Introduction
The animals belonging to the phylum Mollusca have soft-bodies, triploblastic and bilaterally symmetrical and coelomate. The study of Mollusca is called Malacology. They are sluggish invertebrates, with a thin fleshy envelope or mantle covering the visceral organs.
The term Mollusca was derived from the term given by Aristotle to cuttlefish. Mollusc means soft.
These organisms are found in the terrestrial as well as in deep seas. Their size ranges from microscopic organisms to organisms 20 metres long.
They play a very important role in the lives of humans. They are a source of jewellery as well as food. Natural pearls are formed within these molluscs.
The bivalve molluscs are used as bioindicators in the marine and freshwater environments. But few of them such as snails and slugs are pests.
Mollusca Characteristics
The organisms belonging to phylum Mollusca exhibit the following characteristics:
1.They are mostly found in marine and freshwater. Very few are terrestrial and found in moist soil.
2.They exhibit organ system level of organization.
3.Their body has a cavity.
4.The body is divided into head, visceral mass, muscular foot and mantle.
5.The head comprises of tentacles and compound eyes.
6.The body is covered by a calcareous shell.
7.The muscular foot helps in locomotion.
8.They have a well-developed digestive system, the radula is the rasping organ for feeding.
9.Respiration in Mollusca occurs through the general body surface, gills or pulmonary sac.
10.The blood circulates through the open circulatory system.
11.They have a pair of metanephridia that helps in excretion.
12.The nervous system in Mollusca consists of number of paired ganglia and nerves.
13.The tentacles, eyes, osphradium, and statocysts act as the sensory organs.
14.The sexes are separate in most of the molluscs but some species are hermaphrodites. Fertilization may be external or internal.
15.They are generally oviparous with indirect development.
Classification of Mollusca
Following are the classification of Mollusca:
Aplacophora or Solenogasters
The body is cylindrical or bilaterally symmetrical.
These are devoid of the head, shell, mantel, nephridia, and foot.
The digestive ceca is absent.
Spicule-bearing cuticle covers the body.
It contains a dorsal longitudinal keel or crest.
E.g. Neomenia, Chaetoderma
Monoplacophora
The body is bilaterally symmetrical.
The head is devoid of eyes and tentacles.
Respiration occurs through gills which are externally located.
The nitrogenous waste is excreted out through nephridia.
E.g. Neopilina
Polyplacophora
Their body is dorsoventrally flattened like a leaf, and are bilaterally symmetrical.
The shell is composed of 8 longitudinal plates.
They have a well-developed radula.
The ventral foot is flat.
E.g. Chiton, Cryptochiton.
Gastropoda
They are found either on land or in fresh and marine water.
The head bears tentacles, eyes, and a mouth.
The shell is spiral in shape.
The foot is flat and large.
E.g. Haliotis, Pila
Scaphopoda
Found in the marine environment.
The eyes and tentacles are absent.
The foot is reduced.
The body is bilaterally symmetrical.
Pelecypoda
They reside in aquatic habitats.
The body is bilaterally symmetrical and compressed laterally.
The body has no distinct head.
They usually burrow in mud and sand.
E.g. Mussels, Unio
Cephalopoda
They are mostly found in the marine environment.
The shell is either external, internal, or not present at all.
They have separate sexes.
The development is direct.
E.g. Octopus, Spirula
Mollusca Examples
Following are a few examples of the animals belonging to phylum Mollusca:
Pila (Apple snail)
Limax (Slug)
Unio (Freshwater mussel)
Turbinella (Shankha)
Helix (Garden snail)
Octopus (Devilfish)
Loligo (Squid)
Key Points on Mollusca
1.Phylum Mollusca is the second largest phylum.
2.A few molluscs such as Unio possess green glands which mimic the liver in vertebrates.
3.They possess osphradia to test the chemical nature of water.
4.The statocysts maintain body equilibrium.
5.Octopus has 8 arms and contains no shell. They also possess ink glands for protection.
6.The blood is blue or green in colour due to the presence of a pigment containing copper called haemocyanin.
7.They possess a myogenic heart.
Blue Dragon
Blue Dragon Sea Slug Scientific Classification
Phylum. : Mollusca
Class : Gastropoda
Family. : Nudibranchia
Order : Galucidae
Genus : Glaucus
Species : Glaucus atlanticus
There are two species seen on this blog; G. Atlanticus and G. Marginatus. There is debate in their classification as to whether they are two species in the same genus Glaucus or if G. Marginatus is in a separate genus Glaucilla. This webpage will focus specifically on G. Atlanticus, the larger of the two species discovered by Forster in 1777.
G. Atlanticus has many common names including the blue sea slug, blue sea dragon and the sea swallow. They are members of what is known as the ‘blue fleet’ consisting of all the nasty marine stingers including blue bottles and violet snails’ that beach goers’ fear in the summer months in Australia. This community of animals living afloat are commonly washed ashore between August and April on Australian East coasts from the Cape all the way down to Sydney.
G. Atlanticus is pelagic, living life on the ocean surface traveling where the wind and currents take them. This species of nudibranch are very different to their bottom dwelling relatives in the way they live; they are held a-float by an air pocket they maintain in their stomachs by gulping air. They also have the odd trait of spending their entire adult life with their ventral side up due to the placement of their air sack.
