Author: Tokioka & Baba, 1964

Order: Cephalaspidea  Family: Gastropteridae

Maximum Size: 20 mm

Sightings: Sunshine Coast

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Sagaminopteron ornatum Tokioka & Baba, 1964

The distinctive Sagaminopteron ornatum is a head-shield slug (cephalaspidean) belonging to the Gastropteridae family. It is a small sea slug growing to 20 mm in size but is most usually sighted smaller at 12 to 15 mm.

The species is instantly recognizable by its purple/violet (sometimes called pink) to ultramarine or cobalt blue body and parapodia colour. The parapodia are often trimmed with a thin white (sometimes yellow) border. Contrasting in golden-yellow (at times tending to orange) are the head-shield funnel with its medial crest, a large posterior filament and the posterior edges of the mantle.

The head-shield is broadest anteriorly narrowing posteriorly to be triangular in shape and is considered short, being approximately one quarter of the animal’s overall length (when the funnel faces forward). Anteriorly the lateral corners are developed into short tentacles. Posteriorly, the lateral edges of the head-shield are curved under medially forming up into a funnel that is held erect, facing forwards over the head-shield often past the anterior edge. The funnel is supported internally by the dorso-medial crest, the tip of which just projects past the funnel folds. Behind the head-shield is the large visceral hump, elongate-ovoid in shape and bearing a number of posterior protuberances the largest of these being distinctively golden-yellow in colour and is referred to as a flagellum, being a continuation of the right lateral ridge or edge of the mantle cavity. Lateral extensions of the foot, the parapodia, are large and fold up over the visceral hump to meet or even overlap on the midline. The posterior shield overlies the internal shell remnant. The tail, continuous with the foot sole, extends well past the visceral hump before quickly tapering to a blunt point thus forming a triangular shape. The gill is large, translucent white and can often be seen extending above the top edge of the right parapodium. It may carry up to seven branches, unattached except at their bases.

When disturbed Sagaminopteron ornatum is able to swim strongly for quite some time by flapping its large parapodia. This behaviour has earned it the common name of Bat-winged Slug.

Sagaminopteron ornatum like all the sea slugs is a hermaphrodite, possessing both male and female reproductive organs. In this species, the male genital pore containing the penis is located anteriorly on the head just to the right of the mouth and receives sperm from the hermaphroditic genital pore via a ciliated external groove running along the right side of the body. The hermaphroditic genital pore is located to the rear right of the body under the right parapodium. Copulation may occur by one partner acting as a male and approaching the other from the rear to access the hermaphroditic genital pore for insemination, or they may act reciprocally by forming a circle to simultaneously inseminate. The spawn is laid as a number of small, flat but discrete masses, white in colour, on the substrate (often on its food source) and attached thereto at one end. This species grazes on a grey sponge with a granular composition that appears to be from imbedded grains of sand (personal observation). Distribution is tropical & temperate Indo-Pacific. It has been recorded from all around the coastline of mainland Australia.

When this species was described in 1964, a new genus, Sagaminopteron, was raised at the same time. It was named after the collection site of the holotype made by the Emperor of Japan (Biological Laboratory of the Imperial Household) – Sagami Bay, south of Yokohama, Honshu, Japan. Sagaminopteron ornatum is the type species of the genus. The genus was separated, at the time, from Gastropteron on the basis of radula structure. Since then, as more species have been described and their anatomy examined more closely, other features such as the number of gill branches, male reproductive anatomy and shape of anterior siphon and visceral hump have also contributed to the distinction. The genus now holds six described species.

David A. Mullins – September 2020

References:
– Tokioka, T. & Baba, K. (1964). Four new species and a new genus of the family Gastropteridae from Japan (Gastropoda: Opisthobranchia). Publications of the Seto Marine Biological Laboratory, 12(3): 201-229.

– Gosliner, T. M. (1989). Revision of the Gastropteridae (Opisthobranchia: Cephalaspidea) with descriptions of a new genus and six new species. The Veliger. 32(4): 333-381.

– Burn, R., Thompson, T. E. (1998). Family Gastropteridae. Pages 952–954 in P.L. Beesley, G.J.B. Ross, and A. Wells, eds., Mollusca: The Southern Synthesis. Fauna of Australia. 5, Part B. CSIRO Publishing, Melbourne.

– Rudman, W. B., (1998, June 18) Sagaminopteron ornatum Tokioka & Baba, 1964. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/sagornat and associated messages.

– Marshall, J. G., Willan, R. C. (1999). Nudibranchs of Heron Island, Great Barrier Reef: A survey of the Opisthobranchia (sea slugs) of Heron and Wistari Reefs; Backhuys: Leiden, The Netherlands.

– Wells, F. E., & Bryce, W. (2000). Sea Slugs of Western Australia. Western Australian Museum.

– Ong, E., Hallas, J. M. & Gosliner, T. M. (2017). Like a bat out of heaven: the phylogeny and diversity of the bat-winged slugs (Heterobranchia: Gastropteridae). Zoological Journal of the Linnean Society. 180(4): 755-789.

– MolluscaBase eds. (2020). MolluscaBase. Sagaminopteron Tokioka & Baba, 1964. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=531565 on 2020-05-03

– This Species Profile has been modified from a previously published article in Dive Log Magazine’s – NudiNotes Column, Issue: #382 (June 2020): 14 by David A. Mullins