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. Variety of habitats. Deep sea where the currents transport them over long distances. Amongst hydroids and bryozoan colonies, sea plants and sponges. 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. Preys on sponges, hydroids, sea anemones, polychaetes and bryozoans. The predators of sea spiders are poorly understood. 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 Brine shrimp (Artemia salina) exist in large numbers in brine pools and highly saline inland waters such as the salt lake in Utah, USA.* 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. Feeds on phytoplankton that is filtered from the water by setae on the limbs. 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. 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. 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. 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. 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. Carnivorous molluscs and reef fish prey upon barnacles. 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 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. 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 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. Mantis shrimps are eaten by some species of fish, decapods (10-legged Crustacea) and some species of Molluscs. 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. Isopods: Found in all marine habitats from the intertidal to the deepest sea. Amphipods: Most crawl or burrow amongst seaweeds or in sediments. ISOPODA e.g. Beach-lice SIMILARITIES No solid exoskeleton Jointed segments Two unstalked eyes Seven pairs of legs. Certain species eat kelp. Thoracic brood pouch in female. AMPHIPODA e.g. Sand-hoppers 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). Certain isopods and amphipods eat kelp. Some feed on detritus, while others filter food particles from the water. 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. 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. 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. Lobsters do scavenge, but seem to prefer fresh food. They live on mussels, limpets, sea urchins and scraps of food lying around. 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. Most important commercial species. SOUTH COAST ROCK LOBSTER (Palinurus gilchristi) Body orange. Legs and antennae have alternating pinky-orange and white bands. Broad plate between eyes flanked by two outer horns. Second most important commercial species. EAST COAST ROCK LOBSTER (Panulirus homarus) Brown to brick red; orange spines and blue-green markings on the head. Two sharp horns project forwards between the eyes. The abdomen has a scalloped groove on each of abdominal segments 2-5. Readily sheds some of its legs to distract predators. These legs can be re-grown later. The South and West Coast rock lobster fisheries are worth about R150 million rand a year and about 4 000 people are employed in the industry. Rock lobsters are not farmed, as it is not possible to rear the larvae. How far rock lobsters can travel One theory is that larvae are carried in the currents in a big circle around the Atlantic Ocean, north to the equator across to South America, south down the American coast and back across the ocean to South Africa. Many of them also probably remain and develop off the coast of South Africa. * Extract from Fact Sheet Crustacea 1 by Margo Branch, March 1997. 38 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
HERMIT CRABS These soft-bodied crabs are well known for their habit of using an empty mollusc shell for a house. They rarely leave the security of this borrowed home. Hermit crabs are found in rock pools and on sandy beaches. Bodies are extensively modified. They have legs and claws very much like those of crabs. Soft-skinned asymmetrically shaped abdomen makes them vulnerable when not protected by the shell. As they grow, hermit crabs have to move into progressively larger homes. They carefully feel inside the proposed new home before making a rapid change. Most species are scavengers and feed on any dead animals they can find. Bigger fish and octopus and other hermit crabs prey them on. 1 nipper; doorstop 2, 3 walking 4, 5 reduced in size, grip the mouth of the shell Did you know Hermit crabs are not true crabs? Their body shape more closely resembles the lobster. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 39
