Dilatatiocauda bipartita ( Kim & Kim, 1997 ), 2002

Harris, V. A., 2002, A New Genus Belonging to the Family Porcellidiidae (Crustacea: Copepoda: Harpacticoida) With Three New Species from Australia, Records of the Australian Museum 54, pp. 1-24 : 18-22

Dilatatiocauda bipartita ( Kim & Kim, 1997)

Figs. 3, 6, 11, 12

Porcellidium bipartitum Kim & Kim, 1997 , 142–148, figs. 1–3.

Material examined. Specimens from Tei, Kochi Prefecture, Shikoku, Japan (33°32'N 133°45'E), (sample 951017) collected by N. Iwasaki: 30 adult ♀♀ (26 ovigerous) , 11 adult 33 (4 coupled with juvenile ♀), 6 33 + 4 ♀♀ copepodites, 88 nauplii. Specimens deposited at NSM Tokyo: ♀ Cr 13412, 3 Cr 13413, 2 ♀♀ Cr 13414; AM P58800, 1♀ ; BMNH 2000.1255 – 1256 , 1 ♀ , 13.

Diagnosis. Adult female: dorsal ornamentation of crescentic pits, no reticulate ridges or honeycomb structures, hyaline border granular; area of hair-like setules anterior to labrum, labrum with kidney-shaped area of denticulate setules in front of comb plates, comb plates with many ridges (>8); sternal plate of metasome segment 4 without fimbriate setules or ridges; caudal rami elongate, rectangular (length c. 3 × width), and setae not very close, seta about 1 3 way down ramus, terminal seta T1 pinnate, setae T2, T3 and T4 plain, terminal fringe of 16 triangular setules; antenna with seta on segment 1 of endopod, geniculate setae not longer than segment 2 of endopod; maxilliped coxa with fimbriate border; lateral denticulate area on P1 endopod small (1 5 width of endopod), small medial denticulate area present; P4 endopod segments 2 and 3 fused, no chitinous striations along anterior-lateral border of P5. Adult male: dorsal pits and hyaline border as for female; terminal border of caudal ramus with numerous fine setules; antennule coupling denticles not denticulate, dactylus with terminal hook and single tooth; P2 endopod with two plumose terminal setae; P4 endopod segments 2 and 3 fused; P5 almost rectangular, setae pinnate lanceolate without row of setules at base.

Dimensions. Females: mean length (anterior of rostrum to posterior extremity of urosome) 1.45 mm (SD 0.047, N = 16), mean width of cephalosome 0.94 mm (SD = 0.026, N = 16), body length to width ratio 1.54. Rostrum 0.25 mm, ratio of body width to rostrum 3.76. Urosome width to length ratio 1.44. Caudal ramus length to width ratio 2.86, dilation index 8.2. (A single large female measuring 1.65 mm long, 1.07 mm wide and rostrum 0.027 was washed from Chondrus giganteus at Iwaya Port, Awajishima, Hyogo Prefecture, Japan, sample 980525/6, N. Iwasaki). Males: mean length 0.98 mm (range 0.97–1.0 mm, 5 animals), mean width 0.79 mm, (range 0.78–0.81 mm, 5 animals).

Description. Adult female ( Fig. 11A). Pale yellow or colourless. Anterior of cephalosome semicircular, rostrum prominent (projects 1 3 of its width). Crescentic surface pits (2–3 µm diameter) near edge of cephalosome, on urosome, caudal rami and P5. Dorsal fold parallel to anterolateral edge of cephalosome ( Fig. 11D). Hyaline border appears granular (22 µm wide). A pad of filiform setules lies anterior to the muscles of the labrum ( Fig. 11C). A reniform (kidneyshaped) area of short denticulate setules lies anterior to comb plates on labrum, comb plates with more than 8 ridges. Sternal plate of metasome segment 4 without hair-like setules or ridges. Urosome broad ( Fig. 11J), almost semicircular. No notch or cleft but scar indicates boundary between anterior and posterior lobes. Fine setules on distal half of anterior lobe. Posterior lobe wide, medial corner tightly rounded, fine setules along border which pass round medial corner into caudal arch (Korean specimens lack border setules). Two sensory setae present on posterior lobe, but set in from border. Caudal arch deep (58% of urosome length). Anal segment with two dorsal setae ( Fig. 11J). Caudal rami ( Fig. 6E) elongate, rectangular, sides almost parallel, terminal border between T3 and medial corner convex, short setules along posterior half of lateral and medial edges. Alpha seta medial (Hicks’ index 62%, Korean animals 55%). Alpha and beta setae not very close (L/[] = 7). Terminal seta T1 pinnate, setae T2 & T3 very close, plain, T4 plain, set in from medial corner (1 5 ramus width, Fig. 11G), terminal fringe of 16 large triangular setules. Limbs typical of family. Antenna exopod with five plumulose setae and one spinous plumulose seta. Basis and segment 1 of antenna endopod with U-shaped row of triangular setules ( Fig. 12A), endopod segment 1 with small plain seta on anterior edge, endopod segment 2 with three lateral setae and six terminal setae, one plain, one long articulate plumulose seta (twice length of longest geniculate seta) and three plain geniculate setae which are shorter than segment 2, the claw is a long comb (as long as shortest geniculate seta). Posterior lobe of mandibular palp with wing-like hyaline expansions to base of the four bulbous setae (cf. Fig. 10E). Medial lobe of maxilliped coxa fimbriate ( Fig. 11F). Lateral triangular area of denticles on P1 endopod small (1 5 width of endopod), small area of denticles on medial edge of endopod ( Fig. 3E). Spinous seta on segment 2 of P3 endopod ( Fig. 12B) strong, serrate, almost as long as endopod (0.9:1), J-shaped spinous seta on segment 3 serrate, longer than endopod (1.3:1), straight spinous terminal seta serrulate, much longer than endopod (1.6:1), internal and external seta on endopod segment 3 short plumose ( Fig. 12C). P4 endopod segments 2 and 3 fused (i.e., endopod has only two segments, Fig. 12D), fused segment with three internal serrate spinous setae and long straight serrulate spinous seta. P5 basis with area of short setules on anterior edge ( Fig. 11H), fringe of larger setules along posterior edge, dorsal seta plain, two ventral setae (one small plain seta plus longer plumulose seta, Fig. 11I), exopod ovo-lanceolate, anterolateral border without setules or chitinous striations, rest of border with setules. Apical seta present plus one large and two small dorsal setae ( Fig. 11H). Females with 16 to 36 eggs in ovisac (mode 24, N = 18).

