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Butterflies Underwater ….

You may call them sea slugs or nudibranchs or sea butterflies, there are very few parallels to these remarkable creatures, for their beauty, for their adaptations and for their survival strategies. From the great depths to pelagic, from tiny to over 60cm gigantic, from sap sucking herbivorous to most ferocious hunters, from nocturnal to diurnal, from slow moving to incredibly fast swimmers, from tropical to temperate distribution, from reefs to mud flats, from palatable to highly toxic, these remarkable creatures have occupied every niche of oceans and seas across the world. Yet we know so little about them.

Sea slugs are nothing but relatives of seashells. There are not more than 5000 species known worldwide. Yet they have most remarkable evolutionary history, as few freaks among seashells thought of giving away its armored defense like shells, an evolutionary gamble they seems to have won handsomely. However, most of them have shell in larval stage called ‘valiger’.

The primitive opisthobranchs such as bubble shells have a shell into which animal can withdraw entirely. However, in most of the species, the shell has lost completely. There are few species that resembles to sea slugs such as Onchidium. Earlier they were considered as opisthobranchs are now placed under pulmonates and are not from the classical sea slugs group. Lamellariids also in many ways resemble sea slugs. However, they are infact prosobranch snails. The three illustrations shown here can help amateur to distinguish the difference between pulmonates, opisthobranchs and lamellariids. 

Coriocella sp1Onchidium verruculatumPhidiana militaris 1

Lamellarid                                    Onchidium                                   Sea slug


Sea slugs show remarkable and variety of adaptations of which many are similar to butterflies and thus they are also commonly called as ‘sea butterflies’. The only difference that they lake true wings as in butterflies. As butterflies, many species of sea slugs believe in camouflage (Aplysia dactylomela) as primary way of defense. However, many show various chemical defenses (Chromodoris fidelis stores neatocysts, Dendrodoris tuberculata secretes Hydrocynic Acid), few species believe in mimicry. Some make perfect disguises (Notodoris citrina.) resembling a piece of sponge. As butterflies, many are brilliantly coloured, few can swim fast (Euselenops luniceps) and can migrate to short distances. Most of the slugs have a short life not over one and half year.

A brief account of about 100 common sea slugs is given in the book Field Guide to marine Life of India and will help you explore your sea shores. Please remember that these are delicate animals thus handling is not advisable. It is best to capture them on camera. They look better in photographs than in jars.

Where to find sea slugs?

Sea slugs are found virtually in all habitats. For beginners it is best to search shallow tidal pools. Most of the species are nocturnal thus you will have to see them under rocks or dead coral slabs. However, make sure you keep the rock back to its original position, since life that grows under rock cannot survive direct sunlight exposure.

What they eat?

Most nudis are carnivorous and feed on variety of diet such as tunicates, hydroids, sea anemones and even coral polyps. Few from genus Elysia and Thuridiella are sap sucking and feed on green algae from genus Caulerpa. Some are cannibalistic.

How they communicate?

It is one of the most complex processes and there are several research studies on the way they communicate. They use chemical cues to find partner.

How they reproduce?

All sea slugs are hermaphrodite i.e. each animal having both sexes. Applysia and bubble shells however have interesting behaviour. They form long chain in which each animal except the first and last, acts as a male to the animal ahead and a female to the animal behind. All the sea slugs have a free-floating stage called veliger. In most of the species, veliger stage has a shell, which however, is lost during the development.

Common Sea slugs or Opisthobranchs

Bubble shells

These are the most primitive opisthobranchs. Most of the species are nocturnal and remain buried under sand during daytime. Breeding congregations are common in winter months. 

Haminoea cymbalum2Haminoea sp.Gujarat

 Haminoea cymbalum                                                       Haminoea sp.

Sea Hares

All the species form these families are herbivores and remain hidden under or among algal mass in shallow waters. They can grow to large size (up to 60cm). If noticed and attacked by predators, they release dark purple ink that makes surrounding murky thus the animal buys time to escape from predator. 

Aplysia oculiferaAplysia parvula Morch 1863

Aplysia oculifera                                                  Aplysia parvula

Side-gilled Sea Slugs

The gills of these animals are placed on the right side of the body thus the group is commonly called as pleurobranchs. The individuals from family Pleurobranchidae are known to secrete acid against predators. Few species are cannibalistic and others feed primarily on sponges, ascidians and sea anemones.

Pleurobanchus peroniPleurobranchus forskalii.2

Pleurobanchus peroni                                        Pleurobranchus forskalii

Sap-sucking Sea Slugs

Species from family Plakobranchidae (Elysiidae) are herbivorous and feed predominantly on Caulerpa, a green algae.

Elysia expansaElysia rufescens Pease 1871

Elysia expansa                                                    Elysia rufescens

Nudibranchs or true sea slugs

Nudibranchs or true sea slugs are by far the largest group of opisthobranchs. Nudibranchs are formed of two major groups, Dorids and Aeolids.

Atagema spongiosa1Dendrodoris tuberculosa 2Sakuraeolis gujaratica1

Dorid: Atagema spongiosa           Dorid: Dendrodoris tuberculosa   Aeolid: Sakuraeolis gujaratica

Sea slugs are presently unexplored frontiers of marine biodiversity. We know so little about them that we cannot even say how many species we have in India, let alone its conservation. Yet they have shown great promise for medical frontier in the form of active substances they posses.

For a naturalist, it is a sheer delight to see them, yet at times searching them is totally frustrating so tiny they are. They are little masters of ocean realm. They have million stories to tell and many lessons to teach. One undoubtedly is their shrude survival strategy that teaches us a great lesson, ‘Survival of Fittest’, Darwin’s yet undisputed theory of Evolution.

So next time when you are on a beach, do spend time to find one. Who knows that one experience will evoke you to be a totally ‘nudi’ fan.



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