CRABS They are the most specialised crustaceans. Some scuttle around rocks, while others hide in rock cavities in the balanoid zone. Still others are found swimming in deeper rock pools or under rocks. Head and thorax covered by a shield-like carapace. Two stalked eyes. Abdomen is reduced and tucked beneath the carapace. Abdomen of female much broader. Holds the eggs when in berry. The tail fan has disappeared completely. The first pair of walking legs bears nippers; the remaining four pairs are used for walking. Crabs scuttle sideways. In swimming crabs the back limbs have flattened ends to use as flippers for swimming and to flick over their backs when they settle. Some feed on seaweed. Others are ferocious predators and feed on fish, molluscs and other small animals. Octopus, reef fish, catsharks, eels. http://www.aquarium.co.za/blog/entry/giant_spider_crab_comes_out_of_her_shell/ Sponge crabs hold sponges or ascidians on their backs to give them added protection. Decorator crabs cut off pieces of seaweed and plant them on their backs to provide camouflage. 40 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
Quick Review 1. What are the key characteristics (name 2) of the group Arthropoda? 2. These animals need to undergo moulting? Give an explanation for this? 3. Identify the following animals and give two interesting facts you would share about each of the animals to visitors. a). b). Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 41
PHYLUM: BRYOZOA MOSS OR LACE ANIMALS The name Bryozoa means moss-animals. There are over 4,000 species. They are not well known. These moss or lace animals reveal a marvel of miniature architectures when viewed through a microscope. Large stones, flat algae, and mollusc shells can be encrusted with bryozoa. All bryozoans form colonies. One single larva settles and then divides into two which then divides into four and so on until a large colony exists. The colony lays down an external skeleton of chitin or lime to encase each animal in a coffin-like box. (see different shapes on the right) The head bears a ring of tentacles, which filter particles from the water. Retractor muscles can pull the head rapidly back into its box for safety and the operculum (or door) closes on it (illustrated below). Bryozoans are filter feeders. Some species of nudibranchs and sea spiders prey on them. Figure: Microscopic photograph of a colony 42 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
PHYLUM: MOLLUSCA The word mollusc comes from the Latin mollusc = soft. Although a hard shell is the hallmark of the molluscs, the phylum contains many other species with a reduced shell or the shell may be absent. CHARACTERISTICS They are all soft-bodied animals and unsegmented. All molluscs can be divided into four regions: o a usually well-developed head with tentacles and eyes (this has been lost in the bivalves); o a ventral, muscular foot on which molluscs crawl on hard surfaces; o a dorsal visceral mass (body mass) in which the internal organs are concentrated and o a mantle consisting of a sheet of tissue which grows over the visceral mass and secretes the shell. The shells assume an extraordinary variety in shape and colour. They may become enclosed in the mantle and become reduced, as in slugs and squids. The radula or ribbon tongue (lacking only in bivalves) is a horny ribbon that bears rows of teeth and is capable of wide modification for scraping, piercing and seizing. Between the mantle and the body mass is a hollow space filled with seawater - the mantle cavity in which the gills are situated. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 43
PHYLUM MOLLUSCA CLASS GASTROPODA BIVALVIA POLYPLACOPHORA CEPHALAPODA Snails Slugs Mussels Oysters Chitons Octopus Squid Cuttlefish Paper nautilus Most are shelled. Some have forsaken and lost their original shells. Shell consists of two valves. They are hinged together dorsally and extend down laterally on either side of the body to encase it. Shells are split into eight dorsal plates that cover the centre of the body. Foot modified into eight or ten long tentacles armed with suckers. Eyes as complex as those of humans. 44 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
CLASS: GASTROPODA SLUGS AND SNAILS CHARACTERISTICS Most have a special shell and, since shape and texture vary widely from one species to the next, the shell provides a ready means of distinguishing the species. The visceral mass lies on top of the foot, giving the group their scientific name: gastropoda = gut-footed. The visceral mass contains the gut, reproductive organs, blood systems and kidneys. The foot and head can be withdrawn into the mantle cavity and in some is protected by an operculum or door. WINKLES From the littorina zone to the lower balanoid zone, on rocks, beneath rocks, among sea plants in rock pools. However, the alikreukel or giant periwinkle (see diagram below) can be found down to a depth of 8m. There are many different types. They have coiled shells with a round opening. Many species have an operculum, which is a trap door made of shell-like or horny material. When attacked the winkle withdraws into the shell and a perfectly fitting trapdoor slams shut. They are herbivores (plant eaters) and graze on algae. They scrape rocks and rasp sea plants with their tongues (radulae). Fish and crustaceans. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 45
