Stylactaria munita, Calder, 2010

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 35-37

Stylactaria munita , sp. nov.

Figs. 21 –23

Material examined. Oahu: Hawaii Kai , on pilings of bridge over Highway 72, 21º17’06.60”N, 157º43’07.21”W, 0.1 m, 28.vii.2009, on small oyster shell, one colony, to 1.6 mm high, without gonophores, coll. D.R. Calder, ROMIZ B3825 [HOLOTYPE] .

Etymology. The specific name is taken from the Latin word “ munitus,” meaning fortified, armed, or protected, in reference to distinctive nematocyst batteries arming the body together with a dense band of nematocysts around the hypostome of gastrozooids.

Description. Hydroid colonies stolonal, polymorphic, inconspicuous, with zooids arising from a creeping hydrorhiza growing over an oyster shell; stolons of hydrorhiza branching and anastomosing in a loose meshwork. Perisarc thin, investing hydrorhiza, terminating at base of zooids, not forming prominent cup-shaped perisarcal collar; spines absent. Polyps in present material of two types, gastrozooids and dactylozooids (occurring as tentaculozooids). Gastrozooids small, widely spaced, varied in shape from bowling pin-shaped to almost columnar, up to 0.75 mm high, 0.30 mm wide; body column with oval warts containing dense aggregations of nematocysts (comprising large heterotrichous microbasic euryteles only), each aggregation usually containing>25 nematocysts; hypostome dome-shaped to bulbous to almost cylindrical, distal end with a dense refringent band of nematocysts (all large heterotrichous microbasic euryteles); tentacles gradually tapering from base to tip, amphicoronate, in two close whorls around distal end of hydranth, about 10–16 in number. Tentaculozooids frequent, slender, elongate, each resembling an exceedingly long tentacle, tapering gradually from base to distal end, appearing slightly enlarged at tip due to an especially dense aggregation of nematocysts (heterotrichous microbasic euryteles), up to 1.6 mm long in preserved material, usually located near a gastrozooid. Colour of gastrozooids white when alive.

Gonozooids and gonophores not seen.

Nematocysts. Gastrozooids (tentacles):

desmonemes (4.9–5.2 µm long × 3.0–3.2 µm wide)

heterotrichous microbasic euryteles (7.3–8.5 µm long × 2.6–3.1 µm wide)

Gastrozooids (hypostome):

heterotrichous microbasic euryteles (9.2–10.0 µm µm long × 3.3–3.9 µm wide)

Gastrozooids (body nematocyst patches):

heterotrichous microbasic euryteles (9.0–10.0 µm long × 3.0–4.0 µm wide)

Dactylozooids (tentaculozooids):

desmonemes (4.5–5.2 µm long × 2.7–3.3 µm wide)

heterotrichous microbasic euryteles (7.0–9.0 µm long × 3.0–3.9 µm wide)

Remarks. Stylactaria munita , sp. nov., differs from other known species of hydractiniids in having gastrozooids with nematocyst warts on the body wall as well as a refringent band of nematocysts on the hypostome. Nematocysts arming both hypostome and body wall are large heterotrichous microbasic euryteles. FIGURE 22. Stylactaria munita , sp. nov.: tentaculozooid from holotype colony, ROMIZ B3825. Scale equals 0.25 mm. FIGURE 23. Stylactaria munita , sp. nov.: nematocysts from gastrozooid of holotype colony, ROMIZ B3825. a, desmonemes from tentacle. b, small heterotrichous microbasic euryteles from tentacle. c, large heterotrichous microbasic eurytele from hypostome.

