Behaviour
Large juveniles and adults of Terebralia palustris feeding on a fallen mangrove leaf, near Pemba, Mozambique © David Reid, Natural History Museum, London
These mud creepers are amphibious - they crawl over the mud surface both while submerged and while emersed at low tide, although they withdraw into the shell when the surface of the mud dries out.
They congregate preferentially in the shade of the mangrove tree canopy and avoid open sunny situations. The adults can survive out of water and without food for 3–4 months.
Their feeding behaviour and associated morphological adaptations are remarkable.
The smaller juveniles (less than about 30mm long) are exclusively deposit feeders, ingesting detritus from the mud surface, whereas the larger juveniles and adults eat freshly fallen mangrove leaves and fruits, as well as detritus. Correspondingly, the feeding apparatus is modified during growth:
- in juveniles, the radular teeth are typical of detritivorous potamidid snails, with cusps on central and lateral teeth, and long marginal teeth
- in adults the central tooth is reduced, the laterals become massive, cusps are lost and marginals remain small
- the entire muscular buccal mass and jaws become relatively large, appropriate for breaking down tough mangrove litter
The snails detect mangrove leaves on the substrate by chemical cues transmitted in both air and water:
- snails are not initially attracted to intact leaves, and appear to find these by chance
- they respond to damaged leaves from which odours are released - these are usually leaves that are being eaten by other mud creepers, and aggregations of up to a dozen snails can form around a single leaf
While this behaviour leads to competition for food, it may also have an advantage. At low tide, the snails compete for leaf litter with the more agile sesarmid crabs, which steal leaves from individual snails, but cannot do so from the centre of a feeding aggregation.
Feeding aggregations may also prevent leaves from being washed away during high tide. Damaged leaves can be detected from a distance of up to 40cm, and snails show a preference for leaves of Rhizophora trees above those of other mangrove species, and for freshly-fallen leaves.
At high tidal levels of mangrove forests, T. palustris is the most important macroinvertebrate to contribute to plant litter degradation, and therefore makes a major contribution to entrapping primary production in the mangrove ecosystem.
Like all members of the family Potamididae, Terebralia palustris has a light-sensitive organ on the mantle edge in the region of the inhalent siphon. In Terebralia species this is a simple pit, homologous with the more elaborate ‘pallial eye’ of Cerithidea species.
This organ probably allows the animal to detect shadows and movement when the edge of the mantle is protruded through the anterior canal of the shell, before the head and eyes are extended beyond the shell margin.
