Marine slugs refer to slugs living in the marine environment, such as those living in the coral reef, seashore, mangroves or seagrass meadow etc. Most marine slugs breathe through gills, but some living in the intertidal zones may breathe through lungs and are more closely related to land snails.
All marine slugs are hermaphrodites - they have both male and female reproductive organs - and perform internal fertilisation. When two slugs mate, they can either fertilise each other, or one may take on the role of the male, and the other, the female. Sometimes, they form mating chains, and perform different sexual roles to the animal in front and behind.
Here are the different groups of marine slugs that we have in Singapore:
A) Nudibranchs (Clade Nudibranchia)
Nudibranchs (clade Nudibranchia) are soft-bodied marine slugs which have a shell in their larval stage, but lose it as they mature. Many species have an exposed flower-like gills on their back, and hence the name “nudibranchia”, which means “naked gills”. They possess club-shaped rhinophores for detecting odours, and most have oral tentacles to touch and taste. There are two main groups of nudibranch - from the clade Euctenidiacea and the clade Dexiarchia. The former include the nudibranchs with the exposed gills on their back and their close relatives, while the latter include the ones with cerata (finger-like structures) on their back and their close relatives.
B) Headshield Slugs (Clade Cephalaspidea)
Headshield slugs (clade Cephalaspidea) are marine slugs with a well-developed headshield, which is a broadening at the head. This headshield prevents sand from entering the slug's mantle cavity as it burrows. Most headshield slugs have a greatly reduced and thin shell or an internal shell, though some species may lack a shell altogether. Many headshield slugs are often also called bubble snails, as their thin shell and round shells appear like bubbles.
C) Sidegill Slugs (Family Pleurobranchidae)
Sidegill slugs or pleurobranchs (family Pleurobranchidae) are marine slugs with a feather-like gill on the right side of the slug's body between the prominent mantle and foot, and hence the common name. They have a pair of rolled rhinophores on the head, acting as scent receptors to seek out their prey. Most species have a small and greatly reduced internal shell, though some may lack one as they mature.
D) Sap-sucking Slugs (Clade Sacoglossa)
Sap-sucking slugs (order Sacoglossa) are marine slugs that feed on algae. As such, they are usually found with the algae they feed on, and their populations usually boom with algal blooms, and decline when the algae recedes. To feed, sap-sucking slugs use a special feeding structure to pierce algal cells and suck out the content. Many species are able to retain the algae's chloroplast in their body, using them for photosynthesis!
E) Sea Hares (Clade Aplysiomorpha)
Sea hares (clade Aplysiomorpha) are marine slugs with a rounded body and two long rhinophores projecting backwards on their heads (resembling rabbit’s ears). They somewhat resemble rabbits or hares, and hence their common name. Sea hares have a soft body with an internal shell under a fleshy mantle.
F) Onch Slugs (Family Onchidiidae)
Onch slugs (family Onchidiidae) are air-breathing marine slugs mostly found in intertidal areas, and are more closely related to air-breathing land snails than other sea slugs such as nubibranchs, sea hares and headshield slugs etc. They breathe air using a simple lung modified from a section of the mantle cavity. The lung opens through a pore alongside the anus at its back end below the mantle. A few can also breathe through its skin (or mantle) or special breathing structures on their back under water. Most onch slugs hide in air pockets among rocks or burrow into the substrate during high tide, and emerge during low tide to feed.
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