LIMPETS They form the dominant communities in the intertidal zones. Each limpet has a home scar where its shell grows to fit the shape of the rock. Flat molluscs with cap-shaped shells and a wide opening. They have a large foot and a head with feelers and eyes. Different species of limpets are adapted to different lifestyles. All limpets are herbivores. They have a great influence on the rocky shores by their grazing activities. They control seaweed growth and the settling of certain larvae. The limpet scrapes the rocks and rasps the seaweeds with its tongue (radula). Fish, especially the Rocksucker fish, lobsters and whelks, eats them. The radula of a limpet may be twice as long as the body The gut of a limpet can be up to eight times the body length to cope with its sea plant diet. Keyhole Limpet 46 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
ABALONE - PERLEMOEN AND OTHER EAR SHELLS Juvenile perlemoen occur under intertidal boulders or beneath certain species of sea urchins. Adults have a very limited range and occur in kelp forests to depths of about 9m between Saldanha Bay to as far as East London in the lower intertidal and sub-tidal zones. Venus ear or siffie lives in rocky crevices and amongst red bait. Spiral - ridged siffie lives under stones in rock pools or on shallow reefs. It is a type of sea snail with a large muscular foot, which grips tightly to rocks. The perlemoen has a small head with two feelers and two eyes. The shell is shaped like an ear and forms a very flattened spiral with a row of small holes along the left side. As the shell grows, new holes are created and old ones filled in. Reaches up to 18 cm in length. Abalone is a much sought after seafood fetching high prizes on the overseas market. Why does the shell have holes? Water is circulated through the shell to supply oxygen to the gills, which lie beneath the row of holes, and to carry away waste products. The water enters at the front of the shell and leaves through the holes, ensuring that the intake water is separated from the wastewater. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 47
Siffie or Venus ear Haliotis spadicea Smooth shell with a red stain beneath the spire. It reaches up to 8 cm long. Spiral-ridged siffie Haliotis parva Has a conspicuous spiral ridge around the shell. Small - only up to 4,5 cm. It lifts its shell and part of its foot to trap kelp fronds and other seaweeds that are swept under it by the waves. It then uses the long ribbon-like tongue (radula) to rasp off the food. 1 Man and some reef fish feed on juveniles and adults. The perlemoen is the largest ear shell in South Africa. It has beautiful mother of pearl lining on the inside of the shell. Shells of 18 cm are approximately 30 years old. The perlemoen resource is on the verge of collapse due to uncontrollable poaching and export to the Far East. It is easy to dive and retrieve these animals from the shallow waters where they live. Perlemoen farming is now in progress but it has high start-up costs and it is 5 years before they can be harvested. 1 Extract from Fact Sheet Mollusca by Margo Branch August 1996 48 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
WHELKS Great variety of habitats; under rocks, in shallow water, among redbait or mussels, in the holdfasts of kelp etc. Usually pointed spiral shells with an oval opening. Some shells have a notch or long groove. Most carnivorous species have a proboscis that can be extended along a notch or groove to catch prey. A siphon is present. It directs the water into the mantle cavity allowing the animal to sample the water from a different direction. It also aids in the location of its prey. Feed on dead or live animals. Some species use their hard tongue (radula) to drill a hole in the shell of live animals. Fish, starfish, crabs, lobster and other predatory snails feed on whelks. Plough Snail Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 49
