Hydrichthys pietschi Martin, 1975

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 26

Hydrichthys pietschi Martin, 1975

Fig. 13

Hydrichthys pietschi Martin, 1975: 4 , figs. 3–5.

Type locality. USA: Hawaii, off Oahu , ca. 95 m ( Martin 1975) .

Type material. University of Southern California, Hancock Parasitology Collection, No. 7115 (HOLO- TYPE) (not seen) .

Description. (From Martin 1975, with measurements in microns): “ Polyps project from basal plate, some bearing buds on the aboral half of body… Polyp 490–1010 long, 112–168 wide. Ectoderm thin (about 3) endoderm 809 thick enclosing many granules 0.8–. 9 in diameter. Nematocysts and mesoglea not seen. Medusa bud … stalked, 87–106 long, 58 wide. Mass of cells at distal end probably developing manubrium. Bud endoderm also contains granules. Bud stalk 40–50 long; 47 wide.

Basal plate …approximately 4,800 in diam. with outer thin, 3, and inner thick, 16–22, ectoderm. Thickened inner ectoderm contains granules and may function in digestion … Endoderm thickness 8–25, with numerous granules, encloses shallow gastrovascular cavity. Maximum plate thickness 137 … about equal to the greatest penetration into the host. In this penetration the pigmented epidermis and some of the underlying tissues of the host are eroded. The granules of the hydroid and of the host epidermis are very nearly the same size. Possibly the hydroid obtains its granules from the host. There is a definite tissue response of the host to the parasite with a massing of cells beneath the inner ectoderm.… which is not seen in uninfected skin…”

Remarks. Martin (1975) described Hydrichthys pietschi as a parasite on the myctophid fish Ceratias holboelli from waters off Oahu. It is known only from the type locality, and only from the original description. This parasitic species was reported to lack both nematocysts and mesoglea. It would be remarkable indeed if nematocysts are completely lacking in polyps of this cnidarian, and the claim needs verification. While not found elsewhere on the polyp, euryteles and desmonemes were reported from the mouth region in a species of Hydrichthys from Papua New Guinea ( Boero et al. 1991: 191). While scarce or sometimes apparently absent, nematocysts have also been reported in hydroids of both H. sarcotretis ( Jungersen, 1911) and H. cyclothonis Damas, 1934 from Europe (Jungersen 1913; Schuchert 2007).

Reported distribution. Hawaii. Oahu : leeward side, 95 m ( Martin 1975) .

Worldwide. Known only from Hawaii.