Gastropods and Cephalopods: Feeding Habits and Classification PDF

Gastropods display as great a variety of feeding habits as are there shapes and habitats, but all include some adaptation of the radula. Most are herbivorous, rasping algae off a hard substrate, or are grazers on larger plants, or plankton feeders. Haliotsis (the abalone) holds seaweed with its foot...

Gastropods display as great a variety of feeding habits as are there shapes and habitats, but all include some adaptation of the radula. Most are herbivorous, rasping algae off a hard substrate, or are grazers on larger plants, or plankton feeders. Haliotsis (the abalone) holds seaweed with its foot and breaks off pieces with its radula. Some feed on decaying flesh; others are carnivorous (Melongena feeds on clams, Urosalpinx cinerea the oyster borer, drills holes in the shells of oysters to gain access to their prey). Species of the genus Conus inject prey with poison via a modified radula, and several species are fatal to humans. Currently, Gastropod classification is in flux, with disagreements about the inclusion of certain groupings being disputed. However, three main groups (subclasses) are recognised. Prosobranchia (largest class, almost all marine, torsion is extreme), Opisthobranchia (sea slugs, sea hares, nudibranchs, all marine) and Pulmonata (most freshwater and terrestrial species, possess a lung.) Class Cephalopoda contains squid, octopodes, nautilus, cuttlefish, comprising 786 species, which are all marine and carnivorous. The modified foot is concentrated in the head area, and takes the form of a funnel for expelling water from the mantle cavity, with the anterior edge drawn out into a circle of tentacles. Fossil records of cephalopods date back to the Cambrian period with early examples having straight shells, which later coiled, resulting in the only living member of the Nautiloid group, Nautilus. Cephalopods without shells, or internal shells (e.g. octopuses and squids) evolved from a straight-shelled ancestor. Many ammonids (all extinct) had elaborate shells. Cephalopods are sensitive to salinity (are not found in the Baltic due to its low salinity), and range from the intertidal zone (octopuses) to the deep abyss (5000m). Movement is achieved by a from of 'jet propulsion' by expelling water through a ventral funnel, so it's mobile direction can be controlled, as can speed by the force of the water expelled. Squid and cuttlefish are excellent swimmers, but cuttlefish are slower. Nautilus uses gas filled chambers within its shell to provide buoyancy. Octopuses can swim backwards using the same jets of water as other cephalopods, but is better adapted to crawling/walking over rocks, using suction discs on its arms to anchor itself. The nervous and sensory systems are the most advanced in the phylum, the brain is the largest of any invertebrate, consisting of several lobes with millions of nerve cells. Sense organs are well developed, apart from the Nautilus cephalopods, and have highly complex eyes with cornea, lens, chambers and a retina, which are always held horizontally. Octopodes are apparently colour- blind, but have good shape discrimination, and can memorise a particular shape for a considerable time. Their behaviour in a laboratory is easy to influence with rewards and punishment. They are also capable of observational learning, i.e..if one individual observes another being rewarded for a certain behaviour it then repeats the favourable action. They seem to lack a sense of hearing, but tentacles can discriminate between textures (although not shapes), and they also have chemoreceptors. Shallow water species have been extensively studied and visual stimulation seems to be the most important method of communication. These signals consist of arm, fin and body movements, as well as colour changes, which are produced by chromatophores. These are cells in the skin containing pigment. Small muscle cells surrounding the chromatophore cause the cell to expand, diffusing the pigment, causing colour changes. It is believed the chromatophores are under hormonal and nervous control, and elaborate changes of colour and pattern can occur, including general lightening and darkening, flashes of pink, yellow and purple, and the formation of stripes, blotches and spots. Deep-water cephalopods use luminescence in a similar way to communicate. Most cephalopods produce ink from an ink gland that empties into the rectum, in response to a threat, hoping to confuse the predator. Sexes are separate; one arm of adult males is modified into a hectcotylus, which is used to transfer a sperm packet to the female's mantle. Eggs are fertilised as they leave the oviduct and are then attached to stone, etc. Some octopuses exhibit parental care. Juveniles hatch from eggs and there are no larval stages in this phylum. There