Sea stars (class Asteroidea) are animals with a somewhat star-shaped body, made up of several arms extending from a central disc. They belong to a bigger group of animals called the spiny-skinned animals (phylum Echinodermata - "echino" roughly means "spiny"; "derma" roughly means "skin"). Examples of other echinoderms include sea urhins (possibly where the term "spiny-skinned" come from?), sea cucumbers, and feather stars.
Like other echinoderms, sea stars generally have a five-part body plan with radial symmetry (i.e. pentaradial symmetry), at least in some stage of life. In other words, you can divide a sea star into 5 equal parts (imagine a star-shaped pizza...). Also, any keen follower of "SpongeBob SquarePants" will probably know that echinoderms are brainless (like Patrick Star). But despite that, they can still perform their daily functions - they can move, they can eat, they can shit, and they can reproduce - all these without a brain!
One other characteristic that makes the echinoderms so different from us, is that instead of blood vessels, they have a water vascular system. This system is essentially a network of water-filled vessels used for internal transportation of oxygen, food and waste. The sea star can also move by changing the water pressure, which extends or contracts the tube feet on its underside. The tube feet may also be used for breathing, sticking to hard surfaces, and feeding - they work like a conveyor belt by transporting food from the tip of an arm to the mouth in the middle.
Sea stars are also known for their amazing ability to regenerate lost body parts - such as an arm bitten off by a predator. Still, it would take lots of energy and resources to regrow lost body parts, and in the meantime, the sea star will be moving slower, making it more vulnerable to predation and less efficient in finding food.
More than 30 species of sea stars have been recorded from Singapore, but unfortunately, many are threatened by loss of habitats due to reclamation and coastal development, and some are collected by poachers.
Members of this family are usually rather flat below, but convex on the top side, with tapering arms. The arms' marginal plates are well-developed but may or may not be conspicuous looking from the top, and may be concealed by thickened skin.
They also have cylindrical tube feet with a terminal disc. They are usually rather massive - adults are usually more than 10cm wide - with a massive central disc.
The Knobbly Sea Star (Protoreaster nodosus) from this family is one of the bigger sea stars found in Singapore, and some can be more than 35 cm wide! This is perhaps also one of the prettier sea stars, as they occur in various colours, ranging from bright red, orange and pink to dull colours such as brown and beige. They have a hard, calcified body with large nodules on the top surface, which protects them from most predators except fish with sharp and powerful teeth, such as pufferfish and triggerfish. Indeed, every now and then I will see individuals with broken nodules or arms.
Despite the big size, this sea star feeds mainly on microorganisms, although it has also been observed to feed on snails, clams, soft corals and sponges. Like most other sea stars, they feed by everting their stomach over their food and digestion takes place externally.
The Cushion Star (Culcita novaeguineae) is another member of this family, and is possibly the heaviest sea star in local waters, weighing to more than 5kg! Mature ones are more pentagonal in shape and appear somewhat bloated - like a cushion!
Juvenile Cushion Stars appear very different from the mature ones though. They are a lot thinner with more obvious arms. Juvenile cushion stars usually feed on encrusting algae, while mature ones preys on corals, other small organisms and tiny decaying particles (detritus). They usually feed at night, and in order to maintain its huge body mass, they are continuously moving in search for food. It can grow to about 40cm across.
The Mammillated Sea Star (Pentaceraster mammilatus) is another of the bigger sea star that can be found here. It's about the same size as the closely related knobbly sea star, and is usually found in the same habitats. "Mammillated" means "having nipple-like projections", and is certainly a rather appropriate description of this sea star. It was first recorded from Singapore only in 2008, and is likely an introduced species, possibly from ballast water.
Since then, we have been seeing sea stars that suspiciously appeared like hybrids between this sea star and the knobbly sea star. It's probably too early to determine the ecological implications though.
The Rough Sea Star (Anthenea aspera) can grow to more than 20cm across, making it one of the bigger sea stars too. "Aspera" means "rough" in Latin. Indeed, if you run your fingers over the surface of this sea star, it feels rough, despite the "smooth" appearance, due to the numerous tiny spinelets on the top surface.
