Tuesday, October 16, 2012
This is the largest and most successful of the animal phyla, with over 1 000 000 species identified to date and an estimated 1 000 000 000 000 000 000 (1 quintillion) individuals alive today. Some scientists estimate that there are over 200 million insects for each single person. That's just insects alone, and doesn't include the countless millions of spiders, crustaceans, millipedes, centipedes, etc that make up the phylum Arthropoda.
Arthropods are the most easily identifiable of all the animals, as they all share several things in common that no other animals share. First, all arthropods are protected by a hard exoskeleton (outer skeleton) composed of chitin (humans and other chordates have endoskeletons). This exoskeleton protects the organs, gives support for the body, and allows for efficient locomotion. However, because the exoskeleton cannot grow, it must periodically be shed to allow for the organism to grow.
All arthropods have segmented bodies divided into a head, thorax, and abdomen. In some cases, such as with the lobster, the head and thorax are fused together. This is called a cephalothorax. Arthropods also have jointed appendages.
Arthropods have a complex internal design, with ganglia for a brain and an organ that acts as a heart. They have a complete digestive and excretory system. They also have well-developed sensory organs, that include antennae, eyes, and in some cases internal ears. Respiration occurs through the body surface, gills, trachea, and/or book lungs. The oxygen is carried in an open circulatory system (humans have a closed circulatory system).
Arthropods are very diverse, and therefore can locomote in a wide variety of ways. They can walk, crawl, climb, hop, fly, glide, swim, skate, dive — almost every type of locomotion imaginable (other than drive!).
Arthropods can be found on every continent and in every ocean, from the harsh climate of Antarctica to the dryness of the desert to the dampness of the rainforests, and even to the darkness of the ocean's depths. They can be found in ponds, on plants, in and on other animals, and even in your own house!
Because this phylum is so large, there are numerous ways of classifying it. Below is the most widely-accepted 4-subphylum method of dividing this large phylum. While there are several other methods, including one in which Diplopoda and Chilopoda are divided into four separate classes, I believe this one to be the most accurate to date:
This large subphylum consists of the trilobites, which are only known by the fossil record. This subphylum therefore is extinct.
Consists of arthropods that have 6 pairs of appendages, four of which are legs (or five in the case of the horseshoe crab). They lack antennae and mandibles. There are 3 classes:
Arachnida (spiders, ticks, mites, scorpions)
Merostomata (horseshoe crabs)
Pycnogonida (sea spiders)
This subphylum contains 30 000 species of arthropods that lead mostly aquatic lives. They have two pairs of antennae and mandibles. There are 5 classes:
Branchiopoda (fairy shrimp, water fleas)
Cephalocarida (small crustaceans)
Malacostraca (isopods, amphipods, krill, crabs, shrimp, lobsters)
Maxillopoda (copepods, barnacles)
Remipedia (small, recently discovered crustaceans)
This is the largest of the subphyla. These arthropods have one pair of antennae and one or two maxillae. They also have mandibles. Respiration occurs through the trachea, body surface and/or gills. There are 3 classes:
THE OLD 4-CLASS TAXONOMIC SYSTEM
Arachnida (spiders, sea spiders, ticks, mites, scorpions)
Crustacea (crabs, lobsters, horseshoe crabs, krill, shrimp, barnacles, fairy shrimps, etc)
Myriapoda (centipedes and millipedes)
|There are 5 related classes in the phylum Echinodermata (the Latin name means "spiny-skinned"). For a detailed list with all classifications click here:|
Characteristics of EchinodermsEchinoderms are characterized by radial symmetry, several arms (5 or more, mostly grouped 2 left - 1 middle - 2 right) radiating from a central body (= pentamerous). The body actually consists of five equal segments, each containing a duplicate set of various internal organs. They have no heart, brain, nor eyes, but some brittle stars seem to have light sensitive parts on their arms. Their mouth is situated on the underside and their anus on top (except feather stars, sea cucumbers and some urchins).