are three known subclasses of cephalopods, the entirely extinct ammonioda (ammonites, a familiar, common fossil), the Nautiloidea (only two genus, five to six species, distinguished by having two pairs of gills) and the Cleodidea. The Cleodidea has four orders: Sepioidea (cuttlefishes, eight arms, two tentacles, rounded body with fins), Teuthoidea (squids, more cylindrical bodies, again eight arms, two tentacles), Vampyromorpha (deep sea squid, one deep water species) Octopoda (octopuses, eight arms, no tentacles, short sack like bodies, no fins). Phylum Annelida (segmented worms e.g. lugworm, earthworm, leeches). This is a large phylum, with approximately 15,000 species including the familiar earthworm (class Oligochaetea) and leech (class Hirudinea), although 2/3 of the species are within class Polychaeta, which are marine worms. Annelids are true coelomates and belong to the super phylum Lophotrochozoa, and share the characteristic of spiral cleavage with the molluscs. They are a highly developed group in which the nervous system is more centralised and the circulatory system more complex than in the groups discussed previously. Annelids are worms whose bodies are divided into similar segments, arranged in a linear series and externally marked by rings (annuli). Individual segments contain similar organs of each system. In annelids, the segments (also called metameres or somites) are separated by septa, Metamerism is not limited to annelids, but is shared by arthropods (insects, crustaceans, etc.) and vertebrates, in which it evolved independently. The appearance of fleshy parapodia with their respiratory and locomotor function, introduces the beginnings of the paired appendages and specialised gills found in arthropods. A well-developed excretory system will be composed of nephridia in each segment, which means that waste is removed from the coelom and the blood. Annelids are the highest developed group to exhibit complete regeneration, although this depends on species. The body plan typically consists of an anterior prostomium, a segmented body, and a terminal portion (pygidium), which bears the anus. The prostomium and pygidium are not considered metameres, but anterior segments often fuse with the prostomium to form a head. New metameres differentiate during development just in front of the pygidium; thus, the oldest segments are at the anterior end. The body wall has strong longitudinal and circular muscles, adapted for swimming, crawling and burrowing. Class Polycheata. Mostly marine creatures with a distinct head with eyes and tentacles. Most segments have parapodia, bearing tufts of many setae (bristles), and no clitellum. Sexes are separate, asexual budding in some and trochophore larval stage, e.g. Nereis - ragworm. Class Oligochaeta. Conspicuous segmentation, few setae per metamere, no parapodia, no head, hermaphroditic, clitellum present, direct development (no larva), mainly terrestrial and freshwater habitats, e.g. Lumbricus (earthworm), tubifex (Bloodworm - due to red colour). Class Hirudinea. Many annuli, oral and posterior suckers, clitellum present, no parapodia, hermaphroditic, direct development, e.g. Hirudo (medical leech). Phylum Arthropoda This is the most extensive phylum in the animal kingdom, composing more than ¾ of all known species (approx 900,000 species currently recorded, probably at least double this number to be described in total). Arthropods include spiders, scorpions, ticks, mites, millipedes, centipedes and insects. Biological contributions of this group include advanced cephalisation, with advanced sensory organs that allow them to be very aware of their environment. Somites are more specialised, forming functional units. The presence of paired appendages, moved by striated muscles specialised for that purpose. The presence of an exoskeleton enabling a wide range of habitats to be utilised, as it is highly protective, but does not sacrifice mobility. However, it does have the disadvantage of needing to be shed (ecdysis) as the individual grows. The evolution of a more efficient breathing mechanism, involving a trachae, (a system of 'air pipes', which directly supply oxygen to the cells) and allows a high metabolic rate when needed, but does limit body size. Social interaction is highly developed throughout the phylum. The developmental process of metamorphosis means that young and adult of the same species are not in competition for the same food sources, thus allowing for greater numbers. Subphylum Trilobita An extinct group for the last 200 million years, which developed before the Cambrian period, when they flourished, ranging in size from 2cm-67cm. The head had compound eyes, antennae and a mouth. They could probably curl up for protection rather like modern 'pill bugs', or woodlice. Subphylum Chelicerata, are an ancient group