Cylichnoidea

Cylichnoidea are a superfamily of marine heterobranch gastropod molluscs that includes generally small species, often characterised by a cylindrical or elongated shell with a very low or almost invisible spire. Their compact shape and reduced spire give them a distinctive appearance, allowing many of their representatives to be distinguished from other gastropods with more globular or conical shells. Although they often go unnoticed, they form an interesting component of the diversity of small molluscs inhabiting the seabed.

Diaphana globosa @ Artsdatabanken - Cephalaspids of Norway by Manuel Malaquias

Cylichnoidea mainly inhabit benthic environments, ranging from shallow coastal waters to deeper areas. They can be found in sandy or muddy sediments, where many species partially bury themselves or move between the grains of the substrate. This lifestyle, combined with their small size, makes them difficult to detect during conventional underwater observations. However, they may be found in sediment samples collected using benthic sampling techniques.

The shell is one of the most useful features for recognising this group. In many species, it is elongated, cylindrical or slightly oval, with a narrow, elongated aperture that may occupy a substantial proportion of the shell’s length. The spire may be greatly reduced or sunken, while the outer surface can be smooth or bear fine striae and other delicate ornamentation. Despite this relatively uniform appearance, detailed examination of shell shape, proportions and anatomical characteristics allows species that look very similar at first glance to be distinguished.

Regarding feeding habits, many members of this group are associated with sediments and exploit the resources available in these environments. Some species feed on small organisms or organic material present in the substrate, although feeding habits may vary among different lineages. In this way, they contribute to the complex food webs of benthic communities, particularly in ecosystems dominated by soft sediments.

The classification of Cylichnoidea and the evolutionary relationships among its representatives have been revised as anatomical and molecular data have become available. The external similarity between many species does not always reflect a close evolutionary relationship, highlighting the importance of combining different types of characters to understand heterobranch diversification.

Despite their minute size and inconspicuous habits, Cylichnoidea are a relevant group for studying the biodiversity of marine sediments. Their distinctive morphology, adaptation to benthic life and the challenges involved in their identification make them an interesting example of the less visible diversity of marine gastropods.

The current taxonomy according to WoRMS is:

Species of Cylichnoidea reported in the Mediterranean or around the Iberian Peninsula: