Hybocodon chilensis Hartlaub, 1905

Material studied. Huinay area, Comau Fiord (Commune of Hualaihué, X–Region of Chile), several stations (Fig. 1) as follows: station 1: north side entrance of Quintupeu Fiord, 42º09.822´S, 72º26.672´W, 17–21 m, 21.i.2006, fertile colony liberating numerous medusae (MHNG INVE 48115); station 2: west of Isla Liliguapi, 42º09.722´S, 72º35.915´W, 20–25 m, 22.i.2006, (MHNG INVE 48116); station 3: east of Isla Liliguapi, 42º09.290´S, 72º35.776´W, 21–24 m, 28.i.2006, (MHNG INVE 48117); station 4: south of Cahuelmo Fiord, 42º17.683´S, 72º28.101´W, 15–23 m, 26.i.2006, fertile colony liberating numerous medusae (MHNG INVE 48118); station 5: steep wall north of Punta Llonco, 42º19.894´S, 72º27.661´W, 21–23 m, 26.i.2006; station 6: Punta Comau, 42º11.347´S, 72º35.452´W, 22–25 m, 28.i.2006 (MHNG INVE 48492); plankton: collected at night off Huinay Scientific Field Station, 42°22´S, 72º26´W, 10–20 m depth, 18 and 19.ii.2006, one adult female and 2 subadult medusae, and two actinulae (only one kept in the collection) (MHNG INVE 48119). If a neotype of H. chilensis Hartlaub, 1905 is eventually found necessary, the fully mature and well­preserved medusa (MHNG INVE 48119) might be the most suitable specimen.

Description

Hydroid. Colony consisting of small number of hydranths (generally 5–15). Hydranths borne on long hydrocauli, up to 5 cm high, arising from creeping, branched stolons (Figs. 2 and 3 A). Hydrocauli solitary or loosely aggregated, unbranched, basal part of some cauli adnate to adjacent ones with only the perisarc fused together, proximal end thin, gradually enlarging towards distal end (approximately 0.45 mm basally and 0.90 mm distally). Perisarc of caulus firm, thick, smooth over entire length, becoming membranous and annulated at base of hydranth, permitting its flexure; neck region tubular, not swollen (Fig. 3 A, insert 1). Endoderm of hydrocaulus with 7–8 longitudinal ridges projecting into lumen (Fig. 3 A, insert 2); ridges visible through perisarc. Hydranth pear­shaped, with very broad base; hypostome rounded distally, capable of much distension; with two types of tentacles. Aboral tentacles 19­23 in a basal whorl, tapering, hollow, long (up to 7 mm), oval in cross section, not very contractile; bases of tentacles continuing as ridges over basal part of hydranth body. Oral tentacles filiform, approximately 1.5 mm long, 20–39 in two closely set distal whorls, those of upper row alternating with those of inferior row. Blastostyles 8­13, long (often reaching beyond margin of hydranth), little branched, arising just above aboral tentacles (Fig. 3 B), bearing clusters of gonophores liberated as free medusae (Fig. 3 C). In life, soft tissues of polyp pink, perisarc brown.

Newly liberated medusa. Medusa on release 0.75–0.85 mm in height, 0.80–1.00 mm in diameter (Figs. 4 A to C). Umbrella quite spherical, bilaterally symmetrical, margin slightly oblique in lateral view; radial canals 4; marginal bulbs 4, with three of them rudimentary (diameter about 80 µm) and devoid of tentacles, the fourth one welldeveloped (about 230 µm), bearing one tentacle, length of tentacle in extension 0.35 mm or more. No nematocyst clusters distinguishable on tentacle. Manubrium cylindrical, normally spanning about 1/3–1/2 of subumbrellar cavity, extending slightly beyond velar opening in full extension. Exumbrella with scattered nematocysts, these organized in five longitudinal tracks, with three tracks arising from bases of rudimentary bulbs and extending upwards over about 1/2 of exumbrella immediately over radial canals, the other two other tracks originating from base of well­developed tentacle, and extending upwards over approximately 1/3 of exumbrella on both sides of corresponding radial canal. In more advanced developmental stages, nematocyst tracks tending to reach apex of umbrella (Fig. 4 D).

Adult medusa. As in other species of Hybocodon, umbrella of medusa with pronounced lateral asymmetry, with margin oblique to vertical axis, sloping toward side bearing well­developed tentacle (Fig. 4 E). Umbrella bell­shaped, about 4 mm high, 3 mm wide, with rounded summit, without apical process; mesoglea moderately thick; velum moderately broad. Manubrium cylindrical, usually spanning length of subumbrellar cavity on short peduncle composed of vacuolated cells; mouth simple, without lips, with narrow ring of nematocyst batteries. Gonad surrounding manubrium, leaving peduncle and mouth free; in female medusae, mature eggs found in epidermis of manubrium. Exumbrella with scattered nematocysts and with 5 meridional nematocyst tracks originating at marginal bulbs and reaching bell apex. Radial canals of different lengths according to length of bell sides: canal leading to tentacular bulb longest, diametrically opposed canal shortest; other two canals of intermediate length. All radial canals narrow, straight, equal in diameter. Single tentacle with a quite large base placed between two lateral bean­shaped swellings studded with nematocysts (Figs. 4 G to I), tentacle moniliform, very contractile, with about 50 regularly­spaced nematocyst clusters; number of clusters increasing with growth of medusa; no sign of asexual budding of medusae from tentacular bulb. In living medusae, tentacular bulbs and manubrium orange to orange­red. Rows of nematocysts on exumbrella having a bright orange tinge. Marginal tentacle endoderm, tentacular bulb, manubrium, and subumbrellar surface pink (Fig. 4 F).

