Halecium Oken, 1815

Vervoort, W., Henry, L. - A. & Nizinski, M. S., 2011, Redescription of the leptothecate hydroid Halecium macrocephalum Allman, 1877 (Cnidaria: Hydrozoa), Journal of Natural History 45 (33 - 34), pp. 2095-2104 : 2096-2102

Genus Halecium Oken, 1815

Halecium macrocephalum Allman, 1877

( Figure 1A–D )

Character measured Range (µm)

Diameter of the main stems 500 – 1600

Internodes of monosiphonic hydrocladia length 750 – 1375 diameter just above node 142 – 198 just under insertion of primary hydrotheca 240 – 250

Primary hydrotheca diameter (measured in lateral view) 142 – 149 height 8.5 – 9.0

Secondary hydrotheca diameter (measured in lateral view) 149 – 156 height 8.7 – 9.0

Female gonotheca total length 1275 – 1675 maximal diameter 545 – 555 thickness, measured in frontal view 200 – 250

Male gonotheca overall length 1125 – 1375 diameter 60 – 65

Type material

The holotype specimen was collected during the Pourtalès expedition (1867–69) to the Gulf Stream, at 120 fathoms (approximately 219 m) off Sand Key, Florida. The species was described by Allman (1877) and figured therein (and reproduced in Fraser 1944); however, the location of the holotype is uncertain, because the label inside what is presumably the holotype (MCZ 9043) is now lost (Ardis Johnston, personal communication). Fraser’s (1943) specimen of H. macrocephalum , collected from shallow wharf pilings on the Tortugas, had degraded over time; the label within was indecipherable and the specimen was subsequently discarded (M. Arai, personal communication). Therefore only non-type materials were examined for this study.

Material examined

Two slides 5467, RMNH Coel. 32359. Single colony. Stetson Banks Lophelia pertusa reef, 31 ◦ 81.95 N, 77 ◦ 61.52 W, 668 m, unknown substratum (all other specimens from this station were archived as USNM 1110805). Reproductive female. Bushy colony composed of individual branches basally fused; branches proximally polysiphonic, gradually becoming monosiphonic, irregularly branched in all directions, branches to 45 mm high. Many female gonothecae.

Two slides no. 5466, RMNH Coel. 31358. Single colony. Stetson Banks Lophelia pertusa reef, 31 ◦ 84.57 N, 77 ◦ 61.16 W, 670 m (all other specimens from this station were archived as USNM 1110815). Occurring epizootically on a living colony of the gorgonian Eunicella . Reproductive male. Colony as in JSLI-04-4689, but height to 30 mm. Many male gonothecae.

MCZ 2299. Off Maryland. 38 ◦ 36.39 N, 73 ◦ 53.33 W, no depth given. Infertile hydroid fragments only. Not H. macrocephalum , but resembles a polysiphonic colony of Halecium beanii .

MCZ 50336. Off Boca Grande. No latitude or longitude given. 229 m. Infertile fragments of H. macrocephalum , but colony of same structure as in the new material examined .

MCZ 9043. Unknown location, but somewhere between Florida and Cuba during the Pourtalès Expedition to the Gulf Stream. Infertile fragments of H. macrocephalum , but colony of same structure as in the new material examined.

MCZ 50340. Single colony of H. macrocephalum . Off Samboes. 24 ◦ 37.78 N, 81 ◦ 58.33 W, 183–185 m. Reproductive male, but with gonophore distally truncated, resulting in a near halving of the gonophore length relative to the new material examined .

SBMNH 346554. Two haleciid colonies. Off Baja California. 28 ◦ 37.17 N, 115 ◦ 18.33 W, 82–101 m, fine sand/broken shell substrata. Not Halecium macrocephalum , but likely Hydrodendron (Dendrophiodissa) stechowi Hirohito, 1995 .

USNM 20687 . Florida-Hatteras Slope , off Georgia. 31 ◦ 43.33 N, 79 ◦ 11.67 W, 505 m.

USNM 28798 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 70627 . From Nutting’s 1893 Bahamas Expedition hydroids ( Nutting 1895), no station data given other than off Sand Key, 183–230 m.

USNM 69050 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 42749 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 21735 . Two miles west of Cape Romano, Gulf of Mexico, southwest Florida. No depth or latitude, longitude recorded .