that include the extinct eurypterids, horseshoe crabs, spiders, ticks and mites, scorpions and sea spiders. They are cauterised by six pairs of cephalothoracic appendages that includes a part of chelicerae (mouth parts), a pair of pedipalps, and four pairs of walking legs. They have no antennae, and most suck liquid food from their prey. Class Merostomata included giant water scorpions, the dominant life form of the Ordovician period. They resembled their living relatives, the horseshoe crabs. The horseshow crab has remained virtually the same since the Triassic period. They have two simple eyes and two compound eyes on the carapace. Females come ashore to lay eggs, which are fertilised externally. Class Arachnida shows large anatomical variations, with over 80,000 species described so far. The group contains spiders, scorpions, pseudoscorpions, whip scorpions, ticks, mites, daddy longlegs, etc. Most are free living, and generally prefer warm dry regions. Most arachnids are predacious, and have claws, fangs (modified pedipalps), poison glands, and stingers. They usually have a strong sucking pharynx with which they ingest the fluids and soft tissues from their prey. Amongst their interesting adaptations are the spinning glands of spiders. Arachnids have four pair of legs, and may be easily distinguished from insects by this fact (insects have three pairs). Arachnids have a total of six pairs of appendages, two of which have become adapted for feeding, defence, and sensory perception. The first pair of appendages, the chelicerae, serves in feeding and defence. The next pair of appendages, the pedipalps are adapted for feeding, locomotion, and/or reproductive functions. In Solifugae, the palpi are quite leg-like and they appear to have ten legs. The larvae of mites have only six legs; the fourth pair appears when they moult into nymphs. Arachnids are further distinguished by the fact that they have no antennae or wings. Bodies are organised into two areas called the prosoma (cephalothorax), which is derived from the fusion of the head and the thorax and the opisthosoma (abdomen). There are three important modifications that are particularly important for the terrestrial lifestyle of an arachnid. Adaptations for terrestrial life include modified appendages, and water conservation is enhanced by more efficient excretory structures (coxal glands and Malpighian tubules). Arachnids have two kinds of eyes, the lateral and median ocelli. The lateral ocelli evolved from compound eyes, the median ocelli developed from a transverse fold of the ectoderm. Most arachnids are beneficial to humans, as they remove harmful insects, although some species can give dangerous bites (e.g. black widow). Scorpion stings can be painful, even fatal to humans, and ticks and mites are carriers of disease as well as being an annoyance. Subphylum Crustacea (lobster, crayfish, crabs, shrimp and krill). The majority are aquatic (fresh water or marine), but a few groups are terrestrial, e.g. land crabs and woodlice. The majority are free living, although a few are parasitic, living attached to their hosts (including sea lice, fish lice, whale lice). Adult barnacles are sessile, being attached headfirst to the substrate. The scientific study of crustaceans is known as carcinology or alternately malacostracology or crustaceology. Crustaceans have three distinct body parts: head, thorax, and abdomen, the head bears two pairs of antennae, one pair of compound eyes and three pairs of mouthparts. The thorax and pleon bear a number of lateral appendages, including the gills, and the tail ends with a telson. Smaller crustaceans respire through their body surface by diffusion, and larger crustaceans respire with gills or, as shown by Birgus latro, (coconut crab) with abdominal lungs. Both systems, diffusion and gills, were being used by various crustaceans, as early as the middle Cambrian. In common with other arthropods, crustaceans have an exoskeleton, which must be shed to allow the animal to grow (ecdysis). Despite their diversity of form, crustaceans are united by the special larval form known as the nauplius. Most crustaceans have separate sexes, which are distinguished by appendages on the abdomen called swimmerets (pleopods). These are specialised in the male for sperm transfer. Many terrestrial crustaceans (e.g. Christmas Island crab) mate seasonally and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit until hatching into free-swimming larvae in damp conditions. In many decapods the females retain the eggs. Six classes of crustaceans are generally recognised: Branchiopoda-including brine shrimp (Artemia) and Triops (Notostraca). Remipedia-a small class restricted to deep caves connected to salt water, called anchialine caves. Cephalocarida-horseshoe shrimp. Maxillopoda-various groups including barnacles