First record of the crawling medusa Eleutheria dichotoma from Victoria.
Two specimens of a marine hydromedusa were recovered from plankton tows in the Fishermens Cut, Queenscliff, Victoria in May 2014. The specimens were identified as the crawling medusa, Eleutheria dichotoma, which is one of a small group of hydrozoan medusae that crawls on algae rather than swimming in the plankton. The Queenscliff specimens were photographed in the laboratory under a Leica MZ12 stereomicroscope, using a (Point Grey) digital camera. The medusae survived for 10 days in the laboratory before disintegrating.
Family Cladonematidae Gegenbauer, 1857
Genus Eleutheria Quatrefages, 1842
Eleutheria dichotoma Quatrefages, 1842
Figs. 1 and 2.
Bouillon et al. 2004: 89, Fig. 48G-H.--Fraser et al. 2006: 699, Fig. 2A-H.--Schuchert 2006: 381, Fig. 19A-C.
Hydroid: stolonal, hydranth 1-6 mm high, with very short pedicel, perisarc smooth. Hydranth almost cylindrical, with an oral whorl of four to eight capitate tentacles. Medusa buds borne low on hydranth. Medusa: width 4-5 mm across extended tentacles, umbrella flattened hemispherical, oral surface more or less six-sided with thickened marginal ring packed with nematocysts. Velum broad and almost closed, opening only when feeding. Manubrium broad, filling most of subumbrella cavity, mouth simple. Usually six radial canals, gonads on manubrium. Tentacles usually five to six, bifurcated about middle, upper branch with terminal nematocyst cluster, lower branch terminating in an adhesive pad armed with stenotele and desmoneme nematocysts. An ocellus at base of each tentacle. Secondary medusae budding from bell margin.
Microscopic examination and digital images of the Queenscliff specimens confirmed them as Eleutheria dichotoma. As the specimens did not survive there is no voucher material. Further plankton tows produced no other specimens. One medusa had eight tentacles, a reddish ocellus at the base of each tentacle and a new tentacle growing from the side of the bell (Fig. 1). A second specimen had 12 tentacles; although not visible in the image, ocelli were present in the living specimen (Fig. 2).
The intertidal and shallow water cosmopolitan green alga, Ulva lactuca, is the favoured habitat of Eleutheria dichotoma (Fraser et al. 2006 and pers. obs.); this alga is common in the Queenscliff Boat Harbour and adjacent Swan Bay. The species is well known from the northern hemisphere where it has been recorded from depths to 20 m in the Mediterranean Sea (Brinkmann-Voss 1970). The small hydranth is seldom found in nature and is known mostly from aquarium studies. The medusa is easily identified by the dichotomously branched tentacles with terminal pads of nematocysts.
The medusa was recently recorded for the first time in the southern hemisphere on intertidal platforms from Bateau Bay to Pebbly Beach on the central-southern New South Wales (NSW) coast. It was abundant on Ulva, the brown alga Sargassum and corallines, with population densities averaging 52 individuals/10 cm2 of substrate (Fraser et al. 2006). Molecular analysis of the NSW specimens showed a close relationship with E. dichotoma from the Mediterranean Sea, differing by as little as 0.4% (Fraser et al. 2006).
Although both Queenscliff specimens possessed more tentacles than reported for some European and NSW medusae the number is considered variable and not diagnostic of the species (see Schuchert 2006; Brinckmann-Voss, pers. comm.). The actual number of tentacles is probably determined by ecological factors. As Briggs (1920, 1931) made no mention of E. dichotoma in his extensive studies of crawling medusae from NSW and Watson and McInnes (1999) did not find it among intertidal algae from Black Rock in Port Phillip, it is likely to have been introduced to southern Australia over past decades, probably in ships' ballast water.
Bouillon J, Medel MD, Pages F, Gili J-M, Boero F and Gravili C (2004) Fauna of the Mediterranean Hydrozoa. Scientia Marina 68 (Suppl 2), 5-438.
Briggs EA (1920) On a new species of crawling medusa (Cnidonema haswelli) from Australia. Records of the Australian Museum 13, 93-104, pls 17-18.
Briggs EA (1931) Notes on Australian athecate hydroids. Records of the Australian Museum 18, 279-282.
Brinkmann-Voss A (1970) Anthomedusae/Athecatae (Hydrozoa: Cnidaria) of the Mediterranean. Part 1 Capitata. Fauna e Flora del Golfo di Napoli 39, 1-96, pls 1-11.
Fraser C, Capa M and Schuchert P (2006) European hydromedusa Eleutheria dichotoma (Cnidaria: Hydrozoa: Anthomedusae) found at high densities in New South Wales, Australia: distribution, biology and habitat. Journal of the Marine Biological Association of the United Kingdom 86, 699703.
Schuchert P (2006) The European athecate hydroids and their medusae (Hydrozoa: Cnidaria) Capitata Part 1. Revue Swisse de Zoologie 113, 325-410.
Watson J and Mclnnes D (1999) Hydroids from Ricketts Point and Black Rock, Victoria. The Victorian Naturalist 116, 102-111.
Received 26 June 2014; accepted 22 November 2014
Richard Emlet (1) and Jeanette Watson (2)
(1) Oregon Institute of Marine Science, Charleston, OR, 97420 USA. Email: email@example.com
(2) Marine Biology Section, Museum Victoria, PO Box 666 Carlton, Victoria 3001. Email: firstname.lastname@example.org
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|Author:||Emlet, Richard; Watson, Jeanette|
|Publication:||The Victorian Naturalist|
|Date:||Apr 1, 2015|
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