Material Examined

Victoria University, Zoology Department, Cook Strait Collections

Coll. VUZ 24 (Station FOR) 41° 36′ 30″ S., 174° 57′ 30″ E., 14/1/56, 1800–1900 hrs., N2M at about 100 fms. over 540 fms.—1♂9 mm.

VUZ 25 (Station FOS) 41° 37′ S., 175° 2′ E., 14/1/56, 2140–2300 hrs., N2M at about 100 fms. over at least 300 fms.—6 ♂♂ 8–10 mm, 7 ♀♀ 7–15 mm, 1 unsexed 8 mm.

VUZ 26 (Station FUG) 41° 37′ S., 175° 4′ E., 14/1/56, 2315–0100 (15/1/56) hrs., BT at about 100 fms. over 150–300 fms.—1 ♂ 8 mm.

VUZ 27 (Station FUD) 41° 37′ S., 175° 5′ E., 15/1/56, 0125–0225 hrs., N2M at about 50 fms. over at least 100 fms.—1 ♀ 9 mm.

VUZ 28 (Station SAR) 41° 29′ S., 174° 57′ E., 15/1/56, 0415–0545 hrs. BT at about 130 fms. over at least 200 fms.—1 ♂ 7 mm.

VUZ 51 (Station DOP) 41° 35′ S., 174° 53′ E., 22/2/56, 1730–1830 hrs., N2M struck bottom at about 200–300 fms.—2 ♀♀ 8.5–9 mm.

VUZ 52 (Station DOR) 41° 35′ S., 174° 57′ E., 22/2/56, 2020–2130 hrs., N2M at about 100 fms. over at least 400 fms.—11 ♂♂ 8–14 ram, 9 ♂♂ 7–14 mm.

VUZ 53 (Station HUL) 41° 41′ S., 175° 17′ E., 23/2/56, 0100–0215 hrs., BT on bottom at about 250–350 fms.—1 ♂ 10 mm, 6 ♀ 7.5–8.5 mm.

VUZ 54 (Station GUL) 41° 39′ 30″ S., 175° 17′ E., 23/2/56, 0300–0450 hrs., BT on bottom between 50–200 fms.—1 ♀ 11 mm.

VUZ 57 (Station BOOS) 42° 1′ 30″ S., 174° 58′ E., 31/3/56, 0230–0540 hrs., N2M at about 300 fms. over at least 1,200 fms.—4 ♀♀ 8–10 mm.

VUZ 81 (Station KUB) 41° 45′ S., 175° E., 17/2/57, 0920–1100 hrs., BT at about 600 fms. over about 870 fms.—1 ♂ 11.5 mm.

VUZ 82 (Station JUG) 41° 42′ 30″ S., 175° 9′ E., 17/2/57, 1200–1355 hrs., BT at about 500 fms. over 550 fms.—2 ♀♀ 14 mm.

VUZ 83 (Station JUG) as preceding, 1430 hrs., BT struck bottom at about 550 fms.—1 damaged specimen.

VUZ 84 (Station CUK) 41° 47′ S., 175° 02′ E., 19/4/57, 1500–1700 hrs., N2M at about 400 fms. over about 800 fms.—5 ♂♂ 9–14 mm, 8 ♀♀ 9–14.5 mm, 2 damaged 10–15 mm.

VUZ 85 (Station CUK) as preceding, 1915–2400 hrs., N4M at about 600 fms. over about 800 fms.—1 ♂ 12 mm, 1 ♀ 8.5 mm.

VUZ 94 approx. 42° 2′ S., 175° 22′ E., 24–25/8/57, 1845-0115 hrs., N4M at 500–600 fms. over about 1,400 fms.—1 ♀ 14 mm.

VUZ 100 (Station FOJ) 41° 36′ S., 174° 44′ E., 29/8/57, 1315–1430 hrs., BT on bottom at about 380 fms.—1 ♀ 14 mm.

Chatham Islands 1954 Expedition (D.S.I.R.)

Station 7, Chatham Rise, 40° 42′ S., 179° 55′ E., 24/1/54, 1880 hrs., BT on bottom at 280 fms.—1 ♀ 9.5 mm.

Station 52, Chatham Rise, 44° 04′ S., 178° 04′ W., 10/2/54, 0632 hrs., BT on bottom at 260 fms.—1 ♂ 8 mm, 1 ♀ 8.5 mm.

Dominion Museum Collections

23/1/57, off Taiaroa Head, Otago, 45° 38′ S., 171° 12′ E., about 0200 hrs., trawl at about 150 fms. over 600 fms.—1 ♂ 7 mm, 2 ♀♀ 5–7 mm.

B.S. 204 (net 4), White Island Trench, Bay of Plenty, 37° 29′ S., 177° 17′ E., 25/2/57, N2M on 500 fms. wire, over about 600 fms. M.V. Alert—1 damaged, about 8 mm.

