Summary
Physical Description
Size and Colouration
External Anatomy
Ecology
Local Distribution and Microhabitat
Life History & Behaviour
Reproduction
Development and Locomotion
Feeding and Respiration
Behavioural Observations
Anatomy & Physiology
Internal Anatomy
Histological Sectioning
Evolution & Systematics
Phylogeny and Kleptoplasty
Biogeographic Distribution
Conservation & Threats
References & Links
Reference List
Useful Links
Glossary | Development and Locomotion
There is a somewhat of a gap in knowledge regarding development in the sacoglossa, and this is perhaps due to the plasticity seen in their life history strategies [2]. Some species are known to be direct developers, whilst others go through a planktonic or breviplanktic larval phase; of these species some have high larval dispersal, whilst for others larvae dispersal is surprisingly low [2]. Following copulation, most deposit gelatinous egg masses in the most elevated parts of their habitat, often on the uppermost tips of their algal prey, presumably where oxygen concentration is highest and/or oxygen uptake by sedimentary activity most reduced [2]. In some species egg laying has been observed under rubble on the reef crest, though it is thought that this is only possible in areas of exceptionally high water quality, with high predation of egg masses also thought to influence this unusual strategy [2]. It is possible that Elysia sp. may also lay eggs in this manner, as the three specimens collected for this study were all found on the underside of a coral rubble boulder, and it is suspected that they may have been aggregating for mating purposes. Egg masses in the sacoglossa are highly gelatinous and provide protection to the developing embryos, but also provide a passive barrier to oxygen diffusion [2]. This may be part of the reason eggs are selectively deposited in high-oxygen areas, with egg masses shaped to maximise oxygen diffusion; this is extremely important in the sacoglossa, perhaps more so than other taxa, as the rate of oxygen diffusion controls the development of the young embryos [2,3].
Following deposition, eggs can take anywhere between a few days to several weeks to develop [2]. As previously mentioned, some species are direct developers and so young may emerge as juvenile versions of the adult, or may emerge as larvae that are distinctly different from their adult counterparts in those species that have a biphasic lifecycle. It is worth noting that even in those species that do have a planktonic phase in their lifecycle, dispersal appears to be extremely low compared to other marine invertebrate taxa, though the reasons for this are unclear [2,27]. As with all gastropods, juvenile sacoglossans undergo torsion as part of their development, though opisthobranchs are known to also undergo a secondary reversion or 'detorsion'.
Locomotion in adult sacoglossans is by the contraction of the muscular foot, as is seen in all gastropods. The Elysiids do appear particularly graceful in their movement compared with other groups of gastropods, owing to their wing-like parapodia, which make them appear as if they are gliding over, rather than on, the substrate. The foot muscle in the Elysiidae is usually split with a transverse groove [1], and runs the length of the body. The video below is a good example of locomotion in Elysia sp., and high resolution microscope imaging of the foot can be seen on the Histological Sectioning page.
Video- Locomotion in Elysia sp.
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