Echinodermata. To familiarize the marine guide with the echinoderms of Southern Africa

Transcription

1 WildlifeCampus - Marine Field Guiding 1 Module # 4 Component # 1 Objective Echinodermata To familiarize the marine guide with the echinoderms of Southern Africa Expected Outcomes: The learner will be able to: Describe the basic structure of echinoderms Explain the feeding mechanisms of echinoderms Recount the reproductive processes of echinoderms Recall the basic types of echinoderms Discuss the ecological importance of echinoderms Pre-test: Test your current knowledge of marine guiding. Question 1. Certain sea cucumbers can regenerate their intestines after deliberately disgorging it. Yes No

2 WildlifeCampus - Marine Field Guiding 2 Introduction The Echinodermata include species like the starfish, sea urchins, brittle stars, sea cucumbers and feather stars. Their name implies that they have spines in their skins reminiscent of a hedgehog (ekhinos = hedgehog and derma = skin). Hedgehog skin of an urchin Further they have a pentamerous radial symmetry (penta = 5, thus a five rayed symmetry), which inevitably leads to the phenomenon of them being called stars like the starfish, brittle stars or the feather stars. Although urchins and sea cucumbers look less like stars they do have an obvious fiverayed structure visible along their bodies. Radial symmetry also implies that they mostly don t have a left, right, front or rear end but they radiate from a central point and mostly have an upside-down where the mouth is situated and an upper side where the anus is situated except in the feather-stars where it is the other way around and in sea-cucumbers who are mostly lying on their sides. Echinoderms appear to be rather primitive due to their radial symmetry and the fact that they lack a heart, brain and eyes (remember the anemones) but the contrary is true about them, they are in fact highly evolved creatures with the necessary internal organs and external appendages that put them a cut above other creatures with radial symmetry. Echinoderms can virtually be found in most of the marine habitats described in this course from the deep sea to the reefs to the shallows on rock, coral, rubble and sand to mention but a few. Echinodermata can regenerate lost limbs, arms, even spines and seacucumbers can regenerate their intestines after disgorging it in order to side-track predators. Another rare feat is that some sea stars can even break up or divide and grow into two separate but fully formed individuals.

3 WildlifeCampus - Marine Field Guiding 3 Structure Symmetry Although Echinoderms are radially symmetric it is only so in their adult stage. In their young or larval stage they are bilaterally symmetric meaning they have a left, right, front and back side (Humans are bilaterally symmetric). The advantages of radial symmetry above bilateral symmetry is found in the animals ability to move in any direction without having to align it s body first (e.g. we have to turn around to move in the opposite direction where starfish for instance can just set off in the new direction without re-orienting its body). Another advantage is that radially symmetric creatures can detect food in any direction. The body itself has a stronger skeletal structure in that the five rays are represented through body plates that are sutured together, so imagine an orange with its total peel in five wedge-like sections that intersect at the top and bottom. If it were only four plates for instance, the suture lines would have gone right through the middle making the structure weaker (the same goes for six or eight body plates). Three or five body plates allows for a stronger radial structure: In figure (a) the suture lines cross in a straight line making it a weaker structure (e.g. can split in two easier) but in figure (b) the suture lines meet in the middle making it a stronger structure Skeleton Echinoderms have, as mentioned before, a very unique skin in that it varies from species to species in the degree of flexibility (sea-cucumbers are soft and urchins are pretty brittle). This is as a result of small platelets in the skin, which are called ossicles. The ossicles are made up of calcium

4 WildlifeCampus - Marine Field Guiding 4 carbonate and are responsible for the knobbly or spiny surface of the echinoderms and they are overlaid sparsely or stacked tightly to form either a soft/leathery or hard/shell-like skeleton. Individual ossicles are sometimes bound together by strong collagen fibers.