USNM 69738 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 28780 . Florida-Hatteras Slope , off Georgia. 30 ◦ 97.50 N, 79 ◦ 64.17 W, 538 m.

USNM 71211 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 20690 . Florida-Hatteras Slope , off Georgia. 30 ◦ 97.50 N, 79 ◦ 64.17 W, 538 m.

USNM 28796 . Florida-Hatteras Slope , off Georgia. 30 ◦ 97.50 N, 79 ◦ 64.17 W, 538 m.

USNM 21734 . Florida-Hatteras Slope , off Georgia. 31 ◦ 15.00 N, 79 ◦ 55.83 W, 644 m.

USNM 68499. From Nutting’s 1893 Bahamas Expedition hydroids ( Nutting 1895), no other station data other than off Sand Key, 183– 230 m. This could be H. macrocephalum but only two fragments of hydroids were in the vial: one was not a haleciid but was otherwise unidentifiable, and the other was a fragment of a hydrocaulus, which was anthoathecate-like with no hydranth. Possibly the specimen of H. macrocephalum had been removed from the vial and subsequently lost.

USNM 69041 . Florida-Hatteras Slope. 29 ◦ 27.50 N, 79 ◦ 60.83 W, 801 m.

USNM 69240 . Florida-Hatteras Slope. 30 ◦ 73.33 N, 79 ◦ 43.33 W, 805 m.

USNM 21697 . Florida-Hatteras Slope. 30 ◦ 73.33 N, 79 ◦ 43.33 W, 805 m.

USNM 20688 . Florida-Hatteras Slope. 30 ◦ 73.33 N, 79 ◦ 43.33 W, 805 m.

USNM 20689. Florida-Hatteras Slope. 30 ◦ 73.33 N, 79 ◦ 43.33 W, 805 m. A strongly polysiphonic hydroid colony, but not H. macrocephalum , possibly an anthoathecate hydroid belonging to the family Bougainvilliidae .

Diagnosis

Colonies arise from a creeping hydrorhiza. Colonies erect and branched, with strongly polysiphonic primary and often secondary hydrocauli. Sessile alternate hydrothecae, often with characteristic steeply angled (up to 45 ◦) hydrothecal rims that do not flare. Extended hydranths are conspicuously larger than in most other haleciids. Species dioecious, with kidney-shaped female gonothecae and characteristically elongated and sigmoidal-shaped male gonothecae.

Description

Colony of H. macrocephalum Allman, 1877 composed of several thick, polysiphonic stems, resulting from forking close to the base of the stem, mainly directed upright, thinning out gradually towards apex, and bearing irregularly distributed, generally polysiphonic side branches or monosiphonic hydrocladia up to 15 mm long ( Figure 1A ). Stems (hydrocladia) with a basal diameter up to 1600 µm, of a chocolate brown colour, grooved.

Primary hydrocladia composed of many internodes separated by almost straight nodes (septa), these are rapidly converted into side branches by the apposition of secondary tubes; secondary and tertiary hydrocladia develop in all directions, originating from a region of the internode directly under and lateral to the hydrophore. Hydrocladia usually start with one to several athecate internodes (i.e. those that do not bear any hydrothecae) of varied lengths; these may also occur in a regular succession of thecate internodes (i.e. those that have hydrothecae inserted along their lengths). Internodes fairly slender, with almost parallel walls, slightly contracted at the node. The thecate internodes widen apically and rather suddenly to form a laterally directed apophysis (hydrophore) supporting the primary hydrotheca; the various hydrophores of a hydrocladium are alternately directed left or right. The apical surfaces of the hydrophore and the hydrotheca it supports are strongly tilted downwards to about 45 ◦ ( Figure 1B ), though this angle with the longitudinal axis of the internode is slightly variable.

Primary hydrotheca is notably low, and in lateral view quite hollowed, slightly widening from bottom to rim. Secondary (renovated) hydrothecae rare, very few have been observed; these have the appearance of a very low collar inside the primary hydrotheca and only just overlap it.

Recently collected specimens that were inspected have many badly preserved hydranths. These, as in other species of Halecium , are much too big to be accommodated inside the hydrothecae but are in comparison to other species such as Halecium halecinum (Linnaeus, 1758) and Halecium beanii (Johnston, 1838) not convincingly bigger. A row of desmocytes is visible in some of the empty hydrothecae.