COWRIES Coral reefs and beyond the lowest level of low tide. There are some temperate species in our region on the South coast. Colourful shiny egg-shaped shells. Operculum absent. The cowrie not only protrudes its mouth and tentacles, but also its mantle which extends right over its shell completely obscuring it and giving it the brilliant gloss so prized by shell collectors. Most of them browse on sponges. Some species of fish. Tiger Cowrie Until recently, the Fulton s cowrie, Cyprea fultoni s, was only found in the stomach of the black musselcraker (Poenskop). As such, it was considered as extremely rare and was the most expensive cowrie in the world. 50 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
SLUGS AND SNAILS (NUDIBRANCHS (naked gills) slugs) Rocky reefs, to a depth of about 15m. They are the most beautiful and highly coloured invertebrates. The colour patterns of many warn predators of their unpleasant taste. Lack a shell but are not entirely defenceless. (see Did You Know?) A second pair of tentacles (rhinophores) occurs on top of the head. The mantle cavity and primary gills may be lost. A plume of secondary gills around the anus. They feed on a variety of foods ranging from marine plants, bryozoa, sea anemones, small crustaceans, hydroids, jellyfish and bluebottles. Nudibranchs are seldom eaten. Aeolid eats hydroids dorid BUBBLE SHELLS are very fragile seaslugs with a shell. Sea hare eats plants Some Nudibranchs secrete toxic chemicals and should not be handled Others obtain second-hand weapons by using the stinging cells of their prey (anemones, hydroids or jellyfish) Some incorporate the chloroplasts obtained from their prey into their body to continue to photosynthesize. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 51
CLASS: BIVALVIA CHARACTERISTICS All species in this class have two shells enclosing the body. Body compressed from side to side. They have gills with cilia around the inner edge of the shell to propel water into the shell. Lack head with tentacles, eyes and ribbon tongue (radula). They are filter feeders. Giant Clam CLAMS, MUSSELS AND OYSTERS Bivalves have a diversity of habitats. Some are attached to rocks in the lower balanoid zone where the waves break at low tide. Some species burrow into sand or mud. of Black or Brown Mussels They have streamlined shells made up of two close-fitting halves. The two shells snap together using a strong ligament and they are very hard to pull apart once closed. The shells are anchored to the rocks by strong byssus threads. On the rocks a great many individual mussels group together to form mussel beds. The largest part of the animal is the gonad (its reproductive organ). Filter the water for minute particles of food (plankton). Many fish with strong jaws feed on them, as well as lobsters, spiny starfish and octopus. The kelp gull also preys on mussels. One mussel can pump as much as two litres of water every hour. Because mussels are filter feeders, they can concentrate pollutants and toxins in their flesh. They should not be collected for food if they are near a city. 52 Gonad largest part of the animal Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
CLASS: POLYPLACOPHORA CHITONS Some on underside of rocks, others on exposed rock surface and the giant chiton partially or totally buried in sand on flat rocky reefs. Oval, flattened mollusc. Living Shores : fig 297 Eight, overlapping articulated plates along the back which can flex the body over the surface Plates surrounded by a tough leathery girdle. Head completely hidden beneath the girdle and lacks eyes in most. Remainder of the underside broad muscular foot rimmed with small gills. When active they creep slowly about, rasping encrusting plants or animals from the rock surface with their powerful file-like radula. Any predator that is able to dislodge them from the rocks, e.g. seabirds, fish, crabs, eats chitons. Once detached from the rock, it rolls up to protect the softer body parts. This reflex action prevents damage should the chiton be rolled around by the waves. The giant chiton, Dinoplax gigas serves as excellent bait for white musselcracker. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 53
CLASS: CEPHALOPODA OCTOPUS, SQUID, CUTTLEFISH, PAPER NAUTILUS Cephalopod means head-footed and they are the top of the mollusc evolutionary tree. CHARACTERISTICS Head and foot fused. Head has highly mobile tentacles with suckers. Most intelligent invertebrates with a well-developed brain. Capable of learning. Image forming eyes, equivalent to those of vertebrates. Complex buoyancy control. All have powerful beaks shaped much like a parrot s beak. Use jet propulsion to escape predators. They release a black ink into the water when fleeing an enemy or predator. Cephalopods include the largest invertebrates the sea has ever known. Deep-sea squids may have bodies 6m long with a total length of 25 m. Squids are fast swimming animals. Can exceed 20 knots (40km/h). 54 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