5 WildlifeCampus - Marine Field Guiding 5 Spines Although spines are featured on most echinoderms they are best illustrated by the sea urchins. These spines can pivot on the hard casing (or shell) of the urchin (much like a ball and socket joint) and can also be used to pole the urchin along. Sea stars mostly have shorter knobbly spines and brittle stars sometimes have tiny spines on their discs and longer spines between the segments on their limbs. The spines of the sea urchin contain venom sacs at the tips. The tips of spines are known to break off in the wound and it is very painful at least to humans. Pedicellariae Urchins and starfish have extraordinary appendages on their skeletons called pedicellariae. Although they are small they are formidable because the short stalk (2-3 mm) is crowned by a formidable clamp with two or three gaping jaws that can snap shut in an instant. Sometimes these jaws are armed with fangs and venom glands, which can be harmful to humans. The purpose of the pedicellariae is probably to prevent organisms (parasites) or larvae from settling on the relatively hard skeletal surfaces of starfish and urchins. They probably also serve to repel predators or competitors. Movement Another unique characteristic of the echinoderms is that they move around by means of seawater-based hydraulics. They have numerous tube feet in rows under their bodies, which together can propel the animal in the desired direction. The tube feet are extended and retracted by seawater hydraulics within a pipeline or duct system. If you consider the shape of a sea star then imagine a circular duct (like a small hula-hoop) inside the central part of the sea star. From his circular duct five elongate tubes (radial ducts) extend all the way into the five limbs of the star. Such an interconnected duct system means that pressure must be maintained and controlled to enable the hydraulic abilities of the animal. This raises another problem How does it regulate internal pressure in an

6 WildlifeCampus - Marine Field Guiding 6 environment where the tides change twice a day? Needless to say the amount of water above the animal has a direct correlation to the internal pressure of the animal and if the level of water changes, so does the internal pressure. Let s say for instance that the tide is going down (the water above the animal is becoming progressively shallower). The pressure around the duct system will reduce and the duct may dilate (expand) losing its hydraulic pressure and be unable to pump. This problem is countered by a special valve system that can allow water in or expels water when required. A short tube runs from the circular duct to the upper surface (near the center) of the starfish where a control system, which works with a sieve plate, regulates the amounts of water that is taken up or expelled to maintain the internal pressure. The sieve plate is called a Madreporite and can be seen on top near the center of a starfish. Tube Feet Starfish, sea urchins and sea cucumbers have numerous tube feet in rows on either side of the radial ducts. These tube feet extend from the inside through the skin of the animal and it is tipped with a sucker. A tube foot is roughly in the shape of an upside down bowling pin with an inflated head innermost and the thin neck extending downwards through the skin to the outside. The foot part can become longer as the head contracts (by circular muscles) and shorter when the head inflates (or hydraulically sucks back the seawater). The tube foot also has a valve between the head part and the radial duct in order to regulate the pressure or to divert the water to another part of the animal. The foot part outside the body of the animal can also bend in the desired direction by using longitudinal muscles. When a tube foot touches an object, suction is created by water being withdrawn from the ducts. Suction is released when water returns and inflates it again. This action of hundreds of tube feet in unison can exert tremendous force from the animal onto the substrate or onto prey. Nervous System Because echinoderms don t have a head with a brain, the nervous system is simple and net-like. Similarly they don t have eyes but they are sensitive to light and may even react if a sinister shadow (potential predator) passes overhead.

7 WildlifeCampus - Marine Field Guiding 7 Excretory Organs For very highly evolved animals it is rare to find that the echinoderms are not issued with excretory organs. Waste products are engulfed by amoeboid cells that wander around in the gut. The amoeboid cells themselves are then excreted through the anus or the body opening (the mouth) in the absence of an anus.

