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Hong Kong Corals & the Associated Marine Life

Hong Kong Corals & the Associated Marine Life

> Other marine organisms  

Phylum Mollusca

Mollusca is the second largest phylum after Arthropoda. It consists of 7 classes and contains about 100,000 living species, plus an additional 60,000 known fossil species. About half of the species are marine and the rest are freshwater or terrestrial.

It is difficult to formulate a simple definition for these molluscs because they are extremely diverse in their forms. Generally, molluscs are derived from a fundamental body plan, a head with well-developed sensory organs and a large soft body mass containing all internal organs. Most molluscs possess a calcium carbonate shell and a muscular foot. However, evolutionary change in some groups has resulted in reduction, internalization or complete loss of the shell, as well as reduction or modification of the muscular foot.

Many molluscs are commercially significant. Cuttlefish, octopus, many bivalves and various gastropods are highly prized food.

Snails & Sea Slugs (Class Gastropoda)

There are about 35,000 described species of gastropods in six sub-classes.

Sea Snails


Sea snails have a muscular foot used for locomotion or attachment. Mucus is secreted at the foot's front end to reduce friction during movement. When disturbed, the foot is completely retracted into the shell. Some snails have a "trapdoor" or operculum to seal the aperture for further protection. 

Most snails have a unique file-like mouth part called the radula. Herbivorous species use it to rasp or cut algae from rock. In carnivorous cone shells, the radula is modified into a harpoon-like structure for injecting toxin into their preys. It could also be modified into a drill for drilling holes on the prey's shell for other carnivorous species.

Some carnivorous snails, such as Conus geographus, can hunt vertebrates. Their neurotoxin venoms are fatal to humans. Some snails in the genus Drupella attack and kill stony corals. They line up around the living coral and consume the coral tissue with their long proboscis.


Spindle Cowrie Phenacovolva brevirostris

Spindle Cowrie
Phenacovolva  brevirostris

This attractive spindle cowrie (or ovulid) is feeding on the white gorgonian, Euplexaura curvata. Although the flesh has been eaten, the gorgonians are not killed as they are able to regenerate the flesh within a few days.

Gorgonian Cowrie Crenavolva cuspis

      Gorgonian Cowrie      Cuspivolva cuspis

Cuspivolva cuspis has three colours to match that of the host gorgonian, deep purplish-brown, pinkish-white and orange. It is reasonably abundant within the Cape d'Aguilar Marine Reserve.



Soft Coral Cowrie Pseudosimnia whitworthi

Soft Coral Cowrie
Diminovula whitworthi
(Ovulidae )

This tiny cowrie, which rarely exceeds 1 cm in length, lives on red soft coral Dendronephthya sp. The cowrie's mantle provides extremely good camouflage for the snail, having white filaments to mimic the tissue and white spicules of the host. While it is a parasite feeding on its host's tissue, it seems to cause little harm to the host.

Arabian Cowrie Cypraea arabica

            Arabian Cowrie                Mauritia arabica    (Cypraeidae) 

The beautiful and distinctive Arabian cowrie is abundant in local waters. The bilobed mantle, which emerges from the shell can be extended to cover the outside of the shell completely. It has special cells that secrete the calcium carbonate matrix of the shell to maintain its rich, glossy appearance.




Conch Shell Strombus luhuanus

                 Conch Shell                   Conomurex luhuanus  (Strombidae)



This conch shell can reach a maximum length of 7 cm. Species in the genus Strombus are herbivorous and have particularly large eyes mounted on stout tentacles.



Drupella sp.
Drupella  sp.


This species of snail feeds on coral tissues, leaving behind the dead skeleton. Overproduction of this snail can cause an entire reef to be killed off, like the Ningaloo reef in Western Australia.




Sea Slugs

The evolutionary trend in sea slugs have been toward a reduction or even complete loss of their protective shells. They include nudibranchs, sea hares, side-gilled slugs and sap-sucking slugs.

All species of sea slugs are simultaneous hermaphrodites. Each individual has both male and female gonads and can produce both sperms and eggs. The genitals, found on the right side of the animal, are highly modified to prevent self-fertilization. When mating, the animals will pair up, facing in opposite directions and touching genitals to allow simultaneous passing of sperms between individuals. Their eggs are laid on hard surface, often in distinctive spiral egg ribbons. Individual eggs are held to each other and to the substratum by a mucus sheath.  



Nudibranchs mean "naked gills", in which most of the animals have the gills or branchial plumes outside the body. They are shell-less and therefore need other means of defence.

Bright colouration of nudibranchs can warn predators that they are noxious or even toxic. The colour is often acquired from their preys. Chemical weapons are another choice of defence. Most of them have deterrent chemicals which make them foul tasting or poisonous. The chemicals are either made by the animals or absorbed and retained from their toxic preys. 

Some species of nudibranchs can borrow the weapons of others and modify them for their own use. Aeolid nudibranchs feed on sea anemones and hydroids. They managed to transfer the undigested stinging cells from the prey to the tips of their tentacle-like cerata for defence.

Most nudibranchs are highly selective carnivores, with each species specificing feeding on a group of prey, such as sponges, hydroids, sea anemones, sea squirts or other sea slugs. Nudibranchs have a short life cycle, lasting from several weeks to a year.

Of nearly 2000 species of nudibranchs known from the Indo-Pacific, around 200 species have been recorded from Hong Kong.  


Chromodoris dianae

Chromodoris  dianae

As shown in the photos, this recently described nudibranch shows great variation in the black lines and the spots between individuals. The gills and rhinophores (antennas) are normally orange and there is always a black spot or line between the rhinophores. This species is only known from Hong Kong, Philippines, Indonesia and Borneo.