This sea star certainly comes in many colour variations - pink, red, orange, green, brown, black, and not to forget the many different patterns on its top side. Indeed, it also may not be easy to spot them among the seagrass or coral rubble if the colours happen to be just right and blend into the surroundings. This sea star has many tiny clip-like structures on its underside, and some on its topside called pedicellariae. Exact functions of these structures are poorly understood, but they are generally thought to keep the sea star's body surface clear of unwanted stuff, such as dirt, algae or encrusting organisms.
Juvenile Rough Sea Stars are often confused with the Smooth Sea Star (Gymnanthenea laevis), a closely related sea star. Unlike the former which feels rough to the touch, G. laevis is covered with a smooth, thick skin on the top side. The marginal plates also form a smooth margin. Hence, the species was named "laevis", which means "smooth". Large groups of small protruding structures (or papillae) can be found on the top side, while big, conspicuous pedicellariae (clip-like structures, as shown in the previous species) can be found both on top and below.
The Scabby Sea Star (Goniodiscaster scaber) is often called biscuit sea star locally, as it has a regular star shape and look as if it's cut out using a cookie cutter. Smaller specimens are more commonly seen, but bigger ones can grow to more than 15 cm wide. The species name "scaber" means "scabby" or "rough", possibly refering to the rough and broad scab-like marginal plates.
Members of this family are usually small-sized or moderate, covered with granules on the top side. The arms are usually flat below, and flattened or rounded above. The marginal plates are inconspicuous. The tube feet are cylindrical with a terminal disc.
The Rock Star (Aquilonastra coronata), also called the crown sea star, which grows to about 2cm wide. This sea star usually hides among crevices or under rocks in the day - and that's why we called it the rock star! It has numerous crown-like spinelets, and hence the species name "coronata", which means "crown". It emerges at night to feed, possibly on algae or detritus, sometimes on the adjacent seagrass meadows. Most of them have dull colours, but we often see brightly coloured ones as well!
Another small sea star found in rocky areas is the Cryptic Rock Star (Cryptasterina sp.). This sea star blends in very well into the surrounding rocks with its mottled patterns, and this certainly helps it to avoid predation. The ones I have seen are not more than 3cm across. So far I have only seen them on some of the Southern Islands. During low tide, it clamps itself tightly against the rocks to prevent desiccation. It feeds on algae on the rocks.
In some of of our seagrass meadows, a few Nepanthia spp. can be found. The arms are finger-like, usually flat below, but somewhat cylindrical above. The top surface has a mottled or blotchy pattern, and can be red or brown. It is not sure if they are of the same species or different species. The red one could be Nepanthia belcheri, but it lacks some of the features description in some of the identification keys, so I am not too sure as well.
The smallest and lightest sea star I have seen is a tiny unidentified one from the family Asterinidae. It is less than a centimetre wide, but we still have not been able to confirm what species it is. I am sure there are even smaller species in our waters though, just that I have not seen them.
This family is represented by just one member in Singapore. Members are of a moderate size, and the top surface is generally quite flattened. They have spines lining the margins of the body. The arms taper to a pointed tip. The tube feet are cylindrical with a terminal disc.
The Sand-sifting Star (Archaster typicus) from this family is possibly the commonest sea star in Singapore, and hence many nature guides also call it the common sea star. It has many common names though, but I personally prefer to call it the Sand-sifting Star, as I thought the name itself tells the story of how the sea star behaves - it sifts among the sand to avoid predation and to forage for detritus to feed on.
Unlike most other sea stars, the Sand-sifting Stars have a rather interesting reproduction behaviour - males are often found stacked on top of the females, and the pairing may last for up to 2 months before the eggs and sperm are released into the water. The reproductive organs do not meet, and hence this behaviour is termed "pseudocopulation" - in other words, "fake sex". This behaviour apparently increases the chance of fertilisation though.
Members of this family superficially resemble those of the previous family - with the margins lined with spines. However, they have tube feet tapering to a knob with no terminal disc.
Also, the marginal plates with the spines are larger than the paxillae (the round, scale-like patterns on the top surface).