Gonothecae of both sexes present, although inserted on different colonies; the species, as in many Halecium spp. , is dioecious. The female gonotheca resembles that of H. beanii , being in general kidney-shaped ( Figure 1C ). It narrows basally and quite conspicuously into a short stalk attaching the gonotheca to the hydrophore just under and lateral to the primary hydrotheca. The apical part of the female gonotheca bulges, in frontal or dorsal view rather strongly compressed. The concave side of the gonotheca slightly under its middle with an adnate pair of hydrothecae, in our specimens with remnants of hydranths. Just above the attachment of the hydrothecae to the wall of the gonotheca is a roughly circular hole, probably evidence of past extrusion of planulae that would have developed inside the gonotheca. Our specimens contain four to six developing eggs. Female gonothecae were found in abundance on certain colonies, attached to successive internodes.

Mature male gonothecae elongated and sac-shaped, almost tubular, being of uniform diameter over the greater part of their length, and in lateral view weakly “S”-shaped, narrowing basally into a short pedicel and rounded apically ( Figure 1D ). Their attachment is as in the female gonothecae: to the internodal hydrophore just under and lateral to the primary hydrotheca. As in the female, the male gonothecae occur in abundance along the length of the internodes, attached to successive internodes and adpressed to the hydrocladium. Shed gonothecae leave a big circular scar on the hydrophore, closed by a thin hyaline sheath of perisarc.

Distribution

Submersible-based collections of colonies from CWC habitats appear to be attached directly to hard reef substrata, including the corallia of scleractinian corals such as Enallopsammia rostrata , Madrepora oculata and Lophelia pertusa . The larger species of Halecium generally prefer attachment to solid substrata, but smaller species may be attached to algae or live epibiotically on other hydroids, bryozoans, etc.

In agreement with Vervoort (1972a) and based on our re-examination of non-type material, H. macrocephalum is restricted to deep-sea (> 200 m) hard-bottom habitats in the subtropical western North Atlantic. The species is distributed from North Carolina to Florida, the Straits of Florida between Cuba and Florida and inclusive of the Bahamas, and the eastern Gulf of Mexico including the Tortugas.

Remarks

To date, Halecium macrocephalum has been recorded in both the eastern Pacific and the western Atlantic. In the western Atlantic, most records are largely from subtropical marine regions including the eastern Gulf of Mexico and the Straits of Florida northward to Cape Hatteras, North Carolina. However, Fewkes’ (1881) specimen was collected further north on the continental slope off the coast of Maryland during the Blake Expedition. Fewkes’ report helped to perpetuate the notion of a possibly more northern distribution including the temperate and boreal regions of the Gulf of Maine and Bay of Fundy, e.g. on floating Sargassum in this region (e.g. Gosner 1971). Our re-examination of non-type museum material leads us to conclude that these northern records are erroneous. In the case of Fewkes’ specimen, this colony was misidentified and appears to be a polysiphonic colony of H. beanii (Johnston, 1838) , a phenotypically plastic species with a globally cosmopolitan distribution ( Schuchert 2005). In the case of Gosner (1971), the figure given for H. macrocephalum is highly ambiguous, and a reportedly epiphytic habit of H. macrocephalum surviving at the water’s surface on floating Sargassum is unconvincing, given the generally bathyal affinity of the species. We therefore regard Gosner’s record as erroneous, and perhaps based on a misidentification of Halecium nanum Alder, 1859 , which comprises part of the pelagic Sargassum hydroid assemblage ( Calder 1995). Hence, the northernmost record of H. macrocephalum at approximately Cape Hatteras complements Calder’s (1992) findings of a sharp biogeographic boundary for the shallow-water hydroid fauna in this same area where hydrographical and oceanographic conditions rapidly change.

Fraser’s (1948) record of H. macrocephalum attached to shell hash at approximately 82–101 m depth off Isla Cedros, Baja California was also found to be erroneous. His two colonies were significantly degraded, but exhibited strong fasciculation and reduced cylindrically shaped hydrothecae, typical of the haleciids. However, upon further inspection, it was clear that Fraser’s specimens were not H. macrocephalum . Although few hydrothecae remained intact, many of these were strongly renovated. Also, both colonies were reproductive, having each produced a large scapus of gonothecae. These structures resembled a lafoeid coppinia, but did not contain any protective elongated polyps. On the basis of the presence of a gonothecal scapus and numerous (but degraded) nematothecae on the SBMNH specimen, we concluded that the Pacific record of H. macrocephalum is actually a Hydrodendron that conforms most closely with Hydrodendron (Dendrophiodissa) stechowi Hirohito, 1995 , a species so far known only from Japanese waters.

