Below you will find a guide to the invertebrate indicator species of Koh Tao, organized by phyla.
Flatworms are indeed flat, and move around using an undulating motion of the body. It is difficult to differentiate the head from the tail end, and they do not have eyes or antennae (although some do have antennae-like folds at the front of the body (referred to as ‘ceros.’)
There are over 20,000 species, and most are very small and parasitic, but the ones found during the EMP are larger (about 2-10cm), and are free swimming carnivores.
Flatworms found on the reefs can be very sensitive to water quality changes and habitat destruction. Seeing flatworms is a sign of good reef health and diversity, having an absence of flatworms could indicate a larger problem on the reef.
Flatworms are mostly nocturnal and are cryptic. They will be difficult to spot, and will generally be hidden during the day.
Drupella snails are carnivorous gastropods that feed on coral tissue (called a corallivore). They are generally small (<5 cm), and are cryptic. They are long-lived (up to 45 years) and generally occur in aggregations (groups).
The three species of Drupella whic.h are coralivores are D. rugosa, D. cornus, and D. margariticola
In small numbers, Drupella help to increase coral reef resilience by introducing small-scale disturbances which improve diversity and open areas for coral settlement. But in high numbers, Drupella snails can consume vast amounts of coral, reducing diversity and abundance, and changing the population structure of the reef. It has also been shown that they are a vector for disease transmission, allowing coral disease to move from one colony to another. For the EMP we want to monitor Drupella numbers to watch for outbreaks or overpopulations.
Because their shells are usually covered in calcareous algae, it is often very difficult to see the Drupella snails themselves. The best way to spot them is to actually look for their effects, the white coral of the reef which has recently been killed due to predation. As you conduct the survey, look for any clean, white coral skeleton, next look around the white skeleton to see if the predator is still around, very often you will find Drupella snails or the Crown of Thorns Starfish nearby.
Also, because it is too difficult to count Drupella, you will estimate their numbers during the EMP survey. At the end of every segment (before moving on to the next segment) estimate the number of Drupella that were in the survey area and write down a number between 0-3:
0 –> None
1 –> A Few (1-50)
2 –> Fair number (50-150)
3 –> Overabundance ( >150)
The Auger Snail is a large carnivorous snail. They have a very effective toxin which they use to neutralize their prey, before consuming it with their powerful mouth, called a radula.
The Auger Snail is considered to be a top-predator, and thus important in top-down controls of the reef trophic structure.
The Auger Snail grows up to about 30 cm long, but lives primarily in the sandy areas on the reef edge. You will most likely not see the snail, but may see the track they leave as they move just under the sand, resembling a trail like in Bugs Bunny Cartoons.
Some areas of the Indo-Pacific, host the Triton Trumpet Snail (Charonia tritonis.) These snails are a top-predator as well, and are one of the predators of the Crown of Thorns Starfish. In such areas, the Auger Snail (Oxymeris maculate) should be replaced with the Trumpet Triton on EMP surveys
The Ramose Murex, or Branched Murex is one of the largest and more ornate carnivorous gastropods in the Indo-Pacific. They tend to be found away from the reef, in sandy areas. You may find a higher abundance of them in areas with many sea urchins and sand dollars, as these are their primary prey sources.
As large carnivores, they are an important higher order predator that has few natural enemies.
They are an economically important species as their large shell is often appealing to the souvenir trade, and numbers are monitored to assess both biodiversity and the threats of over-collection.
Due to their shape, they, unfortunately, are also prone to being entangled in lost fishing nets lying on the sea bed, one of the reasons why such nets should be removed by divers.
The shells are generally covered in filamentous algae or other epibionts and fouling organisms (sponges, tunicates, etc.), and so the snails can often be difficult to differentiate from rubble. This becomes much easier with practice. During times of mating (September-October), large aggregations can be found in and around the reefs. In some cases, over 20 individuals can be found in a 100 m2 area.
Nudibranchs and Other Sea Slugs
Nudibranchs and sea slugs are incredibly diverse; they are a favorite amongst divers for their very striking colors and often ornamental bodies. The diversity of nudibranchs in an area is a good indicator of overall reef diversity. Furthermore, by monitoring the abundance and range of the animals it may be possible to better understand the effects of climate change on water temperatures and chemistry.
You must look closely to find the Nudibranchs and other sea slugs, so take your time when doing the survey. Check around any of their typical food sources (i.e. sponges, hydroids, macroalgae, etc.)
Remember, ‘look but don’t touch,’ you are not permitted to move or overturn objects on the reef while searching for these or any other organisms. Also, be sure to look for the rose like eggs laid by the Nudibranchs, and record that in the notes section when observed.
