Plakobranchoidea

Plakobranchoidea are a group of marine heterobranch gastropod molluscs included within the sacoglossans. They are small to medium-sized sea slugs, generally associated with green algae on which they feed. Like other sacoglossans, some species can retain chloroplasts obtained from the algae they consume through kleptoplasty, allowing them to maintain part of the chloroplasts’ photosynthetic activity within their own tissues for a period of time.

Elysia flava by Enric Madrenas

Plakobranchoidea mainly inhabit shallow marine environments, particularly in areas where the algae on which they feed are abundant. They occur in tropical, subtropical and temperate seas, although their diversity is particularly high in certain warm-water regions. Their distribution is closely linked to that of their food resources, and some species show highly specific associations with particular algae.

Their appearance is highly variable. Some species have elongated, leaf-like bodies, while others possess broad parapodia that can be folded over the dorsal surface. In several Plakobranchoidea, the parapodia can form wing-like structures, giving the animals a distinctive appearance. Their colouration is often related to the algae on which they live and can provide highly effective camouflage. As in other sacoglossans, the radula is specialised for piercing algal cells and extracting their contents.

Kleptoplasty is particularly interesting in this group. Chloroplasts ingested with algae can be retained within the animal’s cells and remain functional for varying periods depending on the species. In some forms, this ability may provide an additional source of photosynthetic products, particularly when food is scarce. However, the importance of photosynthesis varies among species, and not all Plakobranchoidea depend to the same extent on retained chloroplasts.

Plakobranchoidea display considerable diversity in body form and ecological strategies. Some species are very small and difficult to detect, while others can be relatively large and conspicuous. Their close relationship with algae makes them particularly interesting for studying interactions between marine herbivores and their food algae. In some cases, the presence of a particular sacoglossan species may be strongly dependent on the availability of its algal host.

From an evolutionary perspective, Plakobranchoidea are one of the most interesting groups within Sacoglossa. Anatomical and molecular studies have helped clarify relationships among different lineages and have contributed to our understanding of the evolution of kleptoplasty. The combination of highly specialised feeding, morphological adaptations for living on algae and the ability to retain chloroplasts represents one of the most distinctive forms of adaptation among marine heterobranchs.

Although many Plakobranchoidea species are small and difficult to locate, they form an important part of sacoglossan diversity. Their unusual biology, close association with algae and ability to retain chloroplasts make them a valuable group for taxonomic, ecological and evolutionary studies. Detailed observation of the algae on which they live, together with targeted sampling, is essential for improving our knowledge of their diversity and distribution.

The taxonomy of Plakobranchoidea according to WoRMS is:

Species of Plakobranchoidea recorded from the Mediterranean Sea or the Iberian Peninsula: