Above – Eubranchus sp. 11. A beautifully coloured undescribed species of this distinctive genus.

The authentic “Balloon Animals” of the sea.

Have you ever inflated those special sausage-shaped party balloons and twisted them together to make an “animal”? My favourite was the poodle, probably because it was the easiest to make that was recognizable as an animal. Every time I see a Eubranchus nudibranch I’m reminded of those comical little creations.

Above – Clockwise from upper left: Eubranchus sp. 23. A translucent species with the maroon-coloured digestive gland visible through the notum and within the cerata. The cerata are capped with white-frosted cones. This specimen is in the process of extruding and laying its spiral-shaped spawn mass upon the hydroid. Eubranchus sp. 25. This is a Eubranchus putnami look-alike but different in some important respects. Just one member of a taxonomically cryptic clade. Eubranchus sp. 19. A very long species with small stubby cerata that are olive-shaped but still with an inflated appearance. Eubranchus sp. 21. This species has long transparent cerata where the internal golden-coloured digestive diverticula are visible. White in colour but with a longitudinal brown stripe down each side of the notum and also on the dorsal surface of the foot.

Eubranchus is a genus of aeolid nudibranchs belonging to the Fionidae or Eubranchidae family depending upon whose taxonomy is followed. Whilst almost 50 species of Eubranchus have been described, the systematics of the genus requires much work. For a long time it was thought that the genus was primarily populated by temperate species and a quick look at a list of the described species would seem to confirm that view. However, the tropical Indo-Pacific is rich in putative Eubranchus species with more than 30 awaiting description. Also, in the Bass Strait region of Australia for example, Bob Burn asserts that 17 await description. Yet other regions could very well have additional undescribed species. These estimates do not include any taxonomically cryptic species that are sure to be discovered when in-depth reviews, that include molecular sequencing, are eventually undertaken.

Above – Clockwise from upper left: Eubranchus mandapamensis. Recognised by the distinctive colour pattern on the cerata apices and a dense covering of small white spots. As well as circlets of tubercles on the cerata the rhinophores bear tubercles as well. Eubranchus ocellatus. Some of the cerata on this species may be hugely inflated. The narrow digestive diverticulum running up the centre is visible through the mostly transparent walls that bear brown rings – hence its name. Eubranchus sp. 16. This species also presents with distinctive colouration to the cerata apices and a dense covering of small brown spots. The digestive diverticular within the cerata form a small crucifix shape on the distal half. Eubranchus sp. 17. An undescribed species with hugely inflated cerata that have a distinctive colour pattern to the apices. The digestive diverticula is present as a narrow undulating tube.

What we do know with regards to external differentiating characteristics is that Eubranchus are small species but possess large swollen cerata, few in number, often with annular constrictions creating a bulbous presentation that can carry from one to three circlets of tubercles sometimes giving a knobbly appearance. The cerata are usually arranged in simple rows and are most usually held erect rather than laying down along the body. They are often translucent to a degree, though usually with some opaque pigmentation, but still allowing the digestive diverticulum in each to show through. Those diverticula may completely fill the cerata or they may be just a spindly thin tube. Some specimens look particularly ungainly or asymmetrical as one or two of the cerata may be two to three times more swollen than all the rest. Like most aeolids the tip of each Eubranchus ceras contains a cnidosac wherein the nudibranch concentrates, stores and maintains undischarged stinging nematocysts obtained from their hydroid prey. These are discharged in the hundreds for defence when the nudibranch’s cerata are aggravated.

Above – Clockwise from upper left: Eubranchus sp. 20. An undescribed Eubranchus. The body and all of the appendages are covered in small dark spots. The yellow digestive diverticula fill the long tapered cerata entirely. Eubranchus sp. 06. This undescribed but widespread Eubranchus of the Indo-Pacific has unusual colouration and numerous crowded cerata. Eubranchus sp. 03. This species is among the smallest of the tropical/subtropical Eubranchus. It exhibits swollen cerata with annular constrictions. Random orange spots are found on the body. Eubranchus sp. 24. A Eubranchus of unusually fine appearance. The body, cerata and all appendages are translucent. The cerata are long and tipped in yellow.

