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PHYLUM: ARTHROPODA (Arthron = Joint + Podos = Foot)

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PHYLUM: ARTHROPODA (Arthron = Joint + Podos = Foot) PHYLUM: ARTHROPODA (arthron = joint + podos = foot) Arthropods are a vast assemblage of animals. At least 750 000 species have been described. This is more than three times the number of all other animal species combined. The tremendous adaptive diversity of arthropods has enabled them to survive in virtually every habitat. CLASSIFICATION OF ARTHROPODS Taxonomists are constantly refining the classification of plants and animals. Some university textbooks now group the crustaceans as a subphylum, because there are more than 42 000 known species in this group. CHARACTERISTICS All arthropods have jointed limbs. The body is segmented and covered by a hard, jointed, external skeleton (exoskeleton) made of chitin, in some cases strengthened with calcium carbonate. The exoskeleton cannot expand; therefore arthropods periodically shed the exoskeleton (moult) and can then rapidly expand. Body is divided into a head, thorax and abdomen. Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES 29 CLASS: PYCNOGONIDA SEA SPIDERS Called sea spiders as they superficially resemble land spiders but are totally unrelated. Rarely seen due to their typically small size and cryptic colouring. Sluggish in their movement. Their many reproductive openings at the base of the legs give them their name pycno = great or multiple and gonas – reproduction. Habitat Variety of habitats. Deep sea where the currents transport them over long distances. Amongst hydroids and bryozoan colonies, sea plants and sponges. Description Small body made up of a head and trunk. o Abdomen reduced to a stub. o Four pairs of long spindly legs. Portions of the alimentary canal and reproductive system are partially displaced into the legs. Use claws on end of legs to cling to substratum. Males gather up the eggs with the ovigerous legs and carry them until they hatch. Growth necessitates periodic moulting. Feeding Preys on sponges, hydroids, sea anemones, polychaetes and bryozoans. Predators The predators of sea spiders are poorly understood. Did you know? Nice Legs, but not just for show Crawling among corals, anemones and sponges to suck their juices, sea spiders range throughout all the oceans. However, sea spiders have a problem. Their bodies are so tiny – usually just a few millimetres in diameter – that there is not room for all their internal organs. Therefore their legs serve as auxiliary storage, housing part of their digestive and reproductive systems. Males carry the female’s eggs on a special pair of legs, called ovigers, until they hatch. 30 Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES PHYLUM: ARTHROPODA SUBPHYLUM: CRUSTACEA A HOST OF SHRIMPS, LICE, CRABS and LOBSTERS. SUBPHYLUM CHARACTERISTICS Most crustaceans are marine. Bodies are normally made up of a fused head (cephalus = head) and thorax - the cephalothorax plus a separate abdomen. They have numerous jointed limbs, which are adapted for walking, swimming, feeding, respiration and reproduction. Usually two pairs of antennae. Complex compound eyes. The exoskeleton may form a shield (carapace) that covers and unites various segments of the body. The life cycle is complex. Most have several larval stages that form part of the zooplankton floating in the sea. The larvae are dispersed by currents. Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES 31 SUBPHYLUM - CRUSTACEA 32 Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES BRINE SHRIMP Habitat Brine shrimp (Artemia salina) exist in large numbers in brine pools and highly saline inland waters such as the salt lake in Utah, USA.* Description Adult size 15 mm. Lacks a carapace but have a cuticle. Has stalked eyes which are constantly on the rove A discrete head. Thorax has flattened, leaf-like limbs covered in hair-like setae. Slender abdomen has no appendages. Unusual for a crustacean it has haemoglobin in the blood, which varies in concentration inversely to the oxygen concentration in the surrounding water, i.e. when there is little oxygen there is much haemoglobin. Fertilised eggs can survive long periods of drought. Feeding Feeds on phytoplankton that is filtered from the water by setae on the limbs. Predators Many brine shrimp are bred in the aquarium as food for such animals as: basket stars, seafans, seahorses, pipefish, comb jellies, sea cucumbers, barnacles, juvenile fish, butterfly fish and many more. Did you know? The dried fertilised eggs are purchased from overseas suppliers. They are small and brown and 1 gram contains 300 000 eggs. A 417-g tin of cysts costs around R350 and 2-3 teaspoonfuls are put into an aerated aquarium where they hatch out in 24 -36 hours depending on the water temperature. The animal is an anal water swallower and this is believed to act as an enema to remove unwanted material from the hind gut! The brine shrimp is one of the most important foods in Aquaculture. * Article researched by Bert