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Hong Kong Corals & the Associated Marine Life
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. Squid, octopus, many bivalves and various gastropods are highly prized for food.
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Shell Collecting
Shell collecting is a common hobby for all ages. If you do not do it properly, you may impair our ecosystem.
You are strongly encouraged not to collect live shells. If you really need to do so, it is a good practice to limit the number of live shells taken. You should not collect immature shells or female shells with eggs. Also, please don't collect live shells with scars, holes or other imperfections because they will likely be abandoned subsequently. To minimise unnecessary damages to the natural marine habitats, it is important to return slabs and rocks to their original positions after your search.
It would be the most disastrous if you "collect" shells from souvenir shops. Most of the flawless shells available in the souvenir shops came from live shells, especially the rare and expensive specimens. The more you spend in the shops, the more damages you do to the nature.
The most environmental friendly and hence the recommended approach is to collect dead shells only. The beauty comes not only from the shells collected, but also from your heart.
As you increase your shell collection, please do not forget that some bivalves and gastropods are protected under the Protection of Endangered Species of Animals and Plants Ordinance (Cap. 586). Import, export and possession of those protected species are restricted.
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Snails & Sea Slugs (Class Gastropoda)
There are about 35,000 described species of gastropods in three sub-classes, of which Prosobranchia (i.e. snails) and Opisthobranchia (i.e. nudibranchs, sea hares, bubble shells and others) are marine.
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 in the shell. Some snails have a "trapdoor" or operculum to seal the aperture.
Most snails have a unique file-like mouth part called radula. Herbivorous species use it to rasp or cut algae from rock. In carnivorous cone shells, the radula could be modified into a harpoon-like structure for injecting toxin into their preys. It could also be modified into a drill for drilling holes in the prey shells.
Some carnivorous snails, such as Conus geographus, can kill vertebrate prey. Their neurotoxin venoms are fatal to human. Snails in genus Drupella attack and kill stony corals. They line up around the living coral and suck the soft tissue with their long proboscis.
Sea Slugs (Sub-class Opisthobranchia)
The evolutionary trend in opisthobranchs has been toward a reduction or even complete loss of the protective shell. The animals may have an external or internal shell, or even have no shell at all. They include nudibranchs, sea hares, side-gilled slugs, sap-sucking slugs and so on.
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| Mating |
All species of opisthobranchs are simultaneous hermaphrodite. Each individual has 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 direction 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.
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| Spiral egg ribbons |
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| Spiral egg ribbons |
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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 weapon is 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. Eolid nudibranchs feed on sea anemones and hydroids. They managed to pass the undigested stinging cells from the prey to the tips of their tentacle-like cerata for defence.
Most nudibranchs are highly selective carnivores. They prey on sponges, hydroids, sea anemones, sea squirts and other opisthobranchs. Nudibranchs have a short life cycle, lasting from several weeks to a year.
Of the nearly 2000 species of nudibranchs known from the Indo-Pacific, some 230 species have been recorded from Hong Kong and 5 of them are new to science.
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 longest opisthobranchs on the reef.
All sea hares are herbivores, feeding on algae and sea grasses. They usually inhabit silty weedy area in shallow waters.
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 carnivores, feeding mainly on sponges, tunicates, and other sessile invertebrates.
The chemical weapon they used is an acid secretion that can cause a burning feeling to skin.
Like the nudibranchs, the body form of sap-sucking slugs is highly specialized and variable. Their oral tentacles are very small or absent, and the gills are usually missing. The most obvious external feature is 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.
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.
Many bivalves are filter-feeders, obtaining food particles from surrounding water. When seawater passes over the gills for gaseous exchange, food particles are captured and passed to the mouth by cilia. Some bivalves possess symbiotic algae (zooxanthellae) which provide at least a portion of their nutrients. The best example is the Indo-Pacific giant clams in genera Tridacna and Hippopus. The symbiotic relationship is the major reason that the clams can grow to 1.5m in diameter.
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 the colour changes. These colour changes, which can occur within seconds, are caused by special cells in the skin called chromatophores that contain granules of pigment, 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 forcibly expelled through it from the mantle cavity. Streamlined squids can swim faster than any other invertebrates, up to 25 miles per hour.
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