Adult male ( Fig. 11B). Colour and surface pits as for female. Anterior of cephalosome truncated, shoulders sharply rounded with epaulet, lateral angle of antennule socket projects and is visible form above ( Fig. 11E), rostrum narrow (0.075 mm), cephalosome width to rostrum ratio 10.6, rostrum does not project anteriorly. Crescentic pits near edge of cephalosome and on dorsal surface of caudal rami and P5s. Caudal ramus ( Fig. 12G) short (L/W = 0.9), lateral edge convex, dilation index = 45, fine setules along posterior half of medial and lateral edges, terminal fringe of numerous fine setules, setation as for female except for terminal border. Antennule ( Fig. 12I), anterior lobe and seta lie parallel to anterior edge of compound segment, seta projects laterally (not anteriorly). Three coupling denticles are present, they appear to be folded with a finely ridged or crenulated edge, there are two associated articulate plumose setae. Two long strap-like setae bordered with very short plumulae (marked * on Fig. 12I) are found on the ventral surface of the compound segment, one originates among the setae, the origin of the other is obscured by the proximal coupling denticle. Their appearance is unique and suggests that they may be sensory structures ( Tiemann (1977) illustrates a similar flat seta on his P. planum ). A chitinous ventral process or blade is present. The aesthetasc is short (not longer than the dactylus), it does not have a bulbous base. The dactylus has a large basal lobe, a tooth on the lateral edge and a strongly hooked apex ( Fig. 12H). Endopod of P2 with two plumose terminal setae ( Fig. 12F). P3 and P4 as for female, P4 with segments 2 and 3 fused. P5 ( Fig. 12E) almost rectangular, terminal setae pinnate lanceolate, long row of ventral setules (>20) associated with first (lateral) seta, no setule rows at base of other setae.

Remarks. Dilatatiocauda bipartita is unique among the Porcellidiidae in having only two segments to the endopod of the fourth peraeopod in both male and female animals, a fact indicated in the specific name. It is the largest species recorded for the family. Animals from Japan appear to be smaller than those from Korea. Kim & Kim (1997) only give a single measurement for length (♀ = 1.71 mm) without indicating either the range of variation or the mean value for the population. The mean length of Tei females is 1.45 mm (SD = 0.047, N = 16), but a single female from Awajima measured 1.65 mm in length.

Kim & Kim (1997) state that the second limb (P2) endopod of male animals terminates with 2 or 3 setae. Data is not given as to the frequency of the 3-seta condition in the population, nor is it stated which of the four setae found in females has been lost. All Japanese specimens have only two plumose terminal setae on the male P2 endopod and this must be considered the normal condition.

Dilatatiocauda bipartita shows a remarkable resemblance to the animal described by Tiemann (1977) as Porcellidium planum . The two animals share the following unique features: the antennule of males have two long straplike plumulose setae and similarly shaped dactylus; antennae have an anterior seta on endopod segment 1; the coxa of maxilliped is fimbriate and the area of denticulate pegs on P1 endopod is similar; the unusual shape and setation of P3 endopod and male P5 are virtually identical. However, the following significant differences show that the two species are distinct: P. planum has the seta on caudal ramus proximally situated and very close to (L/ = 35); the maxilliped basis has a fimbriate border; the terminal segment of male P2 has four setae (one spinous, three plumose); segments of P4 endopod are not fused (there are three segments). The significance of these differences will be considered in the discussion.

Distribution. Animals collected by N. Iwasaki at Tei, Kochi Prefecture were washed from Ecklonia cava .A single female was found in washings from Chondrus giganteus collected at Iwaya Port, Awajishima, Hyogo Prefecture. The species has also been collected from Ecklonia cava at Chojagasaki, Kanagawa Prefecture by Yuka (Tadasugi) Sasaki and at Hamajima, Mie Prefecture by Hiroshi Ito (specimens in possession of Yuka (Tadasugi) Sasaki examined by author). Despite extensive sampling at Kadonohama Bay, Ofunato, Iwate Prefecture, D. bipartita has not been found there. This suggests that it is a southern species distributed along the southern coast of Honshu, Shikoku and in the Korea Strait.