Actinula larva. Actinulae (two of them observed) possessing one whorl of 11 and 15 tentacles, respectively (Fig. 3 D). Oral pole of first actinula with 7 developing oral tentacles; aboral pole flask­shaped (Fig. 3 D, inserts 1 and 2). Actinula white, endoderm of tentacles pink.

As in Hybocodon prolifer L. Agassiz, 1862, eggs of H. chilensis probably develop in situ and reach the actinula stage while still in the subumbrellar cavity. However, this was not observed in the small number of adult female medusae caught in the plankton.

Nematocysts

Small and large stenoteles; microbasic euryteles; heterotrichous anisorhizas; desmonemes; microbasic mastigophores (rare) (tables 1 and 2).

Ecology

The hydroid was observed on various inert hard substrates (e.g., polychaete tubes, dead gorgonians, bivalve shells) at depths between 15–35 m, always on vertical slopes, and in zones with significant water currents.

Distribution

Originally found and described by Hartlaub (1905) from Calbuco, Chile, this species had not been collected from the region again until being rediscovered here in abundance in Comau Fiord, southern Chile. Notably, Schuchert (1996) identified a hydroid and its medusa from New Zealand as Hybocodon prolifer L. Agassiz, 1862. The medusae lacked medusa buds, and the species corresponds exactly with H. chilensis Hartlaub, 1905 from Chile, as described here. Moreover, the cnidomes of both polyps and medusae from Chile and New Zealand are identical. That material is assigned here to H. chilensis (with the agreement of P. Schuchert, personal communication). This extends the distribution of the species to New Zealand.

Taxonomy

Due to relative uniformity in hydroid morphology among species of the genus Hybocodon, together with the absence of information about that stage in H. atentaculatus Uchida, 1947 and H. octopleurus Kao et al., 1958, it is more convenient to distinguish the various species by their medusa stage. A compilation of available data from the literature, listing the main morphological characters of both polyps and medusae of Hybocodon L. Agassiz, 1862, is presented in Table 3.

Medusae of Hybocodon chilensis Hartlaub, 1905 can reliably be separated from those of their congeners using certain important characters. In possessing one well­developed marginal tentacle, this species differs from H. atentaculatus Uchida, 1947 which lacks any tentacles. Moreover, the latter species has a very thick mesoglea and its umbrella is several times larger than H. chilensis. Unlike in H. chilensis, nematocysts on the exumbrella are not organized in longitudinal tracks in H. cryptus Watson, 1984 and H. unicus (Browne, 1902) (see also Browne & Kramp 1939 and Millard 1975; respectively). In another species, H. octopleurus Kao et al., 1958, the number of nematocyst tracks is 8, and not 5. The medusa of H. chilensis differs from H. pendulus (L. Agassiz, 1862) in several ways: the tentacle

bulb of the former is flanked by two large, bean­shaped swellings; the umbrella of the latter is much thicker at the apex and the tentacles at the bases of the two intermediate radial canals are quite developed (see Mayer 1910); moreover, medusae of H. pendulus seem not to be liberated: they mature and release gametes while still attached to the blastostyle (see L. Agassiz 1862). Only one species in the genus, H. prolifer L. Agassiz, 1862, is known to form medusa buds at the base of the moniliform tentacle, thus differing from all other congeners.

Stage Localization Heterotrichous Stenoteles Desmonemes Microbasic Microbasic anisorhizas mastigophores euryteles

Medusa Exumbrella + – – – –

Longitudinal + + – – – tracks

Tentacles – + + – +

Polyp Aboral tenta­ + + + + + cles

Oral tentacles + + + + + Nematocyst type Polyp Newly liberated medusa sources cited in references.

Species Hydroid stage

Caulus Number of tentacles Number of blastostyles

Height Diam. prox. Diam. dist. Aboral Oral

H. atentaculatus unknown

Uchida, 1947

H. chilensis Hart­ up to 5.0 ~ 0.45 mm ~ 0.9 mm 19–23 20–39 8 –13

laub, 1905 cm

H. cryptus Watson, up to 3.0 ~ 0.50 mm? 12–18 18–30?

1984 cm

H. octopleurus Kao, unknown

Li, Chang & Li,

1958

H. pendulus (L. up to 12.5?????

Agassiz, 1862) cm

H. prolifer L. Agas­ up to 5.0 ~ 1.0 mm up to 3.0 mm 25–30 32–50?

siz, 1862 cm

H. unicus (Browne, up to 1.9?? 12–19 15–28?

1902) cm

Continued

Species Medusa stage

Dimension Nematocysts on Number of Tentacular Medus Length of manu­

exumbrella tentacles bulb a buds brium

Height Width Scattered Longit.

tracks

H. pendulus (L. 5 mm 3.5? 5 reaching 1 long + 2 moderately no extends a short Agassiz, 1862) mm apex short at enlarged distance beyond

90° velar opening

H. prolifer L. 5 mm 4 mm? 5 reaching 1 long enlarged yes, up 1/2–2/3 the sub­ Agassiz, 1862 apex to 3 umbrellar cavity

gener­

ations

H. unicus 3 mm 2 mm yes no 1 long between 2 no as long as sub­ (Browne, 1902) large swellings umbrellar cavity