Physical description
A clear defining visual characteristic for G. Atlanticus is its cerate (finger like appendages) which are arranged in a single row at the end of its three pairs of appendages where as in G. Marginatus there are multiple rows which look more like clusters at the end of its appendages.
Atlanticus also has a long cerate which looks like a tail that Marginatus lacks. It is usually also much larger than its cousin averaging 5cm while Marginatus reaches only 2cm on average. Colouration is also an easy identifying feature; the side toward the ocean surface (ventral side) has rich blue and silver colouration with a striking stripe down the middle of the body.
The underside of the animals is a silvery grey which is used to camouflage from any predators from below. This countershading on both the dorsal and ventral sides as a great defense for these nudibranchs and is used with other members of the clade.
Habitat
Glaucus spend their entire lives in the open oceans and so not much is known about their habits or living conditions. It can be expected though that as they live in temperate waters it is likely they would be prone to violent storms.
These nudibranchs are at the will of the currents and wind and it is these forces that push them inland and onto coastal beaches throughout the year. The specimen studied for this page were collected at the Gold Coast Spit mid-May of 2015 but are found all throughout the tropics of Queensland during summer months, including coastal islands such as Herron Island.
It would be easy to assume that these tiny delicate little slugs would have significant bodily harm after being pushed around and tumbled in the shore break. This is not the case.
During extreme aggravation Glaucus constrict themselves into a tight ball, release their air bubble so that they sink and escape the worst of the waves without major harm. Although they look delicate they very rarely experience dramatic damage and if they do they are able to regenerate most areas of the body.
Defense
Sea swallows are unique in the way they defend themselves. These sea slugs hunt other venomous pelagic organisms which drift nearby such as the violet snail (Janthina janthina), the blue button jellyfish (Porpita porpita) as well as blue bottles (Physalia utriculus) and then utilise their stinging cells as their own after eating them. The stolen stinging cells are known as kleptocnide and are transported through the gut without being digested in a still unknown mechanism to be stored in the cerate.
Cerate – are blood filled finger like appendages on the Glaucus found on the end of its appendages. As with other Aeolids, Glaucus cerate contain a duct connected to the digestive tract which terminates with a cnidosac at the very tip of the ‘finger’.
This sac is where the nematocysts from the Physalia are stored for defence. Interestingly the cerate of Glaucus are not in fact blue as they appear but actually transparent; the cause of the dark blue colouration comes from the blue Physalia on which they feed.
Locomotion
These slugs are at the mercy of winds and currents in the open ocean however they are able to swim by flapping their ‘arms’ in a coordinated manner to rotate themselves in a clockwise or anticlockwise direction. It would seem though from observation that they are unable to use their appendages or tail like structures to move in a purposeful or directional way and simply flap about until they bump into either a prospective mate or prey.
Hunting
Blue sea slugs use rhinophores (from the Greek words rhino meaning nose and phore meaning carrier) to essentially smell their way to their venomous prey in the open ocean. These smelling receptors send the information to the slugs’ brain, just as our olfactory system does for humans, and then they attempt to travel in the direction the scent is coming from.
Once they have found their prey they latch on with razor sharp teeth and immediately begin to feed on the living organism. This would be a challenge of not for the design of their jaws and teeth which allow them to latch onto even the most gelatinous of victim.
Their teeth are made of chiton which they also steal from one of their prey; the violet snail. All of their tiny teeth (denticles) are serrated and lock together like a zipper would once Atlanticus closes its jaws.
Anatomy and physiology
Glaucus have a relatively simple true gut, food travels from the mouth to the gastric cavity and then into the hind-gut before excreting at the anus. The mechanism in which these nudibranch use to distinguish between food and nematocysts is still unclear but it is known they travel from the gastric cavity to the hepatic lobe before being used as defence. Somewhere during this process the cells are enhanced and made more potent than they were when ingested, again this process is still unclear.
Unlike other nudibranchs, Glaucus mate with their ventral sides facing rather than the right sides. This is likely to avoid any nasty stings from their cerate and possibly so that they don’t get fed on by their mate during reproduction. They are similar to the nudibranch clade in the fact that they are hermaphroditic and oviparous. During mating the slugs act as both male and female producing both sperm and eggs and exchanging with one another.
The female reproductive organ on the left of the Glaucus (the genital opening) produces a string of up to 20 eggs. The penis of Glaucus compared to its body size is immense, sometimes stretching even longer than the body itself and is supported by what is known as the penis spine. As the sea swallow is pelagic there are some modifications to where the eggs are laid however. Most often they are laid into the mucus membrane of the slug and carried around or the string of eggs can be deposited on the carcass of their prey. Once gestation is complete the Glaucus locotrophic larvae become part of the planktonic community of the open ocean.
Biogeographic distribution
Glaucus inhabit temperate waters globally and drift on the currents of the ocean. As such it depends on what time of the year it is as to where they will be seen washed up on beaches; for example, Glaucus are found on the eastern coast of Australia between August and April when the winds blow predominantly on shore and there is a lot of storm activity in the surrounding oceans. They and the rest of the 'blue fleet' are most commonly seen in these months when there is a south easterly wind blowing
Conservation and threats
This species has no commercial significance other than being an unwelcome visitor to beaches for swimmers. There is also very little know about their behaviours or lifestyles as they are pelagic and live most of their lives in open ocean where it is difficult to monitor / observe them.