and copepods. Ostracoda-small animals with bivalve shells. Malacostraca-the largest class, with the largest animals, e.g. crabs, lobsters, shrimp, krill and woodlice. Subphylum Uniramia The Uniramia are a major group of the Arthropoda, consisting of organisms with an exoskeleton, jointed appendages, and with legs that do not branch, namely the Hexapoda (insects and allies) and Myriapoda (centipedes, millipedes, and related forms). Uniramia are characterised by one pair of antennae and two pairs of mouthparts (single pairs of mandibles and maxillae). Their body forms and ecologies are diverse, although (in contrast to the crustaceans) most unirames are terrestrial. The Crustacea are generally considered the closest relatives of the Uniramia, and sometimes were united taxonomically as Mandibulata (subdivisions being considered as aquatic manipulates and terrestrial manipulates). Myriapoda are a subphylum of arthropods, (millipedes, centipedes etc) reaching back into the late Silurian period. The group contains 13,000 terrestrial species. Leg numbers range from over 750 legs (Illacme plenipes) to fewer than ten legs. They have a single pair of antennae and simple eyes. Myriapods are usually found in moist forests, where they fulfil an important role in breaking down decaying plant material. A few live in grasslands or deserts. The majority are herbivorous, (except the carnivorous centipedes). Although not generally considered dangerous to humans, many myriapods produce noxious secretions, which can cause temporary skin blistering. Class Insecta Insects are the most diverse and abundant group of animals on the planet. The number of species has been estimated at up to 30 million, and evidence suggests that evolution is continuing to produce new species all the time. Like other arthropods, insects possess segmented bodies supported by an exoskeleton. The segments of the body are organised into head, thorax, and abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, one to three simple eyes ('ocelli') and three sets of variously modified appendages that form the mouthparts. The thorax has six legs and two or four wings (if present). The abdomen contains the digestive, respiratory, excretory and reproductive internal structures. The nervous system is divided into a brain and a ventral nerve cord. Insect respiration involves trachae. Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore greatly reduced; it has no closed vessels (i.e., no veins or arteries), and consists of little more than a single, perforated dorsal tube, which pulses peristaltically, circulating the hemolymph inside the body cavity. Most of the higher insects have two pairs of wings located on the second and third thoracic segments. Insects are the only invertebrates to have developed flight, which has played an important part in their success. Primitive insect groups use muscles that act directly on the wing structure. The more advanced Neoptera have foldable wings and muscles act on the thorax wall, powering the wings indirectly. Most insects hatch from eggs, but others are ovoviviparous or viviparous, and all undergo a series of moults as they develop and grow in size. In some species, the young are called nymphs and are similar in form to the adult except that the wings are not developed until the adult stage. This is called incomplete metamorphosis and insects showing this are termed hemimetabolous, e.g. the Endopterygota, and includes many of the most successful insect groups. In holometabolous insects (exhibit complete metamorphosis), eggs hatch producing larvae. The larva grows and eventually becomes a pupa, a stage sealed within a cocoon in some species. The exocuticle is greatly reduced in many soft-bodied insects, especially the larval stages (e.g., caterpillars). In the pupal stage, the insect undergoes considerable change in form to emerge as an adult, or imago. Butterflies are an example of an insect that undergoes complete metamorphosis. Some species (parasitic wasps) show polyembryony, where a single fertilised egg can divide into many, and in some cases, thousands of separate embryos. Many insects possess very sensitive and/or specialised sense organs, e.g. detecting ultraviolet or polarised light, while the antennae of male moths can detect the pheromones of female moths over distances of many kilometres. Most species with well-developed eyes, have reduced or simple antennae, and vice versa. It is thought that the range of frequencies insects hear is quite narrow (and may be limited to the frequencies that they, their predators, or prey produce). Social insects, (certain termites/ants/bees/wasps), live together in large well-organised colonies that may be considered individual superorganisms. It is argued that species of the honey bee are the only invertebrates to have evolved a system of