OCTOPUS Found throughout the world in tropical, warm and cold temperate waters - the octopus is a bottom dweller, spending much of its life in hiding, usually in a hole among rocks. All octopus species have eight arms, each with a row of suckers. They lack a shell. They have a well-developed brain. The two large complex eyes are similar to human eyes and their vision is acute. The animals can change colour and texture of their skin rapidly. When an octopus emerges to find food such as crabs, lobster and shellfish, it often lures its victim by wiggling the tip of an arm like a worm; or it glides near and pounces on a crab, sinking its beak into the shell and injecting a poison that kills. An octopus is preyed upon in turn by a number of fishes, including the moray. http://www.aquarium.co.za/blog/entry/breaking_news_jabulani_the_octopus_gives_spain_the_psychic_edge/ The poison of the Blue-ringed octopus from Australia is dangerous to human beings. Many people either eat octopus or use them as bait. Octopus can recognise and respond to humans, clearly showing their intelligence. The colour changing ability is in cup-shaped cells called chromatophores which, when opened, show their colour. Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 55
SQUID (Chokka) Along the South African coast they are found in the open sea off the West and South Coasts. They aggregate or form large shoals in bays between Cape Point and Port Elizabeth in summer to breed. All squid are more slender and delicate than cuttlefish. They have eight short arms and two very long retractable tentacles round the mouth. All tentacles have suckers. Their shells are reduced to a transparent pen that lies just under the skin of the dorsal surface unlike the cuttlefish, which has a large internal shell. They have relatively long diamond-shaped fins. Squids are very active predators that feed mainly on small fish. Squid forms an important food source for fish, several shark species, sea birds and marine mammals. The diamond squid is one of the largest squid in our waters, with a body length of 1m and a mass of 20 kg. Squid used to be caught only for bait but are now popular for human consumption and are sold as calamari. 56 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
CUTTLEFISH About 100 species of cuttlefish are known. Note : the name cuttlefish is sometimes given in error to the squid. Some live in shallow water on a sandy bottom of e.g. sheltered lagoons and estuaries. Others live in intertidal pools or gullies and reefs. Body elongate with a fin running down each side. Head has eight arms and two very long retractable tentacles, all with suckers. The cuttle bone is used to regulate buoyancy by modifying its gas and liquid content. Some cuttlefish capture fish by shooting out their tentacles with speed. Others swim over the bottom and feed, which are caught in the same manner, on invertebrates especially shrimps and crabs. They are a major food source for fishes, including the fast swimming tunny. The white internal skeletons of cuttlefish are often found washed up on our beaches. People collect these cuttlebones to put in bird cages, so that their birds can sharpen their beaks on them The dried, powdered cuttlebone is used in polishing and other industrial processes. The dried ink sacs of the animals yield a brown pigment called sepia (See species named above). Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 57
PAPER NAUTILUS (Argonauta argo) Open water at varying depths. Sometimes close to the surface. Body with eight arms. Two of which are flattened and responsible for holding the shell. Only the female produces an unusual, semi-transparent floating shell. The naked male is much smaller than the female and often lives parasitically in the female s shell. The shell serves as a float, but its main function is to provide an egg chamber in which the female lays her eggs. Females normally die after reproducing. The paper nautilus is an active hunter feeding on animals like crustaceans. Some species of fish. Female paper nautilus The shell of the paper nautilus is not formed by the mantle, as in other molluscs, but is formed by glands in two of the arms. The very pretty coiled white shell is much sought-after by shell collectors. It is found on beaches along the south coast after storms at certain times of the year. 58 Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES
Quick Review 1. To which group do the following animals belong? Phylum: 2. Most of these animals have shells. Give a detailed reason why? 3. Identify the following animal. 4. Why does this animal not need a shell? Two Oceans Aquarium Volunteer Manual Module 4 INVERTEBRATES 59