Sand Stars (Astropecten spp.) are rather common in Singapore. They usually hide in the sand in the day, and emerge when it's cooler and darker to hunt small prey such as clams and snails.
A few different Sand Stars have been seen in Singapore, but it's a challenge to determine the exact species. Unlike most sea stars which digest their food externally, sand stars swallow their food whole and digest them internally.
Most Sand Stars have whitish undersides, as seen in the above photo, featuring one feeding on some button snails.
One particular Sand Star, however, has an orange underside instead.
This is how the Sand Star with the orange underside looks like with the right side up.
Members of this family are very similar in appearance to those of the previous family, with their marginal spines and tapering tube feet. And similarly, they swallow their food whole and digest them internally.
To differentiate the two families, look at the marginal plates of the luidiids from the top - they are not distinguishable from the paxillae, unlike the broad and conspicuous marginal plates found in the previous family.
The biggest sea star I have seen in Singapore, based on body width, is from this family - the Maculated Sand Star (Luidia maculata). The biggest I had seen was 60cm wide! Anyway, I used to call this the eight-armed sand star, until I started seeing a number of them that had 9 arms. This huge sea star burrows in soft sand and feeds on other echinoderms and molluscs. "Maculata" or "maculated" means "marked with spots or blotches", refering to the darker patches on the surface.
Sometimes, whole populations of Maculated Sand Stars with only 6 arms are seen. It is not known if they are variations of the same species or are of a different species altogether.
The Penang Sand Star (Luidia penangensis) usually comes with 6 arms, and of a dark grey colour.
The madreporite of this sea star is very large and conspicuous. And each paxilla on the top surface has an enlarged wart-like central spinelet.
Penang Sand Stars have orange tube feet, distinguishing them from other similar-looking Luidia sea stars in Singapore.
The above is most probably a Hardwick's Sea Star (Luidia hardwickii). It typically has 5 arms.
The tube feet are slight brownish to white.
Members of this family have large marginal plates, forming a conspicuous side-wall to the body. The top side is usually quite flat. The arc between adjacent arms appears rounded. The tube feet are cylindrical and with a terminal disc.
The Galloping Sand Star (Stellaster equestris) from this family is occasionally seen on sand banks near coral reefs. As the name suggests, it can indeed "gallop" by moving on the tips of its arms. There is a row of spines lining the margins.The underside has obvious blotchy patterns.
In the coral reef, divers often come across the Icon Star (Iconaster longimanus) of the same family on lower reef slopes and seabeds. They feed on algae. While most sea stars which broadcast lots of eggs, icon stars produce small numbers of large eggs. The eggs are orange in colour and apparently contain chemicals that deter fish predators.
Lots of other sea stars can be found in Singapore, but unfortunately I do not have photos of them. Here are a few other interesting facts about sea stars:
1. They have an internal skeleton (endoskeleton) made up of calcium carbonate (ossicles), somewhat like us.
2. Most sea stars have an anus on the upperside, but not the Astropecten and Luidia species.
3. Sea stars have light-sensitive eyespots at the tips of their arms, and they often keep the tips slightly tilted upwards so that they can sense their surroundings better.
4. Like most other marine organisms, the sea stars in Singapore are threatened by loss of habitats and collection.
Quoting from Romeo & Juliet: "A rose by any other name would smell as sweet."
As such, unlike many other local nature guides, I never insist that my participants stop calling the asteroids "starfish", and switch to calling them "sea stars". I guess that's as good as asking them to call a cuttlefish "sea cuttle", a horseshoe crab "sea horseshoe", a jellyfish "sea jelly", a hermit crab "sea hermit", or even a feather star "sea feathers" (and in fact some people do call them as such).
To me, these are just common names, and so it's a matter of personal preference or habit. In my case, ironically though, I personally prefer to call the asteroids "sea stars", not for any scientific reasons, but more because I think it sounds nicer. Haha...
As long as the guide explains clearly that a starfish is not a fish, and a horseshoe crab is not a crab etc, I think it is sufficient. If anyone is so anal about getting it scientifically correct, then they might as well go and lead a scientific group and use scientific names - common names are for the general public and they were never meant to be scientific in the first place.
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