THE MOLLUSK PHYLUM
The first Bivalve discussed here is the Giant Clam. This clam is a filter feeder, which also utilizes a symbiosis with zooxanthallae (the same as in corals) to build a very large and strong shell. You will always see the giant clam exposing its mantle (skin like tissue) up towards the sun, giving adequate light to its zooxanthallae crop. Because it is the zooxanthallae that give the giant clams their nice colors, no two giant clams are ever the same (like snowflakes).
The Giant Clam in Indo-Pacific reefs is considered to be a keystone species or one of the most important species in maintaining ecosystem health. Some of their important traits include:
There are many different types of clams and mussels growing on the reefs, be sure to only count the colorful giant clams and boring clams, both in the Tridacna Family and containing zooxanthallae (distinguished by their brightly colored mantle).
Although Giant Clams are protected by law throughout most of the Indo-Pacific, in many areas, their numbers are in decline due to over-collection for food and shells.
Due to declining populations around Thailand in particular, the Thai Navy, the Department of Marine and Coastal Resources and Royal Family have instituted a captive breeding and release program.
Species: T. Crocea
Boring clams are in the same family as the Giant Clams, but they bore into the rock and corals. So, while the Giant Clam is free living, these clams require substrate in which to grow. They contain the same photosynthetic zooxanthallae as the corals and giant clams, which allows them to be very productive. This also means that you will most likely find them on the top of coral heads where there is strong sunlight.
See Giant Clams.
In the Conservation Diver ‘Giant Clam Nurseries and Population Studies’ course you can learn much more about how to identify the various species within the family Tridacna. However, for the EMP we keep it simply as Giant clams (those free living with their shell exposed) and Boring (those without their shell exposed as they have bored down into the substrate).
THE MOLLUSK PHYLUM
Octopus, Squid, and Cuttlefish
Interestingly enough, cephalopods including octopus and squid are a part of the same phylum as clams and snails, even though they look very different. As mollusks they do have shells, but they evolved it to be internal; it is their beak. The beak of the Humboldt Squid is one of the hardest materials made by any animal, and has no problem biting through nearly any prey species. Cephalopods have also evolved incredible eye sight and intelligence, and also one of nature’s most effective systems of camouflage. Not only are they able to change colors, but are able to change texture as well, imitating almost any reef surface. They also use their color changing ability for communication.
Because all of these animals play a similar predatory role on the reefs, they are lumped into one group together. If you do see any members of this class while performing the EMP also note the species.
The Reef Octopus is very small, and inhabits the shallow reef areas. They are generally very well hidden and difficult to spot during the EMP. Squid and cuttlefish tend to stay in the deep sea during the day, and follow the zooplankton up at night to feed (diurnal migration).
These animals are quite rare, so observing them is a good sign of reef diversity. As a top predator, they require a large supply of food, and allow us to also gain information on the crustacean abundance around the island.
Although rare in the reef areas surveyed for the EMP, if you see any cephalopods, including Octopus, Squid, and Cuttlefish please also take note on their behavior (hunting, hiding, mating, etc.).
Hermit crabs are specially adapted to fit inside the discarded shells of many marine gastropods. They use their specially adapted soft abdomens to grasp the inside of the shell, and thus utilize the shells for protection. The hard front legs of the hermit crab protrude from the shell, allowing it to walk around and feed.
Hermit crabs are important ‘reef cleaners’, much in the same way as ants in a rainforest. Hermit crabs feed on detritus or algae, acting as a major controlling factor on the abundance of macro-algae in the coral reef environment.
You will have to look closely at the gastropod shells that you see during the EMP survey to see if it is occupied by the snail or by a hermit crab. If you cannot tell, take your best guess – DO NOT pick up or overturn the shell to check. Some snail species are poisonous, and a sting can be fatal in only minutes. If you cannot tell, skip it or take your best guess.
Crown of Thorns Sea Star
The Crown of Thorns Starfish (COTs) is a large, corallivorous starfish with up to 19 legs. This very well protected sea star is covered in toxic spines, which only a few powerful predators such as the Triggerfish and Triton Trumpet are able to overcome.
As a large corallivore, in high abundances they can consume and destroy vast areas of coral, leading to decreased diversity and abundance of the reef. In low numbers (less than 2-3 per dive) they help to open clean coral skeleton for settlement of juvenile corals and other organisms, and imcrease reef resilience.
They can sense chemicals released by stressed corals, and thus remove less fit corals from the reef, much like a lion helps to keep zebra populations healthy by removing the weak or sick.
Like Drupella snails, COTs can be found by looking for feeding scars, or freshly killed corals like in the picture at left.
