Candelabrum australe ( Briggs 1928 )

Watson, Jeanette E., 2007, Notes on Candelabrum australe (Briggs, 1928) (Hydrozoa, Anthoathecatae), Memoirs of Museum Victoria 64, pp. 103-106 : 103-105

Candelabrum australe ( Briggs 1928)

Myriothela australis Briggs, 1928: 307 , pl. 32, pl. 33, fig. 3, pl. 34, figs 1–4.— Briggs 1929: 244, figs 1–4, pls 42–44.—Briggs, 1939: 10.– Manton, 1940: 280 et seq. fig. 8a.— Dakin, Bennett and Pope, 1948: 208.–Ralph, 1866: 158, 162.

Candelebrum australe —Segonzac and Vervoort, 1995: 35.— Hewitt and Goddard, 2001: 2280.

Description. Living material (supplementing Briggs 1928). Contracted hydroid cap-like in appearance, the gonophorebearing blastostyle region forming a decorative circular basal rim and the body a tapering crown.

Hydrorhiza comprising numerous short stolons covered by perisarc radiating from below the base of the hydranth but obscured by the living animal; stolons terminating in small circular disks firmly attached to the algal thallus; hydrorhiza entirely covered by a thin greyish layer of mucus. Body not extending into the blastostyle region.

Body tentacles capitate, globular, numbering about 1500 ( Briggs 1928), crowded together on the contracted hydranth but separated when body expanded; tentacles borne on stout transparent peduncles and richly armed with nematocysts. Blastostyles crowded on lower body, bearing clusters of 6–10 apically flattened tentacles on stout peduncles; tentacles armed with 100s of large nematocysts.

Hydranth bisexual, gonophores fixed sporosacs, male and female gonophores borne separately on stout branched blastostyles among clusters of tentacles; blastostyles comprising numerous developing and mature gonophores; mature gonophores of both sexes spherical, c. 1 mm in diameter with a flattened circular apical cap. Several larvae visible inside mature female gonophores. Actinula larvae were liberated after 36 hours in the laboratory; larvae flattened, typically lobate but variable in shape, capable of slow, barely perceptible movement.

At least 9 size-classes of nematocysts in 4 morphological categories in the cnidome of the hydranth, all size-classes occurring in the body tentacles and 6 size-classes in the blastostyle tentacles. Data on the nematocysts are summarized in Table 1.

Remarks. The common brown kelp Ecklonia radiata grows in depths of 1–20 m in southern Australia and is likely the seaweed upon which Briggs found his New South Wales specimens of Candelabrum australe . The New Zealand material provided no information about substrate but E. radiata is also a common coastal kelp near Christchurch in the South Island. Although the few southern Australian records of C. australe suggest it is a rare species, it is probably widely distributed in the abundant Ecklonia habitat.

The smooth, long-lived Ecklonia thallus would provide an excellent substrate for attachment of a large hydroid lacking a firm hydrocaulus. The hydrorhiza is strongly adherent to the alga by radiating disk-like stolonal plugs. At the site of attachment to the algal thallus there is a distinct light-coloured aureole indicating reaction of the hydrorhizal mucus sheath with the alga.

The tapering body of the hydranth is remarkably extensile and prehensile, extending rapidly from 2–6 cm, sweeping back and forth over the algal substrate somewhat reminiscent of an elephant’s trunk. It is highly sensitive to stimulus, contracting rapidly when touched. Authors’ descriptions of species of Candelabrum report a small circular mouth but this is probably an artifact of preservation. In contrast, the mouth of living C australe is widely open when actively exploring the substrate.

Briggs (1929) gave a detailed description of the development of the gonophores and formation of the egg of C. australe but as his specimens were almost dead, he could not provide information on larval development. Schuchert (1996) observed a juvenile specimen of an undescribed species of Candelabrum from New Zealand. The specimen moved about slowly for several weeks in the laboratory before adhering to a suitable surface. In the present laboratory study, following liberation, several flesh-pink larvae of C. australe moved sluggishly for several hours on the Ecklonia substrate before attaching by a plug of mucus. Rapid adherence of the larva to the algal substrate would be a vital adaptation in strong water movement of the Ecklonia habitat.

Several morphological categories of nematocysts comprising the cnidome of Candelabrum have been noted by authors: Jaderholm (1905), Manton (1940), Millard (1975), Segonzac and Vervoort (1995), Hewitt and Goddard (2001). Manton (1940) provides a broad classification and illustration of the cnidome of Candelabrum penola ( Manton, 1940) , Candelabrum capensis ( Manton, 1940) and Candelabrum cocksii (Vigurs, 1849) including heteronemes (= mesoteles), stenoteles, desmonemes and haplonemes (= meretrichous isorhizas). Millard (1975) figures the nematocysts comprising the cnidome of C. capensis and C. tentaculata (Millard, 1966) and Hewitt and Goddard (2001) tabulate the cnidome of known species of the genus. The cnidome of C. australe comprises the same morphological categories of nematocysts as reported by these authors.

Examination of the gross morphological features of the New Zealand material confirmed its identity as C. australe , thus extending the range of the species from 34– 44°S and across the Tasman Sea. As none of the nematocysts were discharged in the New Zealand material and the cnidome had deteriorated in preservation, no detailed examination of the nematocysts was possible .

The arrangement of the body desmoneme tentacles of C. australe is such that they would provide little assistance in capturing and passing prey to the mouth; their function is clearly that of defence. Laboratory observations indicated that the large active mouth of the hydranth is capable of engulfing prey, suggesting the species could possibly be regarded as a grazer rather than a static predator.

In contrast to the body tentacles, the cnidome of the blastostyles is overwhelmingly dominated by meretrichous isorhizas aggressively armed with extraordinarily long barbed threads capable of entangling large predators of the gonophores.

Manton (1940) postulated that the hydranth of C. penola may be long-lived, taking some years to reach maturity. Longevity would be advantageous for a large and active hydranth, allowing time for investment of energy in growth, feeding and reproduction. Such longevity requires a firm, equally long-lived substrate such as Ecklonia for settlement and growth. While the life-span of C. australe is still unknown, it spans several years, as the same 2 individuals were regularly observed over a period of at least 3 years on an Ecklonia plant in a shallow tide pool at Port Phillip Heads (R. Burn, pers. com.).