8 WildlifeCampus - Marine Field Guiding 8 Feeding Some echinoderms feed on the algal crust on rocks; others on detritus, others are filter feeders and some are even aggressive carnivores. Apart from normal regimes of feeding Echinoderms are capable of absorbing organic molecules directly through their skins. Sea cucumbers have tentacles around their mouths that can capture plankton and organic particles. Starfish that feed on micro-algae (like the cushion stars) can do so by expelling their stomachs out over the algal crust and virtually digest the food straight from the rock face before pulling their stomachs back into the body through the mouth. Expelling the stomach can also be a predatory activity like with the starfish Marthasterias that feeds on black mussels. The starfish will huddle over the mussel and it will pry open the mussel with its tube feet working in unison (quite a feat), just enough to expel its stomach through the gap between the valves of the mussel. The stomach fluids will then also weaken the mussel s valve muscle (the thick adductor muscle), which in turn will cause the two valves to open further. It will then digest the mussel and pull back its stomach leaving an empty mussel shell. The crown-ofthorns sea star (Acanthaster planci) achieved infamy through its own brand of stomach extrusion but this time over coral polyps. This sea star is responsible for laying bare and destroying large tracts of coral reef in some areas where, it is argued, their natural enemy (the giant Triton conch shell) has been taken out by collectors and divers for the souvenir market. Reproduction Echinoderms mostly reproduce by releasing sperm and eggs into the water where fertilization takes place. Starfish has a pair of sacs at the base of each arm, which serve as gonads. The gonads produce sperm and eggs, which are released to the outside through pores. Fertilization mostly takes place outside the body but some brittle stars also practice internal fertilization and keep the developing embryos inside their bodies until they release them eventually fully formed.

9 WildlifeCampus - Marine Field Guiding 9 Class: Asteroidea (The sea stars) Sea stars are unmistakable with their central disc and five (star-like) limbs. Some sea stars have more than five limbs however, like the upper east coast Luida sp. that has seven or more limbs. The Dwarf cushion star (Patiriella exigua) can be found in shallow intertidal pools around the whole of Southern Africa. The Spiny starfish (Marthasterias glacialis) is a large sea star with nipplelike spiny protrusions on the upper surface. It s a predator on mussels and so is likely to be found in the same zone (e.g. the upper and lower balanoid) of the south coast and lower west coast. The spiny protrusions are actually short spines surrounded by a circle of densely packed pedicellaria. The Sand starfish (Astropecten irregularis pontoporaeus) is found more or less in the same coastal region as Marthasterias but they live on sand. They are whitish to orange and are identified by a double row of armour-like sideplates along the sides of their limbs. The side plates are actually ossicles and the upper row boasts a single sturdy spine and the lower row of ossicles boasts three to four larger spikes. They feed on small molluscs or crustations, which they dig out of the sand. A similar species (Astropecten granulatus natalensis) is found on the east coast. The Crown of thorns starfish (Acanthaster planci) with its heavily armed poison spines on multiple limbs (more than ten) is found on the upper east coast. They have been known to destroy vast tracts of coral due to a sudden population bloom in certain areas. Although ecologists would want to blame human interference for these population blooms and their resultant destruction, studies have shown that this species was prone to population blooms for eons and long before human interference was significant in the oceans. The story is also told of recreational divers in Mozambique who, in a bid to save the coral reefs, took out just about all the crown of thorns starfishes they could find. They then cut them up in pieces and threw them back into the ocean where of course, as we now know, they regenerated and were more numerous than ever before. Linckia laevigata is another east coast (tropical waters) species. They are identified by their blue (sometimes orange) colour and their extremely long limbs, often more than ten times longer than the diameter of their discs.

10 WildlifeCampus - Marine Field Guiding 10 Because their limbs are so long they are prone to breaking off and often you would see a blue starfish with one or two shorter legs. Other starfish with good distribution around Southern Africa is the red starfish (Patiria granifera) and the maroon coloured reticulated starfish (Henricia ornata) that pretty much look like their English names. Class: Echinoidea (The sea urchins) Sea urchins have nearly round shell-like casings for bodies with five double rows of spines arranged in lines from the top to the bottom around the central point. The central point above is usually their anus while the central point at the bottom is usually their mouth. Their gametes are housed in five gonads lining the upper half of the inside of the shell casing. These gonads and their contents are a delicacy in the Far East and are eaten like caviar. Sea urchins frequent rock pools, sometimes in great numbers where they mostly graze. Some urchins have very venomous spines and can cause severe discomfort to humans, especially if the spines break off in your flesh. The Needle urchin (Diadema setosum) is easily identified by its very long needle sharp spines (poisonous). The spines of juveniles are banded and they are found on the upper east coast. The Oval urchin (Echinometra mathaei) is an east coast species with stout sharp spines in purple, brown or green. They are mostly nocturnal since they become active and feed at night. The Flower urchin (Toxopneustes pileolus) has a deceptive name because the flowering appearance is due to the fact that it is armed with numerous open stalked pedicellaria (snapping jaws with poison fangs). The tube feet extend beyond the short spines and the pedicellaria. The Cape urchin (Parechinis angulosus) with shortish pointed spines is found on the west coast as well as the south and east coasts of Southern Africa. They are mostly purple (however, variable colours occur) and they are important because they control the settlement of kelp plants. The Banded urchin (Echinothrix calamaris) has two types of spines, thick primary spines and shorter needle-like secondary spines. In both cases the spines are banded black and white. Spines contain an irritant toxin.

