The world is largely dominated by invertebrates, far outnumbering vertebrates in both abundance and diversity. The same applies to the marine world and therefore there is great importance given to the process of dividing and classifying the hundreds of thousands of different species. This is done via the ever increasingly complex world of taxonomy.
Marine invertebrates can be initially divided by their morphological symmetry, that is, how an animal’s body can be geometrically be mirrored along a line. The three forms of symmetry accepted for this are:
  • Bilateral symmetry – A body shape that has a single line of reflection
  • Radial symmetry – A body shape that has multiple lines of reflections, often about a central point.
  • Asymmetry – A body shape has no obvious symmetry with no possible lines of reflection.


A relatively small phylum that covers flatworms.

Christmas Tree Worm (Spirobranchus giganteus)

Spirobranchus giganteus is a species of polychaete worm lives within tubes burrowed into solid substrate. The feathery like colourful appendages that are on display outside its tube home are used for both respiration and as a means to capture its microscopic food. These worms comes in a wide variety of colours and can most often be seen on hard corals of the genus Porites on the reefs of Koh Tao. This species is arguably the most commonly seen species of Annelida on our coral reefs.


An extremely large phylum that covers segmented worms in both terrestrial and aquatic environments.

Marine Flatworms

These ornate animals are not the most common organism on the reef but are usually a treat when spotted. Often confused with sea slugs, marine flatworms have a characteristic paper-thin body and can sometimes be seen swimming in the water column by rhythmically waving its body. Marine flatworms have evolved to mimic their body patterning to resemble other organisms for protection. This is known as ‘batesian mimicry’ and is used for camouflage and to confuse predators.


A subphylum within Arthropoda that includes crustaceans. This diverse group make up a large portion of the diversity within a coral reef ecosystem. Some species of note are:


Commonly known as cleaner shrimp, there are actually many species of shrimp in at least 3 families that fit the description of cleaner shrimp. These shrimp fill a similar ecological niche within the invertebrate world as cleaner wrasse do in the fish world, often in conjunction with them. Cleaner shrimp are responsible for the removal of ectoparasites from surface skin layers of fish, often by congregating in cleaning stations. They can usually be seen in darker places, such as in holes, caves and under table corals.


Alpheidae is a family of shrimp, often called pistol shrimp, that are characterised by asymmetrical claws. On the reef they are commonly seen among gobies in their homes in sandy areas. This is an example of a comensal relationship. The shrimp have the capabilities of digging and maintenance of the hole, and the goby stays at the mouth of the hole as a sentry looking out for danger. This relationship benefits both individuals and has been in place for so long that the shrimp have evolved to be largely blind, as the visual requirements are limited due to the vigilance of the goby.


Species of the family Paguroidea are crustaceans that have evolved to develop an uncharacteristic soft abdomen without any exoskeleton for protection. This is due to their reliance on an external shell, within which they live. These shells, usually discarded gastropod shells give the individual protection and improved status within the hierarchy of hermit crabs. As omnivorous benthic detritivores, hermit crabs play a crucial role in the ecosystem as scavengers of organic matter, therefore aiding in decomposition at the bottom of the food web.


This diverse phylum includes sea stars, sea urchins and sea cucumbers among other things. In adulthood many Echinoderm families show radial symmetry in their morphology but as juveniles were only bilaterally symmetrical and are therefore classified as so. Many species of Echinoderm are considered indicator species for coral reef health. These and others are discussed below.



Acanthaster planci is a species of predatory sea star that feeds largely on coral. This is an important indicator species on the reef as it a coralivore (consumer of coral) and in high numbers, such as spawning seasons, can be responsible for large amounts of coral predation. These large echinoderms have up to 21 arms and can reach sizes of half a meter in diameter. In recent decades outbreaks of these coralivores have been experienced throughout the Indo-Pacific Region. The NHRCP regularly conducts Crown of Thorns collections in areas where they are over abundant or where we have reef restoration projects.