COTs are on the rise in many areas due to overfishing and nutrient input. With less fish predation, and more nutrients in the water, more of the pelagic COTs larvae survive to adulthood, causing outbreak conditions in the worst cases. Throughout much of the Indo-Pacific, they have reached outbreak proportions, and are regularly poisoned or removed by teams from the diving community.
Do not try to touch or remove COTs while diving unless you are trained and equipped to do so, their spines are very sharp. In certain instances, harming or disturbing the COTs will cause it to release its eggs or sperm in to the water (spawn) which can lead to increased population levels.
The Cushion Star has short arms which give it the appearance of a pentagonal pillow. Although it does not really look like other sea stars in its adult form, as juveniles they do resemble more typical sea stars. Its mouth and tube feet are all located on the underside of the body.
The Cushion star feeds on detritus, small invertebrates, and also hard corals (Primarily fragments that are already under stress). It is monitored for its role in nutrient cycling on the reef, and also its potential to alter coral population dynamics.
The Cushion Star is primarily nocturnal and will often be hiding under coral heads or rocks during the day time.
Long Spine Black Sea Urchin
Internally, urchins are quite similar to starfish, but instead their arms are fused into a round body. Long Spine Black Sea Urchins are very effective herbivores, able to eat much of the macro-algae on the reef which is unpalatable to fish. In areas such as Jamaica, where disease has reduced urchin numbers, algae becomes the dominate form of life in the area.
Sea Urchins are important in regulating macro-algae levels and contributing to the uptake of nutrients from the reef. They are a prey species for large fish and some other invertebrates (like the Horned Helmet). In high numbers they can also indicate an imbalance in either the nutrient levels (too much food availability) or decreased predator abundance (over fishing).
You will notice many different species of urchins on the reef but be sure to only count the ones with the long, thin spines and black body.
Marbled Sea Cucumber
Species: P. graeffei
Sea Cucumbers are still Echinoderms, made up of many of the same parts as the urchins and sea stars, but they have an elongated body, and less distinctive spines.
Marbled sea cucumbers are important in regulating nutrient levels on the reef. They clean rocks and corals, removing microalgae and improving substrate availability for coral larvae and other organisms.
In some areas of the world sea cucumbers, are considered a delicacy, or are used to supplement diets when food is scarce. This practice is incredibly destructive, as sea cucumbers are obvious, slow moving, and easy to collect. The populations of sea cucumbers of an entire reef can be decimated in a single day by an efficient team of collectors in dive gear.
Black Sea Cucumber
Species: H. atra
The Black Sea Cucumber is smaller and less robust as the Marbled Sea Cucumber. They can be found mostly in sandy areas, or in the reef flat and back reef zones. They consume sand, and remove the organic matter or tiny invertebrates from it as it passes through their bodies.
Black sea cucumbers are an important regulator of nutrients and organic matter flowing from land to the reefs. Their numbers reflect the available food supply, and thus are an indicator of water quality.
To defend themselves, sea cucumbers can release their inner tissues, which are sticky and often toxic. This can protect them from predators, but comes at a large cost, as it may take over a month for the tissues to regrow.
Pinkfish Sea Cucumber
Species: H. edulis
The Pinkfish Sea Cucumber (also known as the edible sea cucumber) is very similar to the Black Sea Cucumber, but has a pink strip down the belly. They can be found mostly in sandy areas, or in the reef flat and back reef zones. They consume sand, and remove the organic matter from it as it passes through their bodies.
Much like Black Sea Cucumbers, it is responsible for removing organic matter and biofilm from the sand to prevent algal overgrowth. Furthermore, as it feeds it aerates the sand, improving the conditions for burrowing invertebrates and allowing for aerobic conditions in which bacteria grow.
The Pink Fish Sea Cucumber is differentiated from the Black Sea Cucumber as it is much more likely to be collected as food, especially in China or Indonesia.
Orange Spiked Sea Cucumber
Species: S. chloronotus
The Orange Spiked Sea Cucumber is more robust, larger, and stronger looking than the Black Sea Cucumber. They inhabit sandy to rubble covered areas. Like the other sea cucumbers, they are detritivores who eat anf filter the sands.
Much like the other sea cucumbers, they remove organic matter, biofilms, and small organisms from the sand as it is processed by their digestive system. They also help to aerate the upper layer of sand to prevent it from becoming anoxic.
Sea cucumbers are important hosts for many parasitic or commensal fishes, shrimps, crabs, and worms. Sea cucumbers are also some of the fastest healers of the animal kingdom. This species can actually reproduce asexually when it is separated longitudinally due to its ability to heal and regenerate.