The Octopodous Mollusca of New Zealand.—V.
[Read before the Otago Branch, May 5, 1944; received by the Editor, May 15, 1944; issued separately, December, 1944.]
In my previous articles (Trans. Roy. Soc. N.Z., vols. lxxii, lxxiii, lxxiv) I have described species of Octopods belonging to the Family Octopodidae. I now put on record as occurring in our coastal waters certain other species belonging to other families of the great Order Octopoda. Although I have nothing to add to our knowledge of this octopod my excuse for cumbering the pages of the Transactions is the fact that the literature and figures of Tremoctopus such as those of Naef are not readily available to zoologists in the Dominion, and the only figure that is available is erroneous.
Tremoctopus violaceus Della Chiaje. Plate 40, Figs. 1–3.
This widely distributed species, originally described from the Mediterranean somewhere about the year 1830, has already been recorded from our seas by Suter (1913, p. 195) who obtained a specimen from near the Great Barrier Island; this was identified by Hoyle, who had worked on the Cephalopoda collected by H.M.S. Challenger. I have endeavoured to trace this specimen, but the directors of the museums consulted have not been able to find it in their respective collections in Auckland, Wellington, Christchurch or Wanganui.
The figure, purporting to represent this animal, given by Suter (Pl. 30, Fig. 4) is not only misleading but erroneous in some respects. For example the web or umbrella is represented as being produced into long points between the arms 1 and 2 on each side, whereas it is at these places that it is shallower than it is at the ends of the arms. I therefore give a drawing of the specimen examined by me. It is true that a good though small figure is given by Naef (Fig. 142, p. 741). But Suter was not the first to record the occurrence of Tremoctopus, for T. W. Kirk mentions it in 1883 where, before the Wellington Branch, he gave an abstract of a paper entitled “On a New Cuttle-fish, Tremoctopus robinsonianus” which in the next line he refers to as T. robsoni, of which he had received three individuals from Napier, two of which were males. But beyond mentioning the colour he gives no details. It was no doubt T. violaceus. At any rate the specific name given by him does not stand. He named it after Mr. C. H. Robson, of Mahia, who supplied him with other cephalopods.
The genus Tremoctopus belongs to the family Tremoctopidae, which is closely related to the Paper Nautilus, Argonauta; indeed by some zoologists—e.g., Suter, the genus is placed in the same family. But from Argonauta it differs not only in the fact that the female does not produce an external shell for the transport and protection of her eggs, but, secondly, in the possession of circular pores or apertures
Plate 40Fig. 1.—Tremoctopus violaceus Della Chiaje. Ventral view to show the characteristic features. The arms are numbered 1, 2, 3, 4, but the suckers are omitted; e, eye; f, funnel; ap., mantle aperture; wp., ventral water pores; u, umbrella or web. The dotted extension of arm 1 indicates the relative length of this arm in the young, but this region degenerates as the animal grows.
Fig. 2.—The arrangement of the suckers on Arm 2 (enlarged).
Fig. 3.—The arrangement of the suckers on Arms 3 and 4 (enlarged).
(Gk. tremata) at the base of the web both dorsally and ventrally which lead into chambers in the head.
The specimen hereunder described I owe to the courtesy of Dr. Oliver, who entrusted it to me for confirmation of his identification. In his letter he writes: “It was found floating in the sea at Crater Bay, White Island, on December 6, 1912. Apparently it had been killed by the water discharged from the Crater lake, which contains 5% of hydrochloric acid.”
The animal is a female. The body ovoid, widest at about the middle and bluntly pointed at the hinder end (Fig. 1). The colour after preservation in formalin for 30 years is still purplish-brown dorsally, which colour is continued on the arms and web (umbrella); the ventral surface is of a paler tone, as is the colour of the inner surface of the arms and web.
