Runcinida

Runcinida are a small group of marine heterobranch gastropod molluscs traditionally associated with the sea slugs. They are small animals with soft bodies and no visible external shell in the adult stage. Their appearance is inconspicuous, and their anatomy includes several specialised features that distinguish them from other groups of heterobranchs.

Runcina coronata by Luis Ángel Díaz Álvarez

Runcinida mainly inhabit shallow marine environments, where they can be found on rocky substrates, among algae, beneath stones or in other sheltered benthic habitats. Their small size and cryptic behaviour make them difficult to detect during routine surveys. They are known from different marine regions, although their actual distribution may still be poorly documented.

One of their most distinctive characteristics is their small, elongated and generally flattened body, adapted for movement across the substrate. The head bears sensory structures, while the foot allows the animal to move along the surface. Adults lack a developed external shell, although their anatomical characteristics place them among heterobranchs with an evolutionary history related to shelled forms.

The biology of Runcinida remains poorly known. They are benthic animals, and some species are closely associated with particular algae or other organisms growing on the substrate. Their feeding appears to involve small organisms or organic material available on the surfaces where they live, although detailed information about their diet and feeding behaviour remains limited.

From an evolutionary perspective, Runcinida are particularly interesting because of their combination of ancestral and specialised characteristics. Their position within Heterobranchia has been investigated through anatomical and molecular studies, contributing to a better understanding of their relationships with other groups of heterobranch gastropods. Their small size and simple morphology illustrate how different gastropod lineages have evolved towards highly specialised benthic lifestyles.

Although runcinids are rarely noticed by divers, they form part of the poorly known diversity of marine heterobranchs. Their small size, inconspicuous appearance and still poorly understood biology make them an interesting group for taxonomic and evolutionary studies. Targeted sampling, particularly among algae and in other shallow benthic habitats, can help improve our knowledge of their diversity and distribution.

The taxonomy of Runcinida according to WoRMS is:

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

Runcina adriatica

Runcina africana

Runcina avellana

Runcina bahiensis

Runcina brenkoae

Runcina capreensis

Runcina coronata

Runcina ferruginea

Runcina hansbechi

Runcina ornata

Runcina zavodniki

Bibliography

1246567 Runcinida 1 eunomia-revista-en-cultura-de-la-legalidad 50 creator asc 1 7365 https://opistobranquis.info/wp-content/plugins/zotpress/
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Bibliography based on the works by Steve Long, 2006. Bibliography of Opisthobranchia 1554-2000 and Gary McDonald, 2009. Bibliographia Nudibranchia, with later updates from other resources.