In general, echinoderms are slow moving, benthic grazers. Members of the class Crinoidea, which include feather stars, follow this pattern but for a few exceptions. Feather stars have evolved their arms to filter feed and therefore the feather like arms for which they are commonly named are remain largely in the water column while the rest of the body remains temporarily anchored to the substrate. Due to their light and increased pelagic nature, they are also considerably more mobile than their sea star cousins.


Brittle stars belong to the class Ophiuroidea which describes almost 2000 species. These echinoderms are scavangers and live amongst and under rubble. They are relatively fast moving, especially when disturbed, quickly receding to sheltered and darker areas. Most species seen in the Gulf of Thailand are small, usually less than 10cm across.


Members of the genus Culcita are found sparsely on our reefs but are still noticeable due to their size. The most common species of cushion star on the reefs of Koh Tao is Culcita novaeguineae. It is found in small numbers, and shows no tendency to form aggregations. As one of the larger species of echinoderm on the reef, it is fairly easy to spot and is not easily confused with many other species. The diet of this species is characteristic of its genus, largely detritus and small invertebrates. It is also known to feed largely on hard corals of a variety of genera but has not displayed extensive predation on our reefs.



Commonly found in reef areas worldwide, species from the genus Diadema are keystone species for most reefs. On the reefs of Koh Tao, Diadema setosum is an indicator species that can be found in large numbers and aggregations. Commonly referred to as the Long-spined black sea urchin, this species is a highly effective herbivore, consuming much of the macro algal matter on the reefs. They are considered to be an indicator species for this fact, as, in high numbers, they are an indication of large amounts of food availability, and therefore, high nutrition levels in the ecosystem, which is detrimental to coral growth, favoring algal growth instead. This species is often confused with pencil urchins which are smaller urchins with thicker spines.



Pearsonothuria graeffei is the only species in its genus and is found widely in the Indo-Pacific region. Commonly referred to as the Marbled or Graeffe’s sea cucumber, this species is one of four important sea cucumber indicator species on the reefs of Koh Tao. They are benthic scavangers, and play an important ecological role as decomposers as well as clearing substrate of algae, thereby promoting coral settlement.


Holothuria edulis is the Pinkfish cucumber and is often confused with the black cucumber due to similar size and external morphology. Both species are cylindrical growing to approximately the same size and, when seen from above, look very similar. The Pinkfish has a dark blue-purple upper section and a very pink bottom section and it is from this that it is named and can be differentiated from the Black cucumber. Its role in the ecosystem is largely the same as the Black and Orange Spiked species and is therefore considered to be an indicator species.


Stichopus chloronatus, known as the Orange Spiked or Greenfish sea cucumber is a dark species with a rectangular cross section covered in orange tipped projections known as papillae. It is smaller in size than the Marbled cucumber but larger than the Black and Pinkfish species. It is considered an indicator species for the coral reefs of Koh Tao due to its importance in the ecology of the reef. Unlike the Marbled cucumber, the Orange Spiked lives and feeds largely on rubble and sandy areas where it consumes plant and animal debris, helping to keep the nutrient levels in the water low and aerating the substrate.


Holothuria atra is the black sea cucumber. It is considerably less rigid and ornate than the previous two species but plays an equally important role. The black sea cucumber, much like the Orange spiked and the Pinkfish cucumbers filter organic matter from the sand that they feed on and are therefore incredibly important in controlling nutrient levels and are also an indicator of available food supply.


Synapta maculata is a large species of sea cucumber seen occasionally on reefs, more often in sandy and muck areas. This long sea cucumber is a benthic scavenger like other sea cucumber species and can often be seen with commensal shrimp living on it.


The largest marine phylum, molluscs comprise at least 23% of all marine species. For our purposes we divide molluscs into 3 broad groups. Gastropods are snails and slugs of the sea, cephalopods are complex molluscs including squids, cuttlefish and octopus, and bivalves are shelled molluscs such as clams and oysters.



