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Elysia obtusa

Jeffrey Ikin (2014)


Fact Sheet



Physical Description


Life History & Behaviour

Anatomy & Physiology

Evolution & Systematics

Biogeographic Distribution

Conservation & Threats

References & Links

Life History & Behaviour

Sacoglossan opisthobranchs are suctorial feeders, feeding on the cytoplasm of siphonalean algae. Shelled and non-shelled sacoglossans are likely to vary in diet, with shelled organisms being stenophagous, targeting a small variety of prey algae. After losing the shell, non-shelled sacoglossans undergo a radiation in diets, with some (especially those with temperate distributions) being relatively euryphagous (Jensen 1993). 

The mechanical process of feeding includes the grasping of algal filament with oral and pedal lobes, and buccal regurgitation to mix algal cytoplasm with saliva. Sacoglossans have uniseriate radula with teeth, which are specialized for penetrating algal cell walls, and allowing the animal to then feed on the cytoplasm within. 

Elysiid sacoglossans have also been found to retain foreign chloroplasts, and maintain these chloroplasts for extended periods of up to 3 months, in a process termed kleptoplasty (Brandley 1994). Functional chloroplasts are retained as intracellular organelles  in the cells of the digestive gland for several weeks/months in the absence of any algal nucleo-cystolic influence (Vieira et al. 2009). This may lead to some specificity in the diet of these animals, leading to a preference in algal selection. The ability of sacoglossan sea slugs to retain operational chloroplasts within their gut differs between species, and the chloroplasts are non-autonomous and unable to divide (Evertsen et al. 2007). The ability for sacoglosson sea slugs to retain chloroplasts is also affected by light availability, with high light conditions allowing retention for 6-15 days, and low light conditions allowing retention for 15-57 days (Vieira et al. 2009).

All adult sea slugs are hermaphroditic, however two individuals are needed to reproduce in all species except Berthelinia limax, which is the only extant species able to self fertilise (Behrens 2007). The genital openings can typically be found on the right side of the body of the animal. During copulation, sperm migrates to the receptaculum seminis, which is a temporary storage organ (Behrens 2007). After this event, sperm travels through the hermaphroditic duct to the female gland mass (equivalent function to an ovary/uterus). Within the female gland mass, the albumen gland adds a nutritive layer to a fertilised egg, the egg acquires an outer capsule in the membrane gland and numerous eggs are bonded together into a ribbon in the mucous gland (Behrens 2007). 
Larval development is for opisthobranchs is generally a planktotrophic stage, or a direct development larvae that lack a pelagic life stage.

Locomotion occurs in E. obtusa by the use of crawling on a flat, flexible foot. This foot is composed of two separate muscular bands, a thick outer band that remains in contact with the substrate and an inner section of tissue, the sole (Behrens 2007). The sole makes contact with this substrate when muscular contractions occur, moving the forwards. An example of movement can be seen in the below video. 

Alongside their role in housing branches of the digestive system, the parapodia function in respiration. The large surface area to volume ratio allows the efficient exchange of oxygen across body membranes, and the parapodia may be enlarged by folding along the margin, as seen in the above video. There are no external gills to be observed in any Elysia sp. (Cobb & Mullins 2014). 

E. obtusa appear to lack chemical defences such as acid secretion or stinging capsule defences. Instead, the main defensive characteristics of E. obtusa may be their colouring, where the ingestion of chloroplast matters makes them appear green, camouflaging the animal amongst its food source.