Portobello Marine Biological Station Collection, Dunedin

Station Alert 54.13A, N.E. of Otago Heads, Canyon A, 2/3/54, BT on bottom at about 300 fms.—4 ♀ 8.5–13.5 mm, 1 damaged 14 mm.

Station Alert 54.17 (as for 54.13A)—2 ♀ 9–11 mm.

Specific Diagnosis

A relatively small, fragile shrimp with long appendages and characteristically prominent laterally-directed eye stalks.

Rostrum short, acute, projecting slightly beyond the anterior margin of the carapace and with the dorsal margin weakly convex. Supraorbital crest armed with spine; cervical groove distinct, terminating near the prominent hepatic spine. Suprabranchial groove and ridge well marked.

Eyes large, black, cornea wider than stalk, no protuberance (ocular tubercle) on the latter. Antennular peduncular segments long and relatively slender, second and third together subequal to proximal, third a little longer than second. Third maxillipeds not greatly enlarged, subequal in length to 3rd pereiopods. Only one margin of the two distal segments of the 5th pereiopods fringed with setae.

Male with petasma as shown (Figs. 4, 5); processus ventralis long and armed distally with several small clumps of spiniform projections; lobus terminalis only a little shorter, with several terminal "crochets" (retractible hooks); lobus connectens short, triangular and with anterior face armed with closely packed crochets; lobus armatus curved, long and reaching to the lobus page 11

Figs. 1–5. Sergestes arcticus.—Fig. 1, carapace, lateral view (anterior organ of Pesta represented by broken circle) ♀. Fig. 2—Anterior part cephalothorax, lateral view (setae omitted) ♀ 10 mm. Fig. 3—immature left petasma, posterior view, ♂ 8 mm. Fig. 4—Right petasma. posterior view, ♂ carapace length 12 mm. Fig. 5—As preceding, anterior view.
Figs. 6–9. Sergestes japonicus.—Fig. 6—Carapace, lateral view, ♀ Fig. 7—Anterior part cephalothorax, lateral view (setae omitted), ♀ 28 mm. Fig. 8—Left petasma, posterior view, ♂ 21 mm. Fig. 9—As preceding, anterior view.

page 12connectens, with single row of crochets along inner margin; processus uncifer long, hooked and reaching to the base of the lobus armatus, no lobus inermis present.

Female with two curved teeth medially on coxa of 3rd pereiopod.

Carapace length of New Zealand specimens up to 15 mm, males tend to be smaller than females.

Branchial Formula

I have followed Burkenroad (1937b) in regarding the dorsal series of gills in Sergestes as arthrobranchs rather than pleurobranchs. Thus the branchial formula of the New Zealand specimens of S. arcticus is as follows:

l represents a branchial lamella (i.e., a rudimentary arthrobranch).

This formula agrees with the records, but not the terminology, for S. arcticus given by Smith (1882, 1884) and Hansen (1903, 1922) and also with Caiman's (1909: 280) formula for the genus Sergestes.

In the New Zealand specimens the anterior arthrobranch of the 3rd pereiopod is nearly twice the length of the anterior branch of the 4th. The posterior arthrobranch of the 3rd pereiopod, clearly not rudimentary, is larger than the posterior branch, and only a little shorter than the anterior branch, of the 4th pereiopod.

Colour in Life

S. arcticus in life is transparent with red chromatophores scattered over the body and appendages, these being mainly concentrated on the carapace and mouth parts. Within the carapace itself the black stomach and the bright scarlet organs of Pesta are especially prominent.

In addition to the above general description the following details have been observed from fresh specimens. The chromatophores are especially concentrated dorsally on the carapace and the first two abdominal segments, while the rest of the abdomen is almost colourless except for some patches of chromatophores on the ventral surface of the 6th segment and at the base of the tail fan. The cornea is jet black and the eye stalk bears a few scattered red chromatophores. Of the mouth parts the 2nd maxilliped is the most prominent, many chromatophores occur on all segments especially on the distal two. All the setae of the mouth parts are themselves bright red. The ventral surface of the cephalothorax is also red and there is usually one chromatophore on the coxopodite of each pereiopod. In the male the petasma is sprinkled with small chromatophores at the same concentration as on the pleopods. Of the organs of Pesta only the anterior pair, slightly dorsal and posterior to the base of the mandible, are clearly visible through the carapace. These appear as circular scarlet bodies, becoming opaque yellow after preservation. The more dorsal organs cannot be clearly seen through the musculature of the body, but appear as strongly pigmented scarlet areas within the carapace.