abstract symbolic communication (i.e., where behaviour is used to represent and convey specific information about something in the environment, e.g. location of pollen bearing flowers). Insects were among the earliest terrestrial herbivores and they acted as major selection agents on plants, e.g. plants evolved chemical defences, the insects in turn evolved mechanisms to deal with plant toxins. Conversely, mutualitistic interactions between plants and insects also show co evolution, e.g. flower shaped and particular pollinating species. There are two subclasses of insects, the Apterygota consisting of bristletails and silverfish, and the Pterygota containing all other living insects in 30 or so orders. Other Prot, C-osome Phyla During the Cambrian period, all the major phyla of larger invertebrates became established. However, this 'explosion' of forms contained a number of forms that are living today. Some of these 'experiments of evolution' have a few living examples, which persist today. There are relationships with other major phyla, but the exact evolutionary relationships are much debated amongst zoologists. The first group has more in common with molluscs and annelids, the second with arthropods. Phylum Sipuncula - benthic burrowing worms, small proboscis with tentacles for filter feeding, no metamerism. Phylum Echiura - burrowing marine worms, similar to sipunculans, both share trochophore larvae with annelids and molluscs. Phylum Pogonophora - tube dwellers, no mouth/digestive tract, absorb food directly through tentacles. Phylum Phoronida- tube dweller, tentacles for filter feeding. Phylum Ectoprocta- colonial, filter feeders in secreted exoskeletons. Phylum Brachiopoda. - sessile with dorsal and ventral shell. Filter feeders. Phylum Pentastomida -- worm-like, lung/nasal parasites. Phylum Onychophora -- caterpillar-like, metameric, walk with unjointed clawed legs. Phylum Tardigrada - live in water film surrounding moss/lichen, eight unjointed legs. Phylum Chaetognatha - arrow worms, important component of marine plankton. Phylum Echinoderm (starfish, sea urchins, sea cucumbers). Echinoderms are marine animals found at all ocean depths, appearing near the start of the Cambrian period. The phylum contains about 7000 living species, and they are the largest phylum without freshwater or terrestrial representatives. Echinoderm classes include: the motile Asteroidea (starfish, brittle stars), Echinoidea (sea urchin and sand dollar) and Holothuroidea (sea cucumbers), and Crinoidea (sea lilies and feather stars). Echinoderms evolved from animals with bilateral symmetry, although adult echinoderms possess radial symmetry. Echinoderms larvae are bilateral ciliated, free-swimming organisms. During development, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. All echinoderms exhibit fivefold radial symmetry in portions of their body at some stage of life. Many crinoids and some starfish exhibit symmetry in multiples of the basic five, with starfish such as Helicoilaster spp, known to possess up to 50 arms and the sea lily Comanthina schlegelii with 200. The eggs and sperm cells are released into open water, where fertilisation takes place. The release of sperm and eggs is co-ordinated temporally in some species, and spatially in others. Phylum Hemichordata (early stages in the transition to vertebrate life). Hemichordates are worm-shaped soft bodied marine animals, generally considered as the sister group of the echinoderms, dating back to the lower or middle Cambrian. There are about 100 living species. They were formally considered to be a subphylum of the chordates due to their possession of the chordate characteristics of gill slits and a rudimentary notochord. However, it is now thought that the 'notochord' is actually a buccal diverticulum (mouth cord), and is not homologous to the chordate notochord. The gill slits serve primarily as a filter-feeding device, and only secondarily for respiration. The body of Hemichordates is divided into three parts, proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract, but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that cover its inside surface. The evolution of the hemichordates has long puzzled zoologists, as they share characteristics with both the echinoderms (larval development) and the chordates (gill slits and primitive notochord). The evidence suggests that the echinoderms and hemichordates form sister groups with the chordates. This module has covered a huge range of forms and evolutionary trends. Many of the groups described, have, at various times during the history of the Earth, been the dominant life form. The next module will describe the rise of the life forms, which have dominated the Earth during more recent periods: the chordates, the early vertebrates, fish, and the move to land with the amphibians.