11 WildlifeCampus - Marine Field Guiding 11 The Pansy shell (Echinodiscus bisperforatus) is found on the south and east coast. It is easily recognised because it is flat and has two slits towards the edge of the shell. Below the slits is a five petalled pattern, which is visible once the short fur-like spines drop off after death. Pansy shells bury themselves just under the surface of the sand where they feed on tiny organic particles.

12 WildlifeCampus - Marine Field Guiding 12 Class: Holothuroidea (The sea cucumbers) Sea cucumbers are elongate creatures like fat sausages and they mostly lie on their sides. They also have double rows of tube feet running the length of their bodies with which they can move slowly. They have 10 to 20 retractable tentacles that radiate around the mouth at one end of their bodies with which they feed on detritus or suspended plankton. Their bodies are soft and flexible, unlike the hard casings of sea urchins. The body is eaten in some countries after the innards are removed and the body left in the sun to dry out. They are most commonly found on shallow reefs but some inhabit rock pools and crevices under the water. Sometimes after a storm their bodies get washed out onto the beach in great numbers. They have the ability to disgorge their gut or parts thereof when threatened and some species may even eject long sticky threads from the anus when disturbed. The threads are contained in an organ of Cuvier and they are very tough, sticky and may even contain toxins. The Golden sea cucumber (Thyone aurea) is smallish, soft and goldenorange in colour. Their tube feet are rudimentary and not aligned in rows but are rather scattered randomly over the body. They are mostly buried under the sand with their tentacles exposed above the sand surface. It will disgorge its intestines when handled. They are mostly found on the west coast. The Horseshoe sea cucumber (Roweia frauenfeldii) is found around the Southern African coast and it has a dark body (Grey below) shaped like a horseshoe. Tube feet are arranged in five bands, each with four to six rows of feet. They are also buried with just their tentacles sticking out on one side and their anus sticking out on the other side. The body below the sand is normally attached to a rock surface. The Red chested sea cucumber (Pseudocnella insolens) in the intertidal zone has a dark body with a red chest (upper body), other specimens in the sub-tidal zone are found in dense colonies on shallow reefs and they are bright red all over. Their tube feet are scattered on the upper surface but on their underside they have three bands of tube feet with two or three rows each. They can be spotted on the south and west coasts.

13 WildlifeCampus - Marine Field Guiding 13 Black sea cucumber (Pseudocnella sykion). The upper side have distict bands of tube feet but also some randomly scattered ones in between. The lower side also have bands of tube feet with no more than four rows each. They can be spotted in rocky crevices in the low tide zone on the southern and lower eastern coastlines.

14 WildlifeCampus - Marine Field Guiding 14 Class: Ophiuroidea (Brittle stars) Brittle stars have a disc-like flat central body surrounded by five or more long flexible but very brittle arms. The arms are long and thin (nearly snake-like) compared to sea stars and they are more often than not covered in plates and spines. The disc part are also distinctive in that it can be granulated, shortly spined or leathery/scaly in texture. As mentioned before brittle stars often brood their young larvae internally and eventually release them fully formed through the genital slits (on the disc at the base of the legs). Brittle stars move around using their legs in a snaking way not unlike the movement of octopus over rocks. The Basket star (Astrocladus euryale ) also known as Gorgon s head is found on the South coast regions where it often perches on sea fans (a type of coral) with extended arms making it look like a basket. It s aim is to catch passing animals. The basket-like appearance is enhanced due to the fact that the arms are branched and re-branched into even finer and more delicate structures. They are normally very colourful with dark and light contrasts. The Serpent skinned brittle star (Ophioderma wahlbergi) have genital slits on both sides of the base of the legs through which the fully formed young emerge after being brooded within the granular disc. They are mostly brown-black on the upper surface and have short spines on the arms. They are common on the west coast of Southern Africa. The Hairy brittle star (Ophiothrix fragilis) has a spiny disc. The arms have long perpendicular spines on the sides. They are common on rocky shores around Southern Africa especially below boulders Scaly armed brittle stars (Amphipholis squamata) are very common around the whole of Southern Africa. They are very small, will just about fit on an adult s thumbnail and the scales on the segments of the arm appear to be independent or separate and not interlinked as in most other species. They are found in the intertidal zone where they mostly feed on detritus on mud, sand or gravel.