Echinoderms have tentacle-like structures called tube feet with suction pads situated at their extremities. These tube feet are hydraulically controlled by a remarkable vascular system. This system supplies water through canals of small muscular tubes to the tube feet (= ambulacral feet). As the tube feet press against a moving object, water is withdrawn from them, resulting in a suction effect. When water returns to the canals, suction is released. The resulting locomotion is generally very slow.
Ecology and range of EchinodermsEchinoderms are exclusively marine. They occur in various habitats from the intertidal zone down to the bottom of the deep sea trenches and from sand to rubble to coral reefs and in cold and tropical seas.
Behavior of EchinodermsSome echinoderms are carnivorous (for example starfish) others are detritus foragers (for example some sea cucumbers) or planktonic feeders (for example basket stars).
Reproduction is carried out by the release of sperm and eggs into the water. Most species produce pelagic (= free floating) planktonic larvae which feed on plankton. These larvae are bilaterally symmetrical, unlike their parents (illustration of a larvae of a sea star below). When they settle to the bottom they change to the typical echinoderm features.
Echinoderms are protected through their spiny skins and spines. But they are still preyed upon by shells (like the triton shell), some fish (like the trigger fish), crabs and shrimps and by other echinoderms like starfish which are carnivorous. Many echinoderms only show themselves at night (= nocturnal), therefore reducing the threat from the day time predators.
Echinoderms serve as hosts to a large variety of symbiotic organisms including shrimps, crabs, worms, snails and even fishes.
Characteristics of sea stars (or starfish)Sea stars are characterized by radial symmetry, several arms (5 or multiplied by 5) radiating from a central body. Mouth and anus are close together on the underside, the anus is at the center of the disc together with the water intake (madreporite). The upper surface is often very colorful. Minute pincer-like structures called pedicellaria are present. These structures ensure that the surface of the arms stay free from algae. The underside is often a lighter color.
There are a few starfish that have 6 or 7 arms, for example Echinaster luzonicus or Protoreaster, some even more like the elven-armed sea star (Coscinasterias calamaria). Others normally have 5 arms but now have more arms, because after an injury an arm divided and grew into two arms.
Ecology and range or sea starsThe starfish lives everywhere in the coral reef and on sand or rocks.
Behavior of sea stars
The spontaneous self amputation of an appendage when the organism is injured or under attack. The autotomized part is usually regenerated.
Is asexual reproduction in which an outgrowth on the parent organism breaks off to form a new individual
FissionSelf-division into two parts, each of which then becomes a separate and independent organisms (asexual reproduction)
|The majority of sea stars are carnivorous and feed on sponges, bryozoans, ascidians and molluscs. Other starfishes are detritus feeders (detritus = organically enriched film that covers rocks) or scavengers. Some starfish are specialized feeders, for example the crown-of-thorns that feeds on life coral polyps.|
Starfish have no hard mouth parts to help them capture prey. The stomach is extruded over the prey, thus surrounding the soft parts with the digestive organs. Digestive juices are secreted and the tissue of the prey liquefied. The digested food mass, together with the stomach is then sucked back in. This method can be observed, if you turn around a starfish, that sits on prey or sand - you will see the stomach retreating.
Starfish are well known for their powers of regeneration. A complete new animal can grow from a small fragment such as a arm. In some species (Linckia multifora and Echinaster luzonicus) one of the arms will virtually pull itself away, regenerates and forms a new animal. Autotomy (self amputation) usually is a protective function, losing the body part to escape a predator rather than being eaten. But here it serves as a form of asexual reproduction. In other species of sea stars (Allostichaster polyplax and Coscinasterias calamaria) the body is broken into unequal parts (= fission) then the missing limbs regenerate.
Predators of starfishes
The cushion star (Culcita nouvaeguineae) doesn't look like a starfish at all, more like a large sea urchin without spines. Its pentagonal appearance gives only the slightest indication that this organism is related to other starfish.
