Author: (Allan, 1932)

Order: Nudibranchia  Family: Discodorididae

Maximum Size: 70 mm (Rare sighting of 130 mm)

Sightings: Sunshine Coast

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Halgerda aurantiomaculata (Allan, 1932)

Halgerda aurantiomaculata is assigned to the Discodorididae Family of Nudibranchia. The Halgerda genus (first described by Bergh in 1880) has members that are all quite distinctive and usually cannot be confused with others. The external features normally conform to a familiar arrangement. These include a firm jelly-like body, a broad oval mantle that completely covers the foot excepting the posterior-most portion, usually a high body profile with a notum of ridges (sometimes quite high) joining tubercles (not all species) and often a reticulum of lines on or between the ridges or otherwise spots.

Halgerda aurantiomaculata is broadly oval in shape having a semi-translucent white mantle covered with orange/yellow spots and irregular blotches of varying sizes. The margin carries a narrow, continuous, orange band. This coloured marginal band is repeated on the foot. On the raised centre of the dorsum, between the rhinophores and gills, are three longitudinal rows of distinct tubercles each tipped with an orange spot and joined by low dipping ridges. Most usually, the central row has four of these tubercles whilst the two lateral rows each have three. The extra tubercle in the central row is located anterior to the rhinophores. I have noticed that the relative height of these tubercles varies considerably amongst individuals from almost indiscernible to significant projections. The foot could be considered narrow when compared to the overall mantle width.

The retractile rhinophores are long and translucent. The stalks bear numerous small dark brown spots. The clavus is lamellate, the edges of which carry dashes of dark brown. They taper towards a small translucent white tip and are often carried crooked posteriorly on the stalk. The pocket rims are smooth and very slightly raised.

The four gills (usually) have sturdy stems, are sparsely branched and can be retracted into a pocket below the mantle. Like the rhinophores they are also translucent and covered in brown spots. They are arranged two on each side of the anal papilla. The rim of the pocket is smooth and raised.

As with all species of Halgerda, H. aurantiomaculata feeds upon sponges in rocky habitats.

The spawn is laid as a golden yellow coil upon the substrate. Up until a couple of years ago I had never found this species in the act of spawning. I only came across one in the act when I happened to turn it over to obtain a ventral image. Hidden under the mantle were a couple of whorls unattached to the substrate and more being produced.

It is reported to have a distribution limited to coastal Queensland, the Great Barrier Reef and Melanesia (southern PNG, Vanuatu, New Caledonia and Fiji). Specimens have also been reported as far south as Lord Howe Island.

Originally described as Dictyodoris aurantiomaculata from a preserved specimen collected at North-West Cay, Capricorn Group, Southern Great Barrier Reef. The species author states that she had some difficulty deciding whether to place it in the Halgerda or Dictyodoris genus (both raised by Bergh) the latter since being synonymized with Halgerda.

During a dive excursion several years ago to one of our favourite haunts, The Wall at Old Woman (Mudjimba) Island I encountered an extraordinarily large specimen of Halgerda aurantiomaculata. Found there not uncommonly and normally in the 50 – 70 mm range, this giant example measured 130 mm and even then I don’t believe it was fully stretched out at that length. This is a new size record for the species being almost twice the length of our other accounts.

David A. Mullins – December 2020

References:

– Bergh, R. (1880). Beiträge zur Kenntniss der japanischen nudi- branchien. I. Verhandlungen der Koniglich-Kaiserlichen Zoologisch- Botanischen Gesellschaft in Wien, 30, 155–200.

– Allan, J. K. (1932). Australian Nudibranchs. Australian Zoolologist 7 (2): 87-105.

– Rudman, W. B. (1978). The dorid opisthobranch genera Halgerda Bergh and Sclerodoris Eliot from the Indo-West Pacific. Zoological Journal of the Linnean Society, London, 68, 59–87.

– Willan, R. C. & Coleman, N. (1984). Nudibranchs of Australia, Neville Coleman, AMPI, Sydney.

– Willan, R. C. & Brodie, G.D. (1989). The nudibranch Halgerda aurantiomaculata (Allan, 1932) (Doridoidea: Dorididae) in Fijian waters. The Veliger 32(1):69–80.

– Marshall, J. G. & Willan, R. C. (1999). Nudibranchs of Heron Island, Great Barrier Reef, Backhuys Publishers, Leiden.

– Fahey, S. J. & Gosliner, T. M. (2001).The phylogeny of Halgerda (Opisthobranchia, Nudibranchia) with the description of a new species from Okinawa. Zoologica Scripta

– Coleman, N. (2008). Nudibranchs – Encyclopedia – Catalogue of Asia/Indo-Pacific Sea Slugs. Neville Coleman’s Underwater Geographic Pty Ltd.

– Herve J. (2010). Guide des Nudibranches de Nouvelle-Caledonie et autres Opisthobranches, Editions Catherine Ledru.

– This Species Profile has been modified from Opisthobranch of the Week, Week #815, Halgerda aurantiomaculata (14/01/2013), Mike Miller’s slugsite.tierranet.com by David A. Mullins