Tuesday, April 03, 2012

Echinoderms (Phylum Echinodermata) of Singapore

Before I became a nature volunteer, I never knew that it was so easy to find sea stars in Singapore. And perhaps, it was this fascination with sea stars, more commonly called starfish, that kept me going back to our sea shores. And for the past 7 years, I have indeed not just saw many sea stars, but many of its relatives from the phylum Echinodermata as well, such as brittle stars, sea urchins, feather stars and sea cucumbers. And so finally, I decided to put up what I know about sea stars and their relatives here, just to make it easier for participants of my nature walks to refer to after every walk :)

Starfish (Class Asteroidea)

Pentaradial Symmetry
The phylum Echinodermata comprises marine animals with 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 an echinoderm into 5 equal parts. Their larvae are bilaterally symmetrical though. Echinoderms are also called spiny-skinned animals (phylum Echinodermata - "echino" roughly means "spiny"; "derma" roughly means "skin"). The name possibly originated from the spines of sea urchins.

Water Vascular System
Another important characteristics of echinoderms will be their water vascular system. This system is essentially a network of water-filled vessels within their body, terminating usually at numerous tiny tube feet. These vessels are used for internal transportation of oxygen, food and waste. By adjusting the water pressure, the echinoderm is also able to extend and retract its tube feet or other body parts for locomotion too.

Catch Connective Tissue
Echinoderms are made of a special tissue called catch connective tissue, which allows many of them to harden itself when harassed by a predator, but become soft sluggish enough when necessary to creep into cracks and crevices.

Regeneration & Reproduction
Ehinoderms are also known for their amazing ability to regenerate lost body parts, such as arms bitten off by predators or even internal organs expelled to deter predators. Some species are even able to regenerate a whole animal from a detached armed or split into 2 halves and regenerate the missing halves to become a whole animal. In other words, some echinoderms can reproduce asexually by fission. They also reproduce sexually, mostly by releasing their eggs and sperm into open water for external fertilisation.

Without a Brain
Interestingly, echinoderms are brainless, 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! Some studies have shown that certain species of echinoderms can live for hundreds of years. So, perhaps our brains are really over-rated? Haha...

Here are the various groups of echinoderms found in Singapore. Click on the links for more details:

A) Sea Stars (Class Asteroidea)

Sea Stars (Class Asteroidea)
Sea stars or starfish (class Asteroidea) are animals with a somewhat star-shaped body, made up of several arms extending from a central disc. The mouth of the sea star is located on its underside, and numerous tube feet run along the length of each arm, working like conveyor belts to transport food particles from the tip of the arm to the mouth in the middle. The sea star mostly move on the substrate by gliding along with these tube feet too, instead of swinging their arms like what was usually shown in cartoons.

B) Brittle Stars (Class Ophiuroidea)

Brittle Stars (Class Ophiuroidea)
Brittle stars (class Ophiuroidea) are marine animals typically with five thin, segmented arms extending from a central disc. "Ophiuroidea" means "snake-like", refering to the animals' long and flexible arms, and hence they are also often called serpent stars. When disturbed, brittle stars cast off parts of their arms (thus the common name "brittle star"), not unlike a lizard's tail, to distract any possible predator. Unlike sea stars, they swing their arms to move around.

C) Feather Stars (Class Crinoidea)

Feather Stars (Class Crinoidea)
Feathers stars or crinoids (class Crinoidea) are marine animals with five or more feather-like arms. Each arm carries comb-like structures called pinnules, comprising rows of tube feet used for capturing plankton. On the underside, most feather stars have claw-like arms called cirri to hold on to the substrate. Their skeleton is made of numerous calcareous plates.

D) Sea Urchins (Class Echinoidea)

Sea Urchins (Class Echinoidea)
Sea urchins (class Echinoidea) have a round, rigid skeleton (test) made of interlocking calcite plates. The test is hollow and covered with lots of spines on the outside. The mouth of a sea urchin is on its underside, comprising five elongated vertical jaws held together in a structure known as the Aristotle’s lantern. There are two main groups of sea urchins: the regular sea urchins with spherical tests; and irregular sea urchins with more flattened tests that are bilaterally symmetrical. The latter includes the heart urchins and sand dollars, and they generally lack the Aristotle’s lantern as well.

E) Sea Cucumbers (Class Holothuroidea)

Sea Cucumbers (Class Holothuroidea)
Sea cucumbers (class Holothuroidea) are elongated and somewhat cylindrical animals with distinct front and rear ends. While the pentaradial symmetry is not as obvious, it is still present. Just try to imagine a sea cucumber as a very tall but skinny sea urchin without spines - so tall that it has to lie down sideways and move around like a worm. In other words, the mouth of a sea cucumber is not facing down and the anus up, but rather, they are facing the sides like the 2 ends of a cylinder lying sideways. To protect themselves, some sea cucumbers are able to eject white, sticky threads (cuverian tubules) or even their internal organs to confuse and deter predators too.

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