Photos of sea stars (photo collection) click for enlargement
Characteristics of feather starsFeather stars also known as crinoids. They are characterized by radial symmetry. The body of a typical feather star is cup-shaped, their numerous feathery arms project from a central disc. Some have five arms, others as many as 200. The arms, called pinnules are coated with a sticky substance that helps to catch food. There are appendages known as cirri attached to the underside of the body with which they cling to to sponges or corals. Both their mouth and their anus are situated on the upper side.
Ecology and range of feather starsFeather stars are primarily nocturnal but they are seen in the open during the day with their arms rolled up.
Behavior of feather starsFeather stars can crawl, roll, walk and even swim but usually they cling to sponges or corals. Feather stars are very abundant in areas exposed to periodic strong currents, because they feed on plaktonic food.
|Numerous animals live in close association with feather stars. Echinoderms are hosts to various symbiotic animals such as the crinoid clingfish (Discotrema crinophila), the elegant squat lobster (Allogalathea elegans) or the crinoid shrimp (Periclimenes sp.). These animals receive shelter and food (left over) and also feed on microorganisms living on feather stars.|
Photos of feather stars (photo collection) click for enlargement
Characteristics of brittle starsBrittle stars are close relatives of sea stars. Characterized by radial symmetry with a central body from which five snakelike arms protrude. The arms are highly flexible. There is no replication of internal organs, just one set in the central disk. Compared to starfish, brittle stars have a much smaller central disc and no anus. Wastes are eliminated through the mouth which is situated on the underside center.
On the underside of the body disk there is a splitlike opening at the base of each side of each arm. These ten openings are breathing and reproductive outlets, taking in water for oxygen and shedding eggs or sperm into the sea.
The basket stars are a specialized type of brittle stars. They have a series of complexly branched arms which are used to catch plankton.
Serpent stars are seen coiled snakelike around branches of gorgonians.
Ecology and range of brittle starsBrittle stars are very cryptic and hide in crevices under corals. Best seen at night time, when they emerge to feed on plankton. Usually at places exposed to strong currents.
Serpent stars feed mostly on small invertebrates like mollusks, worms and crustaceans and are generally found in crevices and beneath rocks or in holes in the sand.
Snake stars (for example Ophiothela danae) are found entwined in the branches of black corals or gorgonians where they feed on the rich mucus of their host, in turn performing cleaning functions.
Behavior of brittle starsAs the name suggests, the arms of the brittle stars are rather liable to break. This is actually an escape mechanism. Those arms regenerate quickly and an entire new organism can regenerate, if the broken arm is attached to a seizable portion of the disk. Brittle stars can reproduce asexually by self-division. Brittle stars are the most active and fastest moving echinoderms.
Brittle stars feed on plankton but also on detritus, coral-shed mucus, bottom detritus (detritus = organically enriched film that covers rocks), mollusks and worms.
Photos of brittle stars (photo collection) click for enlargement
Characteristics of sea urchinsRadial symmetrical body with a external chitinous skeleton and a centrally located jaw (called Aristotle's lantern) with horny teeth. The mouth consists of a complex arrangement of muscles and plates surrounding the circular opening. The anus is located on the upper surface. Some sea urchins have a spherical, bulb like cloaca (to store fecal material) that protrudes from the anal opening. It can be withdrawn into the shell.
Depending on the species, movable spines of various sizes and forms are attached to the body. These spines often are sharp, pointed and in some cases even venomous. Pincer like pedicellaria for grabbing small prey. Some pedicellaria are also poisonous.
Ecology and range of sea urchinsRubble and sand. An abundance of sea urchins can be a sign for bad water conditions.
Behavior of sea urchinsLocomotion by tube feet but also by movement of the spines on the underside of the body. Sea urchins are generally nocturnal, during the day they hide in crevices. However some sea urchins such as Diadema sometimes form large aggregations in open exposed areas. Despite their sharp spines sea urchins are easy game for some fishes, particularly triggerfishes and puffers. A triggerfish grabs the sea urchin with its hard beak like mouth by the spines or it blows some water towards the sea urchin and turns it on its back. The underside of a sea urchin has much shorter spines and those are easily crushed. During the breeding season the body cavity is crammed with eggs or sperms. This is one of the main reasons urchins are so attractive to fish predators (Japanese also like them for the same reason).
|Some sea urchins are camouflaged. They hold on with their tube feet onto some bottom debris like rubble or pieces of seagrass and carry them on their back. Some even carry live soft corals or anemones.|
Most sea urchins are algal grazers but some feed on sponges, bryozonans and ascidians and others on detritus (detritus = organically enriched film that covers rocks).
