Moerisia horii ( Uchida & Uchida, 1929 )

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 57-58

Moerisia horii ( Uchida & Uchida, 1929)

Fig. 38

Laccocoryne horii Uchida & Uchida, 1929: 158 , figs. 1–3.

Ostroumovia horii . — Cooke, 1977: 73, figs. 1a, b.

Type locality. Japan: Kahoku and Ochi, in brackish waters ( Uchida & Uchida 1929) .

Material examined. None.

Description. (From Cooke 1977: 73): “The polyp is as long as 2 cm, with most of the length a highly contractile stalk… The hydranth itself is 1 mm to 2 mm long with 4 to 15 tentacles (the number increasing with age) and a well defined hypostome. The tentacles bear the nematocyst rings characteristic of the group and are also slightly capitate. The nematocyst rings are not quite complete, failing to fully encircle the tentacle, although this can be confirmed only in extended live individuals. Scattered among the tentacles of older individuals are medusae buds, as many as 10 in number, in all stages of development. The most complete medusae appear close to release. These are about 0.5 mm tall and 0.4 mm in diameter and bear four moniliform tentacles. Asexual budding occurs with new hydranths being formed from the lower part of the hydranth and stalk. No basal perisarc or chitinous disk was observed…”

Remarks. Uchida & Uchida (1929) described, as Laccocoryne horii , a hydroid from two brackish coves (Kahoku and Ochi) on the west coast of Japan. The hydroid, growing on Potamogeton , was reported to be solitary. No gonophores were observed on their specimens, even though hydroids were kept in culture for at least several weeks. Later, the complete life cycle of the species, including a description of the medusa, was described by Uchida & Nagao (1959). They assigned the species to Ostroumovia Hadži, 1928 , and it was later transferred to Moerisia Boulenger, 1908 by Kramp (1961: 445). Recent authors, including Petersen (1990) and Bouillon et al. (2006), have applied the binomen Moerisia horii to it.

Cooke (1977) studied living and preserved moerisiid hydroids from the Island of Hawaii that were similar to those described by Uchida & Uchida (1929) from Japan, and referred them to Ostroumovia horii . Medusa buds, present in his material, were about 0.5 mm in height and 0.4 mm in diameter prior to release, and four moniliform tentacles were present. New hydranths arose by budding from the stalk and proximal end of the hydranth. Illustrations were provided of two polyps of this species, drawn from living material. No specimens of this species were found in collections at the Bishop Museum.

Moerisiid hydroids are anatomically simple, morphologically variable, inadequately studied, and difficult to identify. The value of some characters used to distinguish taxa, such as solitary or primitively colonial polyps, developmental complexity of the pedal disc, and formation and arrangement of podocysts, is open to question. Life cycle studies and molecular analyses will be necessary to establish whether populations of this hydrozoan from Hawaii are indeed conspecific with those of Moerisia horii from Japan or with another nominal species. In the meantime, the identification made by Cooke (1977) is maintained here in the interests of nomenclatural stability.

Whatever its identity and name, occurrence of this invasive species in Hawaii is a biogeographic enigma. How this brackish water hydrozoan was transported to a pond on a remote oceanic island system in mid- Pacific Ocean is at present a matter of pure conjecture. Carlton & Eldredge (2009) considered it cryptogenic in Hawaii.

Reported distribution. Hawaii. Island of Hawaii: Honokohau, brackish pond in a lava flow ( Cooke 1977).

Worldwide. Japan and Hawaii; shallow waters ( Uchida & Uchida 1929; Uchida & Nagao 1959; Cooke 1977).