No common name

Hydrichthys mirus

ORDER

Filifera

FAMILY

Pandeidae

TAXONOMY

Hydrichthys mirus Fewkes, 1887, New England.

OTHER COMMON NAMES

None known.

PHYSICAL CHARACTERISTICS

Hydroid: hydrorhiza as a thin encrusting plate on the skin of fish host. Polyps of two types: gastrozooids are elongate, tubular, with distal mouth, and no tentacles; nearly all with a small bud at base; mouth with an armature of some microbasic eury-teles and desmonemes; no nematocysts in other regions of colony, apart from medusa buds; gonozooids very contractile, with up to three lateral branches bearing clusters of medusa buds at different stages of development; no sign of perisarc. Medusa buds on gonozooids, hydrorhiza, and possibly also on hydranths; exumbrellar nematocysts present; well-developed buds stiff, with two big tentacular bulbs and no apparent sign of tentacles.

Medusa: umbrella height 0.15 in (4 mm), dome shaped, with apical projection; jelly soft. Manubrium with well-developed folds in stomach wall, bearing the gametes; manubrial folds nearest to mouth much developed, almost covering lips. Above these main folds, two or three additional folds sometimes developed. Gametes almost completely covering interradial portions of stomach.

DISTRIBUTION

Indo-Pacific; very rare. (Specific distribution map not available.)

HABITAT

The hydroid lives symbiotically with fish; the medusa is planktonic, possibly with a tendency to forage near the bottom.

BEHAVIOR

The gastrozooids can bend to touch the surface of the fish. They have a distal annular contractile ring resembling a sucker, but feeding on fish was not observed. Under laboratory conditions, the hydroid do not take Artemia nauplii. After the death of the fish, colonies can detach and become free living. It is therefore possible that they can survive in the plankton if the host dies. In the laboratory, the medusae tend to stay on the bottom of rearing jars, with their long tentacles completely extended, attached to the bottom of the finger bowl, stretching them for a long distance while swimming parallel with the bottom. Tentacles are then contracted and the bell is drawn quickly backwards. Prey is captured in lips during this fast backward movement. Medusae possibly have a benthic feeding habit. After feeding, they swim vigorously for some hours.

FEEDING ECOLOGY AND DIET

The hydroid most probably feeds on the supporting fish, using the mouth as a sucker. The medusa feeds on crustaceans.

REPRODUCTIVE BIOLOGY Nothing is known.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS None known. ♦

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