15 WildlifeCampus - Marine Field Guiding 15 Class: Crinoidea (feather stars) Crinoids are the most elegant of the echinoderms where they appear like feathered flowers on the substratum of the ocean and especially on coral reefs. They have a small central body or disc from which the ten or more feathery arms (tentacles) radiate upwards. Since they are mostly sedentary they need to attach themselves to the substratum. For this purpose they also have a fringe of hook-like segmented limbs (also called cirri ) surrounding the disc and radiating downwards gripping onto the substratum. They mainly feed by filtering or trapping particles with their featherlike limbs that extend into the current. Each feather has numerous side branches or pinnules and they pass food particles onto the axis, which has a ciliated groove on the upper surface in which food is driven down to the mouth on the upper part of the disc. This feather-like filtering method is also used to great effect in the fan worms or Christmas tree worms (annelids) as seen in Module 3 Component 4. Feather stars can also move about should it become necessary. They can crawl or even swim using their feathers, which they rhythmically move, much like the undulations of a jellyfish. Because feather stars have such small bodies with little or no place for anything more than the gut, their gonads are positioned in the pinnules on the lower parts of their tentacles. These can greatly swell during breeding season and even burst to release the eggs and sperm into the water where fertilization takes place. The two species below can be found on the Southern and Eastern coasts of Southern Africa. The Elegant feather star (Tropiometra carinata) has neatly arranged dense feathers. The arms range in colour from darker to purple near the base gradually becoming lighter to even yellow at the tips. They can be found solitary on shallow reefs but their numbers increase at deeper levels. The Common feather star (Comanthus wahlbergi) has less dense feathers and appears to be more frayed. They are more common than the elegant feather stars and are sometimes found in dense clusters on shallow reefs.

16 WildlifeCampus - Marine Field Guiding 16 The more primitive Sea lilies are found in deeper waters and instead of the ring of cirri they are attached permanently to the deepwater substratum by a long stalk.

17 WildlifeCampus - Marine Field Guiding 17 Ecological Importance Echinoderms play an important role ecologically because they have the capability to influence community structure and to determine which other creatures can flourish around them. One such example is a starfish that preys on mussels in the intertidal zone. If you remove the starfish (Pisaster sp.) then the mussels flourish and dominate the zone to the detriment of many other species. This balance between populations is best maintained by the presence of the starfish and is therefore considered a keystone species. On the flipside of the coin echinoderm communities can be threatened or maintained by other creatures. Certain mollusks like limpets (that also feed on encrusting algae) are more successful than cushion stars due to their effective radula s that scrape algae from rocks. In this way cushion star populations show a reduction in growth where they compete with limpets for the same food. Certain sea cucumbers are also a safe haven for other commensal creatures and may even house a small species of fish (Carapus sp.) in its anus. The fish will reverse tail first into the sea cucumber s anus where it will shelter from predators. The marine guides course leans heavily on information found in the books by local marine scientists George Branch et al. Further research and data can be obtained from the following two publications: 1. The living Shores of South Africa, George and Margo Branch,1993, Struik Publishers, Cape Town. 2. Two Oceans: A guide to the marine life of Southern Africa, G.M. Branch, C.L. Griffiths, M.L. Branch, L.E. Beckley, 1994, David Phillips publishers, Claremont.