Tucker Group 1 Volunteer Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES 33 CLASS: CIRRIPEDIA BARNACLES They are highly modified crustaceans. Habitat Along the shore they live permanently attached to rocks in the balanoid zone. Can also attach themselves to other solid substrata, e.g. pier pilings, whales, bottom of ships, flotsam. Description A shield of calcareous plates encloses body. The shells of barnacles dot the rocks in the balanoid zone like small grey volcanoes or white stars. At low tide the barnacles seal the opening at the top of the shell with an operculum of four plates. Feeding Filter feeders. When the tide comes in and they are covered with water, they open up their shells and extend hairy cirri (leg-like structures) to comb food particles from the water. They catch plankton and suspended organic matter in the water. Predators Carnivorous molluscs and reef fish prey upon barnacles. Did you know? Goose Barnacles have a flexible stalk and occur in colonies attached to floating objects in the sea. Acorn barnacles live on ships, rocks and even whales. The animal inside the shell is actually standing on its head. The balanoid zone (an intertidal rocky shore zone) gets its name from a northern hemisphere barnacle, Balanus sp. 34 Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES MANTIS SHRIMPS Habitat Live in burrows or in rock and coral crevices. The Cape mantis shrimp is common in Cape Town where it burrows into the soft sediment. Description Massive raptorial second thoracic limbs. Eyes are large and stalked A short carapace covers the front half of the thorax. They have a large segmented abdomen. Down each side are paddles that are used for swimming. Many are brightly coloured and defend their territories. 1. to hold food, 2. to catch food, 3,4,5, used to shred food Feeding Highly specialised predators Two functional groups occur: o Spearers: impale soft-bodied prey such as fish and shrimps. o Smashers: strike hard animals like crabs and molluscs with the elbow and stun them. Predators Mantis shrimps are eaten by some species of fish, decapods (10-legged Crustacea) and some species of Molluscs. Did you know? The strike force of a ‘smasher’ approaches that of a bullet and can easily crack the glass of a small aquarium. Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES 35 PHYLUM: ARTHROPODA SUBPHYLUM: CRUSTACEA Isopods (iso = same + podos = foot) are a diverse group of small crustaceans with over 270 species in southern Africa. They include the kelp louse, fish louse - parasitic on fish and the pill bug Tylos that burrows in sand. Amphipods (amphi = both + podos = foot) are also a diverse group of crustaceans with over 300 species occurring in southern African waters. They include beach-hoppers. Habitat Isopods: Found in all marine habitats from the intertidal to the deepest sea. Amphipods: Most crawl or burrow amongst seaweeds or in sediments. Description ISOPODA e.g. Beach-lice SIMILARITIES AMPHIPODA e.g. Sand-hoppers No solid exoskeleton Jointed segments Two unstalked eyes Seven pairs of legs. Certain species eat kelp. Thoracic brood pouch in female. ISOPODA DIFFERENCES AMPHIPODA Bodies flattened from top to bottom. Narrow bodies that are flattened from side to side. Shrimp-like creatures. Legs similar to one another. First two pairs of legs generally form nippers – help with feeding. Remaining five pairs end in a simple claw. Some have a well-developed jumping ability (e.g. sand hoppers). Feeding Certain isopods and amphipods eat kelp. Some feed on detritus, while others filter food particles from the water. Predators Fish, crabs and worms. 36 Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES PRAWNS, ROCK LOBSTERS AND CRABS ROCK LOBSTERS Popularly called crayfish, they are correctly termed "spiny lobsters" or "rock lobsters" to avoid confusion with clawed freshwater crayfish. Habitat They spend the daytime hiding in holes under rocks. Small spiny rock lobsters may be seen in rock pools, but they normally stay in deeper water. Description Large, robust crustacean with a long abdomen ending in a well-developed tail-fan. Thorax and head covered by a single shield or carapace. None of the walking legs end in nippers. They have an elaborate life cycle involving 13 larval stages which drift in the sea for nearly a year before assuming the adult form and settling on the sea floor. Feeding Lobsters do scavenge, but seem to prefer fresh food. They live on mussels, limpets, sea urchins and scraps of food lying around. Predators Young seals, dog sharks, octopus, fish and humans hunt them. Living Shores: Figure 264 Two Oceans Aquarium Volunteer Manual – Module 4 – INVERTEBRATES 37 SOME LOBSTERS ALONG OUR COAST* WEST COAST ROCK LOBSTER (Jasus lalandii) Body orange-brown. Tail fan orange, blue and green. Front carapace has two large spines and smaller projections between the eyes. Intolerant of low oxygen levels. They moult once a year, breed after 5 years and live up to 30 years. Males reach a carapace length of 190 mm and females 140 mm.
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