Author: Carlson & Hoff, 1974

Order: Cephalaspidea  Family: Gastropteridae

Maximum Size: 20 mm

Sightings: Sunshine Coast

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Sagaminopteron psychedelicum Carlson & Hoff, 1974

Sagaminopteron psychedelicum is a cephalaspidean, or head-shield sea slug, belonging to the Gastropteridae Family. It is a small sea slug growing to 20 mm in length.

Sagaminopteron psychedelicum cannot be confused with any other member of its family due to its remarkable colouring, after which it is named, and the gill location, being mid-dorsal, not unlike a dorid nudibranch, rather than hidden under the right parapodial fold as in most gastropterids.

The head-shield is triangular in shape being broadest anteriorly with the anterior corners projecting somewhat in a tentacular manner and more so than most species of the family. Posteriorly the head-shield tapers and involutes, that is, it bends anteriorly and at the same time the ventral surface wraps around almost enveloping the dorso-medial crest, forming a funnel or siphon. The dorso-medial crest projects through and well past the folded head-shield funnel terminating as an apical papilla, orange/red in colour. The parapodia (lateral extensions of the foot) are long, extending almost the full length of the animal from the anterior end of the head to near the end of the tail. While they have a greatly convoluted edge they do not quite reach across to cover the dorsum completely. They can be extended to facilitate swimming by the animal. (The nervous system of gastropterids is among the most highly cephalised of the cephalaspideans. It has been suggested it may serve to explain how the complicated swimming motions are coordinated.) The gill is large and well developed and, unusually, is exposed and afforded little protection by the parapodia, as it is situated mid-dorsally. The four rachi are transparent but covered in fine black spots the intensity of which may at times give an impression of grey colouration. There are also larger spots of cream scattered over the gill.  The visceral hump is high, elongated, rounded and separated from the tail. It carries a single conical projection posteriorly (referred to as a flagellum) and slightly to the right of the midline. The anal papilla protrudes from the dorsum and is situated just posterior to the gill on the right side. The genital pore is also located on the right side but anterior to the base of the gill. There is a seminal groove that extends from the genital pore anteriorly to a male genital pore on the right, beside the mouth. There is no distinction between the foot and the parapodia and the foot and the tail. The broad foot bears a medial division – notched – anteriorly. This is a feature of the Sagaminopteron only, across the entire Cephalaspidean order. The tail is bluntly pointed.

The colour and pattern is complex and dramatic. The background colour of the head-shield, parapodia, and dorsum (also called the visceral hump) is light green or light blue or cream of varying intensity. A crowded arrangement of patches is present. These are mostly oval to round but sometimes form an irregular “L” or “C” shape or even a doughnut. These patches are outlined with a well-defined black line with a pale green or cream line inside of that. The interior colour of the patches varies from tan through to pinkish cream even on the one specimen. The parapodial margins carry a broad cream band and a narrow submarginal black line.

A small internal shell is present in juvenile specimens, situated posteriorly in the visceral hump, but is absent in mature specimens.

The sensory Hancock’s organs are located anteriorly in the groove formed between the head-shield and the foot on each side. The conspicuous funnel-shaped siphon with its medial crest is also believed to act in a chemosensory capacity. The anterior dorsal location, the manner in which it is held aloft or jutting forwards and presented to the environment, the increased surface area for sampling and the water funnelling capability all suggest an at-a-distance chemosensory function.

Although specimens have been reported to swim in laboratory conditions I have not been fortunate to see this behaviour, in this species, in the wild. When aggravated it emits a milky substance believed to be of a defensive nature.

Sagaminopteron psychedelicum is found upon sponges of the Dysidea genus, notably Dysidea granulosa. For many years it was thought that they could not feed upon sponges because of their radula arrangement being different to other sponge-feeding sea slugs but somewhat similar to that of bubble shells that prey on hard-shelled species. There are other examples however of species within the same genus having the same radula arrangement as each other but different diets. Sponges possess secondary metabolites of a toxic nature as a defensive method against predators. Dysidea granulosa possesses a number of these but the major compound is 3,5 dibromo-2-(20,40-dibromo-phenoxy)phenol. It is postulated that the cyanobacteria living in association with the sponge, produce this toxic compound. Varying concentrations of this compound, higher than in the sponge tissue, are found throughout the tissues of S. psychedelicum however, in the parapodia the concentration is more than twice that of the sponge. Therefore, it may be deduced, S. psychedelicum is consuming the sponge and selectively concentrating one of the secondary metabolites within its tissues and also discharging a mucus containing the same toxin for its own defence. Consequently the conspicuous colouration of S. psychedelicum is considered to be aposematic or warning colouration signalling to potential predators that it should be avoided.

Sagaminopteron psychedelicum are simultaneous hermaphrodites and mating is performed by a pair forming a circle in a head to tail position. Spawn is extruded as a long single strand of eggs culminating as a tangled mass.

Distribution is widespread in the Indo-Pacific. Specimens from the western Indian Ocean and the Red Sea are now considered to be a different, cryptic species, due to measured divergence in molecular sequencing and consistent differences in colouration, however more work is required to confirm this.

The genus Sagaminopteron was raised in 1964 by Tokioka and Baba. The genus name is derived from the collection location of the holotype of the type species of the genus, Sagaminopteron ornatum – Sagami Bay at the entrance to Tokyo Bay, Japan.

David A. Mullins – February 2021

References:

– Carlson, C. H. & Hoff, P. J. (1974). The Gastropteridae of Guam with descriptions of four new species (Opisthobranchia : Cephalaspidea). Publications of the Seto Marine Biological Laboratory, 21 (5-6): 345-363.

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

– Behrens, D., image by Bidrain, P. (2005). Opisthobranch of the Week, Week: 461. Mike Miller’s Slug Site. Available at: http://slugsite.us/bow/nudwk461.htm

– Becerro, M. A., Starmer, J. A. & Paul, V. J. (2006). Chemical Defenses of Cryptic and Aposematic Gastropterid Molluscs Feeding on their Host Sponge Dysidea granulosa. Journal of Chemical Ecology 32 (7): 1491-1500.

– Yonow, N. (2008). Sea Slugs of the Red Sea. Pensoft Publishers.

– 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, XX, 1–35