Chapter 1 Modes of Life

Lecture 4

Nutrition

The search for food controls the distribution and density of organisms. 1. Autotrophic Organisms Chlorophyll-bearing plants use solar energy to carry out organic synthesis from mineral constituents. Autotrophic forms also exist in the bacteria. Some of them can occur in the absence of free oxygen. They are amongst the first signs of life on earth. All animals are heterotrophic. They feed on organic matter of animal or vegetable origin. 2. Microphagous Organisms Microphagous organisms eat small nutrient particles or small organisms (plankton) either from suspension in the water or mixed with the sediment. a) Suspension Feeders Nutrient particles in suspension in the water can be collected in several ways: - by beating vibrating cilia which produce a water current tending to direct nutrients towards the mouth; for example: flagellar cells of sponges, tentacular plumes of some attached polychaetes (serpulids), the lophophore of brachiopods and bryozoans, the food grooves on crinoids' arms, etc.; - by filter mechanisms; for example: the appendages of trilobites and cirripede crustaceans, the sticky filaments of some polychaetes (terebellids), the gills of bivalves (oysters) and tunicates (ascidians), etc.; - by organs catching food and carrying it to the mouth; for example: the tentacles of fixed coelenterates. The ability to capture larger prey leads to a macrophagous diet.

b) Detritus Feeders Organic matter often accumulates on the surface of the sediment where it forms a thin film. This is actively gathered by detritus feeders in various ways: - grazing made possible by the development of a mouth (gastropods) or labial palps (Nucula — bivalve); - by the elongation of the siphon of burrowing bivalves (tellinids); - scuffling of the sediment with their appendages by many crustaceans (copeopods, amphipods); - by the movement of their ambulacra! feet by various ophiuoids.

c) Mud Feeders Mud feeders eat the organic matter which is disseminated in the sediment by swallowing large quantities of mud or sand. They thus do a considerable amount of re-working of mobile substrates: this is called bioturbation.At the same time they produce a large number of faecal pellets which become mixed with the sediment. Many annelids do this (arenicolids,lumbricids), as do echinoderms (holothurians, spatangoids), enteropneusts etc. The microphagous diet is characterised by the absence of chewable pieces. It is often associated with a sedentary or barely mobile mode of life. Several fixed suspension feeders belonging to various groups develop cone-shaped shells or skeletons whose opening is orientated towards the water surface. Within especially dense populations, such an orientation allows efficient interception of food particles in suspension. This is a good example of morphological convergence (Fig. 10).

Fig. 10A—I. Cone-shaped tests in various suspension-feeding organisms. A Jurassic sponge(Tremadictyon); B Cambrian archaeocyathid;C Palaeozoic conularid; D Jurassic hexacoral (Montlivaltia); E Permian brachiopod (Richthofeniaj; F Cretaceous bivalve (Hippurites); G Tertiary gastropod (Rothplezia); H Tertiary cirripede crustacean (Pyrgoma); I Ordovician echinoderms (Cyathocystis). (Zeigler 1963)

2 3, Macrophagous Organisms The ingestion of large food particles and the capture of prey are essentially attributes of

mobile organisms (Fig. 11).

Fig. 11A—E. Adaptation of the dentition of the higher vertebrates to their diet. A carnivorous diet (lion: Leo); B herbivorous diet (horse: Equus); C omnivorous diet (chimpanzee: Pan); D dentition for grinding (lizard: Dracaena); E dentition for filtering (Permo-Carboniferous reptile: Mesosourus). (Zeigler 1972) a) Herbivores Because herbivorous organisms tear up plants, they need hard parts associated with their mouth. This function is carried out: - by the chitinous jaws of free-moving annelids; they are frequently found separated in sediments: these are scolecodonts; - by the radula of gastropods; - by the Aristotle's Lantern of regular echinoderms; - by the jaws and postbuccal appendages of arthropods (insects, some Crustacea); - by the continuously-growing teeth and cutting ridges of mammals (ungulates, rodents). b) Carnivores Carnivorous organisms feed on living prey which they pursue or trap. Prey is seized by specialised organs:

3 - tentacles armed with stinging (urticant) cells in the cnidarids; - the jaws of annelids; - the radula of some gastropods which bore into shells (muricids, naticids); - the buccal appendages of arthropods: claws (eurypterids, spiders) and mandibles (crustacea, insects); - the arms of echinoderms (asterids); - the tentacles and beak of cephalopods; - the teeth of vertebrates (fish, amphibians, reptiles, mammals) - the carnivorous mammals are the most specialised. c) Saprophages Saprophages feed on dead bodies. This is very difficult to demonstrate in fossils. Examples: worms found in the bodies of insect larvae from the Bunter Sandstone of the Vosges; fungal sclerotes preserved in the rhizomes of Devonian psilophytes from Rhynie (PI. I Fig. 4). d) Parasites In fossils, evidence of parasites can be seen where they have deformed the host (protuberances caused in the body cavity of decapod crustaceans by bopyrids) or where they are found within tissues (nematodes in the cuticle of some Carboniferous scorpions). III. Reproduction Methods of reproduction control the ability of an organism to invade an environment and colonise it. 1. Asexual Reproduction Asexual reproduction occurs in lower forms of life where it allows an exponential development of colonies and in consequence a rapid take-over of the environment (bacteria). In most of the protista (foraminiferids, diatoms), it alternates with sexual reproduction. In some foraminiferids (miliolids, nummulites) the alternation of these two modes of reproduction can be identified by the size of the initial chambers (Fig. 12): the microspheric form produces spores for asexual reproduction, the macro-spheric produces gametes.

Fig. 12A,B. Alternationof generations in miliolids. A macrospheric form (gamont);B microspheric form (agamont)

In more highly evolved organisms, asexual reproduction helps in: the dissemination of the species; for example: plant spores, buds of sponges and hydrozoans, etc.; the formation of colonies where individuals formed by budding do not detach themselves from their parents; for example: coral colonies, bryozoans, graptolites, tunicates, etc.