As with 29 other species of hydractiniids assigned or provisionally assigned to Stylactaria Stechow, 1921a ( Bouillon et al. 1997), S. munita , sp. nov., possesses a hydrorhiza that consists predominantly of perisarc-covered stolons rather than an encrusting mat of coalesced coenosarc. Of these, S. munita closely resembles two Mediterranean species, S. aculeata ( Wagner, 1833) and S. pruvoti ( Motz-Kossowska, 1905) . In the three species, tentaculozooids as well as gastrozooids are present, and spines are either absent or small and only occasionally present. Stylactaria munita and S. aculeata differ from S. pruvoti in having gastrozooids with a refringent band of nematocysts on the hypostome. In turn, S. munita and S. pruvoti differ from S. aculeata in having clusters of heteronemes on the hydranth body, with those of S. munita occurring as slightly elevated warts. Of these species, S. munita is the only one known from a substrate other than gastropod shells occupied by snails or pagurid crabs, and its hydrorhiza is not known to be encrusting. Peña Cantero & García Carrascosa (2002) described and illustrated a hydroid identified as Hydractinia carnea (M. Sars, 1846) from the Mediterranean Sea that also had nematocyst patches on the body column, but no mention was made of a dense band of nematocysts on the hypostome of their species.

The band of nematocysts that surrounds the hypostome in Stylactaria munita may be a more widespread character among species of the genus than currently recognized. It is known to be shared with several other hydractiniids besides Stylactaria aculeata , including S. proboscidea ( Hincks, 1868) , S. inermis ( Allman, 1872) , S. arctica ( Jäderholm, 1902) , S. claviformis Bouillon, 1971 , S. otagoensis Schuchert, 1996 , Hydractinia sarsii ( Steenstrup, 1850) , H. fucicola (M. Sars, 1857) , and H. allmanii Bonnevie, 1898 ( Schuchert 1996, 2008a). Nevertheless, all of them differ from S. munita in one or more characters besides having larger gastrozooids: S. proboscidea has tentaculozooids shorter than gastrozooids; S. inermis lacks tentaculozooids; S. arctica has zooids with basal perisarcal collars and lacks tentaculozooids; S. claviformis lacks nematocyst warts on the body column of gastrozooids; S. otagoensis has hydrorhizal spines and larger nematocysts; H. sarsii and H. fucicola have encrusting hydrorhizae with naked coenosarc, and hydrorhizal spines.

The closest known species to S. munita geographically is S. mar Gasca & Calder, 1993 from the Pacific coast of Mexico (Bahía de Manzanillo). That species differs from S. munita by having long spines, zooids with distinct basal perisarcal collars, and larger gastrozooids (to 3 mm high) with haploneme nematocysts as well as euryteles and desmonemes. It was found on tubes of a sabellid polychaete.

Stylactaria munita is one of the smallest species of the genus described to date, with the largest gastrozooids observed measuring a mere 0.75 mm high. Only Hydractinia cytaeiformis Vervoort, 2006 (referred to here as Stylactaria cytaeiformis , comb. nov.) from deep waters (1200 m) off Cape Verde Island, and the poorly known Stylactaria siphonis ( Stechow, 1921b) , reported once off Plettenberg Bay, South Africa (see Millard 1975), are equally small (0.5 to 0.8–0.9 mm high). However, S. siphonis differs from S. munita in having fewer tentacles (8–10), in lacking tentaculozooids, in kind of substrate (occurring in the siphon of a gastropod instead of a bivalve), and in bathymetric provenance (500 m instead of 0.1 m). The bathyal S. cytaeiformis most notably lacks the dense nematocyst armature of hypostome and body column seen in H. munita , but its tentaculozooids also differ in being small and capitate.

The presence of tentaculozooids, together with the absence of both spines and a perisarcal collar at the bases of gastrozooids, is shared with Stylactaria multigranosi Namikawa, 1991 from Japan. Gastrozooids of S. munita differ from that species in the dense band of nematocysts on the hypostome and nematocyst warts on the body column. Stylactaria multigranosi is also thought to be substrate specific on shells of the gastropod Nassarius multigranosus ( Namikawa 1991) . As for tentaculozooids, their abundance is related in certain species, at least in part, to the proximity of space competitors ( Namikawa et al. 1992).

Material of Stylactaria munita examined here lacked both gonozooids and gonophores, and the nature of the life cycle is as yet unknown.

Reported distribution. Known only from the type locality.

Figures

FIGURE 21. Stylactaria munita, sp. nov.: gastrozooid from holotype colony, ROMIZ B3825. Scale equals 0.25 mm.