Fraser’s (1944) specimen of H. macrocephalum from the wharf piling in the Tortugas became degraded and unidentifiable, and was subsequently discarded. Fraser’s identification of H. macrocephalum seems unlikely, given the strong bathyal affinity of the species and the morphological similarities between H. macrocephalum and other shallower-water haleciids such as H. beanii . On the subject of shallowwater occurrence of H. macrocephalum , we could not locate Nutting’s (1895) specimen from the Bahamas Expedition collected between 5 and 24 m. Nutting’s specimen is more intriguing, because although bathymetrically shallow, the adjacent rugged and often deeply-trenched bottom topography and coral-rich areas on the ridge between Eleuthera and Little Cat that were sampled by Nutting strongly conform to the types of hard-bottom habitat that H. macrocephalum colonizes elsewhere in the subtropical western Atlantic ( Henry et al. 2008). It is therefore highly possible that the depth was recorded erroneously. Although we consider Nutting’s identification to be correct, but with strong reservation, it would be interesting to revisit this rocky, coral-topped ridge in search of H. macrocephalum .

Distinctions from H. jaederholmi

Morphological differences between H. macrocephalum and H. jaederholmi are slight, and have led to taxonomic confusion between these two species (e.g. Ritchie 1913; Rees and Thursfield 1965). These species are also confused with another haleciid, the Antarctic Ophiodissa arborea (Allman, 1888) (e.g. Jäderholm 1905; Naumov and Stepan’yants 1962) as well as Zygophylax pinnata (G.O. Sars, 1874) (e.g. Ritchie 1907), a lafoeid known from North Atlantic deep waters and possibly distributed worldwide. Adding to the confusion, Fraser (1948) seems to have mistaken the Pacific haleciid later known as Hydrodendron (Dendrophiodissa) stechowi Hirohito, 1995 for H. macrocephalum , most probably because the nematothecae of the SBMNH specimen were degraded, and the gonothecal scapus had gone unnoticed by Fraser.

The morphological differences between H. macrocephalum and H. jaederholmi are significant enough to draw contrasts and allow taxonomic resolution between the two species. Halecium macrocephalum differs from H. jaederholmi by having longer, more slender internodes that widen rather suddenly to form the hydrophore. The apertural plane of the hydrotheca in H. macrocephalum slants downward much more strongly than in H. jaederholmi . In contrast, H. jaederholmi possesses a slight constriction of the perisarc some distance under the hydrotheca, which sets off a “chamber” in the hydrophore. Additionally, the secondary renovation of the hydrotheca is cup-shaped in H. jaederholmi and ring-shaped in H. macrocephalum . Differences between the two species in the shape of the female gonothecae are slight, but these may reflect the developmental condition of the planulae within the gonophore. As the male gonotheca of H. jaederholmi has yet to be described and the male gonotheca of H. macrocephalum figured by Allman (1877, pl. 12 fig. 4) in the original description is probably a damaged male or a juvenile female gonotheca, it is unknown whether the male gonophore can be used to distinguish between the two species. We do, however, find significant differences in the geographic distributions of the two species. Halecium macrocephalum is strictly a subtropical western Atlantic species, while H. jaederholmi has a Southern Ocean distribution ( Allman 1876, 1879; Vervoort 1972a, b).

Figures

Figure 1. Distinguishing morphological characteristics of the leptothecate hydroid Halecium macrocephalum Allman, 1877. The colony is densely branched with a thick, strongly polysiphonic hydrocaulus (A; scale bar 10 mm). Notably, many hydrothecae are steeply sloped at an approximately 45◦ angle (B; scale bar 500 µm). The kidney-shaped nature of the female gonotheca (C; scale bar 1 mm). The characteristic sigmoidal and elongated shape of the male gonothecae form the basis for this redescription (D; scale bar 1 mm), a shape that differs significantly from that figured for the holotype by Allman (1877).