Rhinophores and oral tentacles are most usually smooth although a few with annular rings or tubercles on the rhinophores are known. Oral tentacles are usually much shorter than rhinophores. The body is elongate and narrow with a wide foot, often wider than the body; useful for those that need to grip and traverse hydroid branches. The anterior corners of the foot may be rounded or angular or even further developed into propodial tentacles. For those who are interested, the Eubranchus possess a triseriate radula (three toothed), a feature that assists taxonomists to separate them from similar looking species in other related genera.

Above – Clockwise from upper left: Eubranchus sp. 18. An undescribed Eubranchus with long tubular cerata where the circlets of tubercles are crowded and confined towards the apices and resemble hydroid polyps. Rhinophores and oral tentacles are especially long and tapered. The red internal organs are visible through the translucent body wall. Eubranchus sp. 13. A species with sub-apical swellings on the cerata that constrict just before the nipple-like tip. Exhibits a light yellow dusting of pigment on the oral tentacles, rhinophores and dorsum. Eubranchus inabai. The cerata vary greatly in swollen size often giving this species an asymmetrical appearance. Originally described from Japan. Eubranchus sp. 12. A species found living on inter-tidal hydroids that are epibionts on sea grasses and algae. Its mottled appearance provides excellent camouflage in that habitat.

The diet of Eubranchus, where it is known, consists of colonial hydroids, either the type that are free growing or others that are epibiotic on various kinds of algae and sea grasses. Not all species of Eubranchus are observed in the open with many being found in and under coral rubble in the tropic and subtropical habitat. Most are small, less than 20 mm, (but a few do grow over 40 mm) with many being less than 10 mm in length and often quite cryptic on their hydroid food. Often it is their spawn, laid upon the hydroid branches, that will betray their presence to the searching photographer. Knowing food sources is an excellent aid to finding sea slugs and with many species of Eubranchus the same applies. At times you might be fortunate to find several of the one species preying upon a single hydroid.

Above – Eubranchus virginalis. Here, several specimens of the species are feeding together on a hydroid colony. The circlets of tubercles are obvious on the translucent cerata. The body and cerata are covered in black spots that are often ocellated in white. A distinctive white patch is present on the central notum.

The images in this NudiNote are some examples of Eubranchus species from Indo-Pacific tropical to sub-tropical regions. This serves to explain the reason why two thirds of those illustrated here are currently undescribed.

(Note: Species names with sp. numbers in the captions refer to species on this website.)

David A. Mullins – August 2024

References:
– Baba, K. (1949) Opisthobranchia of Sagami Bay Iwanami Shoten: Tokyo.

– Rudman, W. B., (1998). Suborder Aeolidina Onchidorididae Pp. 1011-1017 in Beesley, P. L., Ross, G. J. B. & Wells, A. (Eds) Mollusca: The Southern Synthesis. Fauna of Australia. Vol.5 CSIRO Publishing, Melbourne, Part B 565-1234 pp.

– Rudman, W. B. (1998 – 2010). Family Eubranchidae. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/specieslist.htm

– Valdés, A., Hamann, J., Behrens, D. W. & DuPont, A. (2006) Caribbean Sea Slugs. Sea Challengers.

– Burn, R. (2015). Nudibranchs and related molluscs. Museum Victoria.

– Gosliner, T. M., Valdés, Á. and Behrens, D. W. (2018). Nudibranch & Sea Slug Identification: Indo-Pacific – 2nd Ed. New World Publications: Jacksonville, FL.

– Cunha, T. J., Fernández-Simón, J., Petrula, M., Giribet, G. & Moles, J. (2023) Photographic Checklist, DNA Barcoding, and New Species of Sea Slugs and Snails from the Faafu Atoll, Maldives (Gastropoda: Heterobranchia and Vetigastropoda). Diversity 2023, 15, 219.

– MolluscaBase eds. (2024). MolluscaBase. Eubranchus Forbes, 1838. Accessed through: World Register of Marine Species at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=137954 on 2024-04-20.

– This NudiNote has been modified from a previously published article in Dive Log Australasia Magazine – NudiNotes Column, Issue: #406 (June 2024): pp. 25-27 by David A. Mullins.