The length of the body from the apex to the level of the eyes is 75 mm., and its greatest breadth is 55 mm. This is about the average size of the species. The surface is smooth. The mantle aperture is wide (Robson's “C”). The arms formula is 2:1:4:3, though the last two are almost equal in length. A peculiarity about the pair of the most dorsal arms (No. 1) is, as has been described by Naef (p. 740, Figs. 442, 443), that during the growth of the animal each of these two arms loses more or less of its distal region by a sort of degeneration. When newly hatched and for some little time after, these two dorsal arms are longer than the second arms (2), but as the animal grows the distal. portion slowly rots away. From Naef's article it appears that the reason of this peculiarity is quite unknown, and no explanation has been suggested. In the present individual the length of the second pair (No. 2) is 175 mm., measured as usual from the mouth, hence the total length of the animal is 250 mm., about 10 inches. The dorsal arms (No. 1) are only 150 mm. though one is rather shorter than the other.
The inner surfaces of the arms present different appearances. On the two pairs of dorsal arms (Nos. 1 and 2) the suckers are quite small, except those quite close to the mouth, and are arranged along the edge of the inner face so that the two rows are wide apart and separated by a flat, smooth area (Fig. 2) which in arm 1 is 3 mm. wide and in arm 2 as much as 7 mm. (Naef figures them on p. 741, Fig. 442). In the other arms (3 and 4) the suckers are arranged as usual, as short cylinders of large size arranged close together, as in the genus Octopus, and the space between the two rows is quite narrow (Fig. 3).
I do not recall reading of such an arrangement in ordinary Octopods though it is not unusual for the suckers to differ in size in different arms.
The web, or umbrella, has quite an unusual and therefore a characteristic arrangement. It is highest between the arms 1 and 2, passing up the side of the latter almost to its tip. Between the pair of dorsal arms (No. 1) it is very much shallower, though it passes up the mesial side of the arms as far as the fractured ends and even beyond. The other sectors are about half this height. The different
sectors have the following heights (measured from the mouth to the edge), using Robson's nomenclature:—
A peculiarity already referred to, in which the genus differs from the rest, is the existence of two pairs of “water-pores” as they are termed. These are small, circular perforations of the web close to the roots of certain of the arms (Fig. 1, wp.). One pair is dorsal, close to the roots of arm 2, the other pair ventral, close to arm 4. Each pore leads into a semicircular chamber or head-cavity, one therefore on each side of the head which thus opens both dorsally and ventrally. This cavity passes between the eye and the base of the arm, and sends outgrowths or sacculi between the roots of the arms, and one such sacculus runs above and behind the eye, and still another ventrally between the eye and funnel (I have summarised Naef's account). The function of this series of channels and chambers seems to be unknown. A single pair of “water-pores” occurs on the ventral surface of the head of Ocythoe, and their occurrence here led one author at least (Pelseneer) to group the two genera in a Family Philonexidae. But more modern zoologists place the genus Tremoctopus in a separate Family Tremoctopodidae, of which it is the sole genus. It is sometimes called a “pelagic argonaut” as it is without an external shell and by some authors it has been relegated to the Family Argonautidae.
The species T. violaceus is widely distributed, having been recorded from the Mediterranean, the Atlantic, the Red Sea and the Pacific. In addition to those that have been found on the coasts of New Zealand at Napier, the Great Barrier and Crater Island, Massy (1916) obtained a few very young ones (4 and 5 mm. in length) off the Three Kings Islands during the expedition of the Terra Nova, in 1910.
Kirk, T. W., 1833. On a New Cuttlefish Tremoctopus robsonianus. Trans. N.Z. Inst., vol. xvi, p. 549.
Massy, A. L., 1916. Cephalopoda, British Antarctic Expedition (Terra Nova), vol. ii.
Naef, A., 1923. Die Cephalopoden. Fauna u. Flora d. Golfe di Napoli, Monograph 35.
Suter, H., 1913. Manual of New Zealand Mollusca. Wellington.