal tube (3 µm); the terminal part of its internal sensory organ is visible inside the papilla: it passes through the van der Land’s organ and reaches the terminal tube (Fig. 8C).

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Two New Species of the Genus Rhomboarctus

Tab. 2. Morphometric summary of the specimens of Rhomboarctus duplicicaudatus sp. n. examined and measured in LM. Measurements in µm

Holotype Male

Paratype Female

Paratype Female

Paratype Female

Paratype Larva

Paratype Larva

Paratype Larva

Body length Body width Caudal appendage Height of cephalic fin Buccal tube Pharyngeal bulb Stylet Median cirrus Internal cirrus External cirrus Lateral cirrus Primary clava Secondary clava Cirrus E 1st leg sense organ 2nd leg sense organ 3rd leg sense organ 4th leg papilla Distance anus-gonopore

102 43 10 3 26 15 33 12 14 17 27 9 13 × 7 21 11 9 10 7×3 3

110 46

117 34

136 56

80 37

96 40

90 38

3

3

3

2

2

2

13 15 19 31 10

13 15 19 39 10

8 13 18 26 8

8 12 18 26 8

20

22 13 10 9 7×3

20 7

20

16 20 35 9 16 × 4 22

6×2 10

7×3 7

The cirrus E has a basal accordion–pleated part (10 µm) and a terminal flagellum (8 µm) (Fig. 8D). The entire dorsal cuticle is strongly punctuated with evident small pillars more or less regularly arranged in parallel rows. Irregular metameric dorsal folds with larger pillars are present. The mouth opening is surrounded by a large suckerlike ring, which can be protruded. The pharyngeal apparatus is complete: it consists of buccal canal (26 µm), pharyngeal bulb (15 µm) and stylets (33 µm) with stylet sheaths (10 µm). Stylet supports are evident, but partly disintegrated due to the preparation and therefore not describable. The male gonopore is, as usual, circular, covered with a crescent fold. It is 3 µm anterior to the anus. Allotype: Adult female 110 µm long and 46 µm wide. The sexual dimorphism is limited to the morphology of the gonopore, which in the female is rosette-like (Fig. 9), and the seminal receptacles. The seminal receptacles are very small dorso-lateral vesicles with narrow and slightly sinuous ducts that open ventro-laterally on the body sides (Figs. 8B and 9). In the cephalic region, 8 lipoid droplets are irregularly arranged around the pharyngeal apparatus. These elements, according to Danish researchers BOESGAARD & KRISTENSEN (2001), HANSEN et al. (2001) and JØRGENSEN & KRISTENSEN (2001) are amoebocytes. These cells, variable in number from 8 to 20, were also observed in the other females.

8 14 17 27 7 8×1 18 7 6 7 6×2

6×2

6×3

Larvae (Figs. 8E and 8F): Only 3 first stage larvae have been found. Neither anus nor gonopore is present. They have two toes with very thin sinuous peduncles. The claws are as in the adults. The lateral fins have pillars 4 µm long, whereas in the dorsal cephalic fin the pillars are less than 2 µm long. No caudal appendage is visible. All sensory organs are as in the adults.

4. DISCUSSION R. aslaki and R. duplicicaudatus shares with R. thomassini the convex shape of the head, the presence of the lateral fins and a caudal appendage, the morphology of claws, digits and peduncles, and the morphology of the flagellar portion of the cephalic cirri. Thus, the two new species confirm the diagnostic characters for the genus, set by RENAUD-MORNANT in 1984. Interestingly, R. aslaki shares with R. duplicicaudatus several important characters that indicate that these two new species are more closely related to each other than they are to the type species, R. thomassini. The secondary clavae in R. aslaki and R. duplicicaudatus are large, lenticular and located dorsally between the internal and external cirri, whereas in R. thomassini they are very small ventral papillae. The primary clava and lateral cirrus arise on the same cirrophore in both R. aslaki and R. duplicicaudatus, whereas in R. thomassini the primary clava and lateral cirrus arise separately on a common

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pedestal. Both R. aslaki and R. duplicicaudatus have a dorsal cuticular fin with short pillars between the cirrophores of both primary clavae and lateral cirri. This fin is not present in R. thomassini. The seminal ducts of R. aslaki and R. duplicicaudatus open ventro-laterally on the body sides. In R. thomassini, the ducts open midventrally, close to the gonopore. Finally, the basal part of cirrus E is accordion-shaped in both R. aslaki and R. duplicicaudatus. This character, that is often difficult to see, is not described in R. thomassini. The most important differences between R. aslaki and R. duplicicaudatus are the arrangement of the cephalic cirri and the morphology of the leg sense organs, seminal receptacles and caudal appendage. In R. duplicicaudatus the position of the internal dorsal cirri is almost at the same level as the medial cirrus, whereas in both R. aslaki and R. thomassini the internal dorsal cirri are close to the frontal edge. The sense organs of the second and third leg of R. duplicicaudatus consist of a scapus and a cut off flagellum. In R. aslaki, these sense organs are simpler, without any recognizable segmentation, resembling the shape of curved spines. The seminal ducts of R. aslaki are short and straight and protrude from the body ventro-laterally like small tubes on the body sides. In R. duplicicaudatus the ducts are longer and slightly sinuous and do not protrude from the body. The shape of the caudal process appears to be a valid character differentiating the present species of Rhomboarctus, considering that R. aslaki was collected in four different years, in April, July and September, without showing any crucial variation in the shape of the appendage. It is also interesting to note that R. aslaki and R. duplicicaudatus demonstrate that the genus Rhomboarctus shares certain characters, not only with Raiarctus as stated by Renaud-Mornant in 1984, but also with Styraconyx nanoqsunguak. In S. nanoqsunguak, Raiarctus variabilis D’Addabbo Gallo, Grimaldi De Zio & Morone De Lucia, 1986 and Rhomboarctus aslaki, the genital duct opens ventro-laterally on the body sides and protrudes from the body like a short tube or a papilla. Furthermore, in Rhomboarctus, as well as in Raiarctus, the morphology of peduncles and of the buccal apparatus also resembles those of S. nanoqsunguak (see RENAUD-MORNANT 1981, 1984; KRISTENSEN & HIGGINS, 1984), and it is likely that the genera Rhomboarctus and Raiarctus, both developed from S. nanoqsunguak. Acknowledgements. We sincerely acknowledge Dr. Alberto Troccoli’s work on the beautiful and accurate drawings of

R. duplicicaudatus, and Stine B. Elle, employed by the Carlsberg Foundation, is thanked for her beautiful drawings of R. aslaki. The crew on “R/V Magnus Heinason” is thanked for the help and assistance during the 2001 expedition. Aslak Jørgensen and Reinhardt Møbjerg Kristensen are warmly thanked for assistance with preparing specimens of R. aslaki for scanning electron microscopy.

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Author’s adress: Jesper Guldberg HANSEN, Zoological Museum, University of Copenhagen, Universitetsparken 15, DK Copenhagen Ø, Denmark; Tel.: ++45 3532 1116, Fax: ++45 3532 1010, e-mail: [email protected] Received: 02. 09. 2002 Reviewed: 09. 12. 2002 Accepted: 13. 01. 2003 Corresponding Editor: Reinhardt M. KRISTENSEN