Previous more or less complete descriptions of the colour of this species have been given by Kemp (1910a), Hansen (1922: 63, Pl. I, figs. 1, 2) and Holthuis

Length of Antennal Flagellum

The flagellum is so seldom preserved unbroken that its length is not often recorded. A female specimen of S. arcticus with a carapace length of 14 mm had an undamaged flagellum of 170 mm, the characteristic crescentic bend occurred after 54 mm and every segment of the remaining 116 mm was armed with the pair of curved setae so often described in this genus. This example can be compared to Stebbing's (1905) statement that his S. gloriosus (S. prehensilis Bate), with a carapace length of 15 mm, had an antennal flagellum 163 mm in length.

Systematic Position

This New Zealand form shows all the features of the "S. arcticus group" of the subgenus Sergestes s.s.: 3rd maxillipeds subequal with 3rd pereiopods, the two distal segments of the 5th pereiopods setose on only one margin, 3rd segment of antennular peduncle shorter than 1st, processus uncifer terminally hooked. Within this group it can be fully distinguished by rostral and branchial features from the only other species described, the North Pacific S. similis. Though the petasma of S. similis is undescribed, that of our New Zealand form agrees in every detail with the highly characteristic organ of S. arcticus excellently illustrated by Hansen (1922) and others.

Distribution

This extremely well known and abundant Atlantic species has been recorded in all regions of the North Atlantic from the coasts of Norway and Greenland to the Mediterranean and the Canary Islands, while in the South Atlantic it has been taken off the French Congo, South Africa and Uruguay. However, in the Pacific it has been reported from only two localities, off Chile (Holthuis, 1952) and from Challenger Station 159, south of Australia (Hansen, 1903: 59). Though Gurney and Lebour (1940) state that an acanthosoma larva from Terra Nova Station 251 (54° 2′ S. 177° 0′ W, south of New Zealand) "seems to be indistinguishable from S. arcticus", adults of this species have not been recorded from the New Zealand area until now.

Sund (1920) found S. arcticus to be the commonest species of the genus in the North Atlantic and stated that it had virtually the same distribution in that ocean as the genus Pasiphaea. Also Ekman (1953) mentions that S. arcticus in the Atlantic is one of the few bathypelagic species which are truly cosmopolitan with regard to temperature. By this he means that the species is found in equatorial as well as in both north and south cold-water regions, though it might avoid waters in the highest latitudes.

S. arcticus has almost invariably been taken in Cook Strait in association with an abundant bathypelagic Pasiphaea, very similar to, but distinct from, the North Atlantic P. sivado (Risso).

Bathymetric Distribution

S. arcticus has been taken in nearly every bathypelagic haul made in Cook Strait between 50 and 600 fms. However, as all nets used have been of the "open" type, specimens could have been taken at any depth between the surface and that at which the net was being used. Only four collections (V.U.Z. 25, 52, 53, 84) contain sufficient numbers of specimens to warrant individual discussion. The two most interesting were both made shortly before midnight at about 100 fms., a total of 34 specimens being taken for 2½ hours work. A haul made shortly after 1 a.m. at about 300 fms. took 7 specimens, while another made at 400 fms. after 4 p.m. caught 15 specimens.

Thus it can be said that adult S. arcticus in Cook Strait can be found during the night from at least 50 fms. to about 300 fms. with an apparent concentration at about 100 fms., while during the day there may be another concentration at about 400 fms. Sund (1920) found that during the Michael Sars North Atlantic Expedition adults were not common above about 250 fms. though they had been taken up to 100 fathoms and even less. During work with "closing" nets on the Atlantis in the Western North Atlantic, Welsh and Chace (1938) took S. arcticus between 100 and 500 fms. and later were able to show (Waterman et al, 1939) that though the centre of average vertical distribution was a little above 300 fms. extensive diurnal vertical migration of at least 100 and as much as 200 fms. takes place, some specimens being in less than 100 fms. between about 8 p.m. and 4 a.m.

Size at Maturity and Partial Parasitic Castration

Male specimens with a carapace length of less than 9 mm invariably have a petasma with the various lobes and crochets incompletely developed. The immature petasma illustrated (Fig. 4) is from a specimen with a carapace length of 8 mm and represents an even earlier stage than that figured by Holthuis (1952). Those specimens with carapace lengths of 10 mm and more always have, with certain exceptions mentioned below, mature petasma. Thus males, and we must assume, females, usually mature at a carapace length between 9 and 11 mm.

Many specimens have small or large larval nematode worms loosely coiled in the posterodorsal portion of the cephalothorax immediately beneath the carapace. Collection VUZ 25 had one so parasitised out of 14, collection VUZ 52 had 6 parasitised out of 20 and collection VUZ 84 had 5 out of 15, making a total of 12 of these 49 specimens parasitised (approx. 24%). Specimens with such a nematode in the area of the gonads have the dorsal mid-line of the carapace characteristically weakly convex, and males, at least, show partial parasitic castration. Thus no parasitised male seen had a mature petasma, even specimens with carapace lengths of 14 mm had this organ small and still relatively undeveloped.