Ophiacantha densispina Mortensen 1936

“Ophiacantha densispina, n.sp.

(Plate VII, fig. 1)

St. WS 99. 19. iv. 27. 49° 42' S, 59° 14' W, Falkland Islands, 251-255 m. 1 specimen.

St. WS 248. 20. vii. 28. 52° 40' S, 58° 30' W, Falkland Islands, 210-242 m. 1 specimen.

St. WS 825. 29. i. 32. 50° 50' S, 57° 15' W, Falkland Islands, 135-144 m. 2 specimens.

St. WS 840. 6. ii. 32. 53° 52' S, 61° 49' W, Falkland Islands, 368-463 m. 3 specimens.

The type specimen (from St. WS 248) is a large, coarse specimen, 16 mm. in diameter of disk. The arms are all broken, but judging from their size they must have been at least about five to six times the diameter of the disk; they are somewhat flattened, 3.5 mm. broad at the base. The other specimens are smaller and very poorly preserved.

The disk is covered, both on the dorsal and ventral side, by a rather dense coat of coarse spines, 1-1.5 mm. long; they are smooth, simply pointed in the type specimen, in other specimens more blunt. At most the distalmost part of the radial shields is naked, but there may be a rather distinct naked line proceeding inwards in continuation of the radial shields. There are three to four rather long, slender mouth papillae on each side of the jaw, the outermost not, or only slightly, enlarged, and there are no extra papillae on the distal part of the jaws. The infradental papilla is not at all enlarged, rather smaller than the other papillae, but there are in the type specimen a few small extra papillae placed irregularly at the apex of the jaw. The buccal shields are of a very characteristic shape, almost rectangular, only slightly broader within than without; the adoral plates are small, almost square, sometimes with a more or less conspicuous outward prolongation which separates the buccal shield from the first lateral plate. The ventral plates, which are almost contiguous, have an almost straight proximal edge, the distal edge being convex; they are usually somewhat thickened and elevated in the distal part. A single large, leaf-shaped, more or less pointed tentacle scale. The dorsal plates are almost rhombical, almost contiguous proximally. Arm spines eleven to twelve in the proximal part of the arm, joining in the dorsal median line so as almost to conceal the dorsal plates. They are cylindrical, pointed, rather thorny in the basal part; they increase very gradually in length upwards, the uppermost ones being as long as five to six arm joints. One of the specimens from St. WS 840 is of a dark brown colour, the other specimens whitish, evidently bleached.

This species is viviparous, but appears to have separate sexes. The larger specimens (excepting the type and the specimen from St. WS 99, which are dried and could not be examined as to their sexual character) are males; only a small one, 5 mm. in diameter, from St. WS 825, is a female with young ones in the bursae (I found one large and one very small young one in the same bursa).

An interesting anatomical feature is found in this species, viz. that the two bursae at each arm have coalesced above the dorsal side of the arm. They are heavily plated, some of the plates prolonged into irregular spines, turning into the body cavity — to my knowledge a unique feature (Fig. 6 c).

There is undeniably much resemblance between this species and Ophiacantha rosea, Lyman, taken by the 'Challenger', also off' Patagonia (St. 308). It is, however, beyond doubt that they are not identical, the main differences being found in the covering of the disk (small, short stumps of O. rosea) and in the mouth papillae, O. rosea having a cluster of distal papillae. Further, the bursae are not coalesced in O. rosea, as I can state, having examined a cotype of that species. It appears that O. rosea is not viviparous.

On examining the specimens of O. rosea in the British Museum I found that one from 'Challenger' St. 308 and two specimens from Tom Bay, Patagonia, are infested with Myzostoma, particularly at the bursal slits, one being even wholly within the bursa.”

(Mortensen, 1936; 249-251)