The sexes are separate and the young are formed indirectly by the fusion of sperm and eggs released into the water.
Mandarinfish, dragonet(Synchiropus splendidus)
|Many animals live in symbiotic relation with sea urchins. Even on the poisonous spines of the fire urchin (Asthenosoma varium) small shrimps (Periclimenes colemani) can be found. One shrimp (Stegopontonia commensalis) is striped black and white lengthwise and perfectly camouflaged and lives in spines of the long-spined sea urchin (Diadema setosum). Some cardinalfishes and juvenile shrimpfishes also like to take shelter in-between these spines, but even small cuttlefish hide there. It has been observed, that they change their coloring also to black and white. Some flatworms wrap around the thicker spines of the diadema sea urchin (Echinothrix calamaris).|
There are two specialized types of sea urchins with an unusual appearance: the sand dollar is very much flattened with very small spines and the heart urchin which are oval and have bristle like spines. The both bury in sand. The heart urchin "jumps" out of the sand, when disturbed.
Photos of sea urchins (photo collection) click for enlargement
Characteristics of sea cucumbersUnlike other echinoderms, holothurians don't have a distinct radial symmetry but are bilateral (distinct dorsal and ventral side). Holothurians are also called sea cucumbers. As their name suggests, they are cucumber shaped with an elongated, muscular, flexible body with a mouth at one end and the anus at the other. Around the mouth there is a number of tentacles (modified tube feet) used in food collecting. Sea cucumbers come in many sizes, from small species only a few centimeter in length to long snakelike animals which may stretch up to 2 meter!
Ecology and range of sea cucumbersRubble, rocks and sand. Also seen on some sponges in large aggregations.
Behavior of sea cucumbersMost species feed on the rich organic film coating sandy surfaces. The crawl over the bottom ingesting sand. The edible particles (organic matter such as plankton, foraminifera and bacteria) are extracted when passing through their digestive tract and the processed sand is expelled from the anus (as worm-like excrements).
Sea cucumbers move by means of tube feet which extend in rows from the underside of the body. The tentacles surrounding the mouth are actually tube feet that have been modified for feeding.
Other holothurians feed on current-borne zooplankton. They bury in sand extruding their featherlike tentacles (Pseudocolochirus violaceus, Neothyondium magnum or Pentacta crassa). The tentacles have the same shape as soft corals or some anenemones. Large congregations of some small species are found on sponges. They apparently feed on substances secreted by the sponges as well as detritus from the surface.
Some species of holothurians have separate sexes others are hermaphrodites. The sea cucumbers hold on to exposed rocks or corals, raise their body to a upright position, rock back and forth and release the sperm and eggs into the sea.
|Sea cucumbers have a remarkable capacity for regenerating their body parts. When attacked they shed a sticky thread like structure which is actually parts of their guts. The so called Cuverian threads are toxic (the poison is called holothurin) and can dissuade many potential predators. These structures quickly regenerate. (see photos below)|
Encheliophis homei and mourlani / Onuxodon margaritiferae
|Holothurians host a variety of symbiotic organisms: crabs, shrimps, worms and even a very unusual fish. The pearlfish (Encheliophis homei and mourlani / Onuxodon margaritiferae) has a long slender, transparent body and lives in the gut cavity of the sea cucumber (Boshida argus, Thelanota ananas, Stichopus chloronotus). They also inhabit some starfish as well as pearl oyster shells. The fish leaves and enters (tail first) through the holothurian's anus. They probably feed on the gonads and other tissues of its host. It is said to leave at night to feed on small fishes and shrimps. Sea cucumbers are used in Asia as a base for soups.|
Photos of sea cucumbers (photo collection) click for enlargement