Chromodoris geometrica
Goniobranchus geometrica

Juveniles of this nudibranch often have orange rhinophores and gills, while those of the adults are green. This species has a distinctive habit of raising the front of the head and vibrating the gills as it moves.

Chromodoris lineolata
Chromodoris  lineolata

This species can grow to 5 cm in length. It is an active species and feeds mainly on sponges. In Hong Kong, it primarily occurs in Double Haven and Tolo Channel.



Hypselodoris festiva

Hypselodoris  festiva


This species is widely distributed and can be found under rocks and stones in almost all months of the year. It is active and feeds on ascidians.



Dendrodoris miniata

Doriopsilla miniata


This nudibranch was observed on the stony coral Tubastrea sp.

Dendrochoris nigra
Dendrodoris  fumata (Dendrodorididae)

Colour of this species varies from brown and orange to the more characteristic black with pink edged-skirt. It feeds on sponges. Groups of this species were observed laying eggs in March.



Gymnodoris ceylonica

Gymnodoris  ceylonica


This species has not been recorded in Hong Kong previously.

Mating Nudibrach
Mating Gymnodoris inornata
Sperms of Nudibranch
Sperms of Nudibranch
Egg case of Dendrochoris nigra
Egg case of  Dendrodoris sp.
Egg case of nudibranch
Egg case of nudibranch




Side-gilled Slugs



The plume-like gill of side-gilled slugs is usually located on the right side of the body, between the mantle and foot. Most of them reside in shallow sublittoral area, but a few species may occur in depth more than 30m. They all are carnivorous, feeding mainly on sponges, tunicates, and other sessile invertebrates.

The chemical weapon they used is an acid secretion that can cause a burning sensation to skin. 


Pleurobranchus forskalii (Pleurobranchidae)
Pleurobranchus  forskalii 

This species is nocturnal. It normally grows to 10 cm in length but individuals up to 30 cm have been recorded. It produces an acidic secretion that can cause a burning sensation to skin.



Sap-sucking Slugs




Like the nudibranchs, the body form of sap-sucking slugs is highly specialized and variable. Their oral tentacles are very small or absent, with the gills usually missing. The most obvious external feature are the rolled rhinopores.

Sap-sucking slugs mainly feed on green algae. They have a highly-developed radula and mouth part that enable them to pierce the algal cells and suck out the content.   


Elysia sp.
Elysia  sp. (Elysiidae)

It feeds on green algae by piercing the cell walls and sucking out the contents with its highly-developed radula and mouth part. This species is believed to be new to science.

Thuridilla bayeri
Thuridilla  gracilis (Plakobranchidae)

This elongated cell-sucker was first described from the Marshall Islands in the Central Pacific.




Sea Hares



As the name suggested, sea hares have a rabbit-like appearance. Their shells may be external, internal or absent. They can grow very large and are the largest sea slugs on the reef.

All sea hares are herbivorous, feeding on algae and sea grasses. They usually inhabit silty weedy area in shallow waters.  

Aplysia oculifera (Aplysiidae)
Aplysia  oculifera  (Aplysiidae)

This sea hare can be identified from the small black rings on its body.

Ragged sea hare
Bursatella leachii (Aplysiidae)

This species has branching outgrowths and distinctive bright blue eyespots on its body. They can be found aggregating on protected soft shores especially during spring time for reproduction.

Some species from this family will secrete purple ink from the purple gland upon provoked or stressed, as a defense mechanism to deter predators. Although the ink is considered non-toxic to human if contacted, it might cause irritation to the eyes as well as to other animals. Therefore it is not recommended to touch sea hares.



Clams Oysters (Class Bivalvia)



Bivalves are molluscs with two shells (or valves), held closed by two strong muscles and hinged along one edge. Their muscular foot is compressed and highly modified. In sand-dwelling bivalves, the foot is adapted for burrowing; in sessile bivalves that attach to substratum, the foot is reduced.

Green Mussel
 Perna viridis

Many bivalves are filter-feeders, obtaining food particles from their surrounding waters. When seawater passes over the gills for gaseous exchange, food particles are captured and passed to the mouth by cilia. 



Fan Mussel
Atrina sp.
It occurs in sandy and muddy substrate.
Mimachlamys sp. 
There are small eyes around the margins of the mantle.


Octopus, Cuttlefish & Squids (Class Cephalopoda)



Octopus, squid and cuttlefish are all cephalopods. They are generally adapted for swimming and have the most advanced nervous system of all invertebrates. Their eyes can accurately register shapes, textures and colours. They can find and capture preys by their arms/tentacles.

Cephalopods communicate with each other by movements of the arms and bodies and also through their colour changes. These colour changes, which can occur within seconds, are caused by special cells in the skin called chromatophores that contain granules of pigments, as well as the tiny muscle cells that surround the chromatophores.

Cuttlefish and squids have 8 arms and 2 tentacles while octopus have 8 arms but no tentacles. When stressed, they all are able to release a quantity of ink-like substance from a muscular siphon. The siphon also provides jet locomotion when water is forcefully expelled through it from the mantle cavity. Streamlined squids can swim faster than any other invertebrates, up to 40 kilometers per hour.


Cuttlefish Sepia sp.
Cuttlefish  Sepia  sp. (Sepiidae)

Most cuttlefish are able to alter their colour and texture and thus they should not be considered reliable features for identification.



Each egg capsule contains several developing squids.



Egg Cases of Squids
Egg Capsule of Squid


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