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28.1C: Physiological Processes in Sponges

  • Page ID
    13710
  • Sponges are sessile, feed by phagocytosis, and reproduce sexually and asexually; all major functions are regulated by water flow diffusion.

    Learning Objectives

    • Summarize the physiological processes of sponges

    Key Points

    • Choanocytes trap bacteria and other food particles from water flowing within the sponge: in through the ostia and out through the osculum; particles are ingested by phagocytosis.
    • Sponges reproduce by sexual and asexual methods, which include fragmentation or budding; the production of gemmules is another asexual reproduction method, but is found only in freshwater sponges.
    • Sponges are monoecious; depending on the species, production of gametes may be continuous through the year or dependent on water temperature.
    • In nature, sponges are sessile as adults; however, under laboratory conditions, sponge cells are capable of localized creeping movements through organizational plasticity.
    • Gas exchange, circulation, and excretion are other major body functions in the sponge; these are achieved through the diffusion of water through the sponge body.

    Key Terms

    • oocyte: a cell that develops into an egg or ovum; a female gametocyte
    • gemmule: a small gemma or bud of dormant embryonic cells produced by some freshwater sponges
    • phagocytosis: the process where a cell incorporates a particle by extending pseudopodia and drawing the particle into a vacuole of its cytoplasm

    Physiological Processes in Sponges

    Sponges, despite being simple organisms, regulate their different physiological processes through a variety of mechanisms. These mechanisms regulate metabolism, reproduction, and locomotion.

    Metabolism

    Sponges lack complex digestive, respiratory, circulatory, reproductive, and nervous systems. Their food is trapped when water passes through the ostia and out through the osculum. Bacteria smaller than 0.5 microns in size are trapped by choanocytes, which are the principal cells engaged in nutrition, and are ingested by phagocytosis. Particles that are larger than the ostia may be phagocytized by pinacocytes. In some sponges, amoebocytes transport food from cells that have ingested food particles to those that do not. For this type of digestion, in which food particles are digested within individual cells, the sponge draws water through diffusion. The limit of this type of digestion is that food particles must be smaller than individual cells.

    All other major body functions in the sponge (gas exchange, circulation, excretion) are performed by diffusion between the cells that line the openings within the sponge and the water that is passing through those openings. All cell types within the sponge obtain oxygen from water through diffusion. Likewise, carbon dioxide is released into seawater by diffusion. In addition, nitrogenous waste produced as a by-product of protein metabolism is excreted via diffusion by individual cells into the water as it passes through the sponge.

    image

    Water flow in a sponge: In a sponge, water enters through the body pores and exits in the direction of the osculum (direction of blue arrow). This diffusion of water through the body supports major functions in the sponge.

    Reproduction

    Sponges reproduce by sexual, as well as, asexual methods. The typical means of asexual reproduction is either fragmentation (where a piece of the sponge breaks off, settles on a new substrate, and develops into a new individual) or budding (a genetically-identical outgrowth from the parent eventually detaches or remains attached to form a colony). An atypical type of asexual reproduction is found only in freshwater sponges, occurring through the formation of gemmules. Gemmules are environmentally-resistant structures produced by adult sponges wherein the typical sponge morphology is inverted. In gemmules, an inner layer of amoebocytes is surrounded by a layer of collagen (spongin) that may be reinforced by spicules. The collagen that is normally found in the mesohyl becomes the outer protective layer. In freshwater sponges, gemmules may survive hostile environmental conditions such as changes in temperature. They serve to recolonize the habitat once environmental conditions stabilize. Gemmules are capable of attaching to a substratum and generating a new sponge. Since gemmules can withstand harsh environments, are resistant to desiccation, and remain dormant for long periods, they are an excellent means of colonization for a sessile organism.

    Sexual reproduction in sponges occurs when gametes are generated. Sponges are monoecious (hermaphroditic), which means that one individual can produce both gametes (eggs and sperm) simultaneously. In some sponges, production of gametes may occur throughout the year, whereas other sponges may show sexual cycles depending upon water temperature. Sponges may also become sequentially hermaphroditic, producing oocytes first and spermatozoa later. Oocytes arise by the differentiation of amoebocytes and are retained within the spongocoel, whereas spermatozoa result from the differentiation of choanocytes and are ejected via the osculum. Ejection of spermatozoa may be a timed and coordinated event, as seen in certain species. Spermatozoa carried along by water currents can fertilize the oocytes borne in the mesohyl of other sponges. Early larval development occurs within the sponge; free-swimming larvae are then released via the osculum.

    Locomotion

    Sponges are generally sessile as adults and spend their lives attached to a fixed substratum. They do not show movement over large distances as do free-swimming marine invertebrates. However, sponge cells are capable of creeping along substrata via organizational plasticity. Under experimental conditions, researchers have shown that sponge cells spread on a physical support demonstrate a leading edge for directed movement. It has been speculated that this localized creeping movement may help sponges adjust to microenvironments near the point of attachment. It must be noted, however, that this pattern of movement has been documented in laboratories, but it remains to be observed in natural sponge habitats.

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