The Queen Helmet Conch is actually a large species of murex snail. It is referred to as the Thai or Queen Helmet Conch on Koh Tao. It is one of the largest gastropods in the Gulf of Thailand and feeds mostly on micro invertebrates such as small crustaceans, other gastropods and simple echinoderms. Chicoreus ramosus lives largely in deeper sandy areas but are often seen in sandy muck areas and will occasionally venture into shallow sandy areas and even near coral reefs.


Spider Conches belong to the genus Lambis. Spider Conches are the smallest of the 3 conches seen on Koh Tao and are sometimes confused with small helmet conches due to the finger like projections, known as digitations. The spider conch is largely herbivorous, feeding primarily on red algae among other things.


Oxymeris maculata is a benthic snail that lives in sandy areas, including those within and near reefs. It is an auger snail and is locally known as the Thai trumpet triton snail. It spends most of its life underneath the sand, where it feeds on a variety of smaller gastropods and echinoderms. However the Trumpet Triton snail is also an incredibly important indicator species as it is one of the few remaining species on the reefs that preys on the Crown of Thorns sea star discussed earlier.


The drupella snail is an important indicator species on our reefs. The snail itself is a coralivore (predator of corals), and feeds largely on hard corals. Individuals are small, usually less than 5cm long, and tend to feed in groups. These aggregations can range from pockets of a handful to areas of over a hundred. Monitoring numbers of drupella snails is important as, in extreme cases, aggregations can be responsible for predation of considerable portions of coral on the reef. The NHRCP monitors drupella snail abundance around the island and also preforms regular collections of Drupella snails.



Three main types of cephalopod exist on Koh Tao, Cuttlefish, Squid and Octopuses. Cephalopods are the most highly evolved of all molluscs, and this can be seen by the incredible mental acuity for invertebrate species and their use of chromatophores. They are not common on the reefs of Koh Tao, with cuttlefish being predominantly found on soft substrate such as sand and silt.



Tridacna gigas or the Giant Clam is arguably one of the most important indicator species on any coral reef. The giant clam is a large bivalve that can grow up to 1.2 meters across and weigh up to 200kg, creating dense shells that are exposed. These shells provide high quality substrate for recruits of hard corals when the giant clam dies.


Unlike Tridacna gigas, the species Tridacna crocea (boring clam) does not have an exposed shell, instead, the animal bores itself into a hard substrate such as rock or coral skeleton, hence its name. Boring clams tend to grow to a much smaller size than giant clams but are sometimes difficult to distinguish between giant clams whose shells have been hidden. One simple way of telling the difference is the presence of an obvious excurrent and incurrent siphon on the giant clam mantle, whereas both holes are not exposed in the instance of the boring clam. Unlike the giant clam, the boring clam does not filter as much water nor does it have an exposed shell that can be used for structure and substrate on the reef. However, the boring clam is an indicator species because, like the giant clam, the boring clam stores zooxanthellae in its tissues thereby being an important potential safeguard in times of coral stress.
When it is alive, a single giant clam can filter over 1000 litres a day, helping in the removal of the nutrients from the water which promote macro-algal as opposed to coral growth. Giant clams also support hard coral reefs by storing photosynthetic algae in its tissues which it uses to produce energy from the sun. These same algae, known as zooxanthellae, have a symbiosis with all hard coral species and therefore giant clams can act has small stockpiles of this valuable algae in leaner times such as coral bleaching events. These are just some of the reasons why the NHRCP is actively involved in many different giant clam monitoring, nursery, and restocking projects around Koh Tao.


This purely aquatic phylum exhibits only radial symmetry and is named after cnidocytes, stinging cells used primarily in prey capture.

HARD CORALS (Scleractinia)

The worlds most diverse and beautiful habitats start of with just one type of animal. Coral reefs are some of the most productive and complex ecosystems, yet it is often forgotten that the reefs themselves are animals. Hard coral colonies are comprised of many smaller polyps which are the individual animals, with some of the older, larger colonies being made up of hundreds of thousands of polyps, all genetically identical. The variation in the genetics of a species of coral are seen between colonies as opposed to within a single colony. Colonies grow via asexual reproduction, via a process known as budding, to create complex growth forms which results in all of the beautiful structures of the reef. Hard coral species also partake in sexual reproduction where eggs and sperm from different colonies are released, sometimes post fertilization, and these become pelagic larvae that settle on hard substrate to form new, genetically novel colonies.
Each polyp within a colony is actually a symbiosis. As mentioned earlier, photosynthetic algae called zooxanthellae, live within the tissues of coral polyps, giving them their color. Polyps are largely colourless themselves but are seen to be the colour of the zooxanthellae that live within it. During coral bleaching events, the coral animal disposes of the algae within their tissues due to conditions causing the algae to cause more harm than good, thereby removing all colour from the animal. Since hard coral polyps secrete a calcium carbonate skeleton to give them structure, the corals look white in times of bleaching, when actually what is being seen is the skeleton set down by the polyps and the thin translucent film on the surface are the animals themselves.

SOFT CORALS (Octocorallia)

Soft corals, in the order Alcyonacea, are similar to hard corals in that they are a colony of polyps that form an overall structure. However, unlike hard coral species, soft corals do not lay down a hard calcium carbonate skeleton as substrate. Instead, soft corals use sclerites, which are small skeletal parts, to provide a basic structure. Like hard corals, soft corals utilise a symbiosis with zooxanthellae for most of its energy.

ANEMONES (Actinaria)

Sea anemones are actually large, solitary polyps in the order Actinaria. Most species are sessile, attaching themselves to the substrate via their basal disk, anchoring them to a spot so they can capture passing prey. Many species have a symbiosis with zooxanthellae, much like other species of invertebrates discussed so far, allowing for greater energy acquisition and in turn, protecting the algae from predators.

JELLYFISH (Scyphazoa)

The Floating Bell – Perhaps the most famous type of animal within the phylum known for its stinging cells, jellyfish species are remarkable animals. It is easy to remember the things that jellyfish are commonly known for, pelagic drifting animals with often extremely long tentacles with powerful stings on them. Jellyfish are single animals that are responsible for huge booms in populations, often resulting in dead zones, yet they are also some of the worlds most unique habitats. Many species of jellyfish, both in life and death, host small schools of fish within the spaces of its body. This commensal relationship offers protection for the fish, often juveniles, from predators due to the stinging cells that adorn the tentacles of the jelly. The jellyfish gains little by hosting these fish, and since many of the fish species have mucous based coatings to protect them from the stings, the jellyfish cannot feed on them.
Though population numbers are seen to be growing out of control, it must be remembered that they remain and important food source for many animals such as sea turtles, butterfly fish and various species of shark. Numbers of predators of jellyfish have reduced over the years due to anthropogenic causes so, much like the Crown of Thorns sea star discussed earlier, without natural predators and intervention, the ecosystem cannot be balanced and population growth can excel unchecked.


Showing no symmetry, sponges are easily distinguished from most other phyla of marine invertebrate.This ancient phylum is older than all other multicellular phyla. Sponges are very old, considered simple yet highly evolved organisms. Most species are marine, save for a small number of freshwater species, and all have a relatively similar and basic morphology. On coral reefs, sponges, especially larger ones such as the barrel sponge, are water filtration powerhouses. They depending on filtering the water for food, respiration and even waste removal, therefore, they process huge amounts of water on a daily basis.

Some large sponges on coral reefs create a symbiosis with Cyanobacteria and Eubacteria that live within its tissues. This symbiosis allows for these sponges to produce up to 3 times more oxygen than they use in respiration, making them significant contributors to oxygen levels in reef systems. However there is also a less productive side to the effects of sponges on coral reefs. Certain species of sponges grow over substrate and are hardy enough to grow over coral, with some species particularly adapted to growing over hard coral colonies, thereby smothering them and killing the polyps. These species grow at a faster rate with increase in nutrient levels causing corals to compete with growth boosts in both suffocating sponges and macroalgae.

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