Sponge morphology

Sponges are characterized by four important cell types. Arch-aeocytes are cells shaped like amoebae, able to move within the colony and lacking a fixed shape. These cells are feeding cells and can also change into another cell type if required. Sclerocytes secrete mineralized elements of the skeleton, while spongocytes secrete the organic parts of the skeleton. Cho-anocytes are the cells that generate feeding currents through the sponge. They have a funnel-shaped end, with a long flagel-lum, or whip-like filament, extending through the funnel and into the water beyond. The movement of many flagellae, from many choanocytes, helps to move water through the colony.

Sponges have a simple body shape, characterized by their functional needs as filter-feeding organisms (Fig. 3.1). They construct a skeleton with narrow openings, called ostia, through which water can enter and a broad opening through which water leaves, called the osculum. This has the hydro-dynamic effect of facilitating a good flow rate with less work from the flagella-bearing cells. Feeding occurs in the walls of the colony. In the simplest case, archaeocytes line apertures in the wall of the cup-shaped sponge (this is known as ascon-grade organization). In more complicated morphologies, the feeding and water-moving cells are arranged in multiple chambers, linked to a common central area or paragaster (this is the sycon grade of organization). Most commonly, networks of such chambers are developed within the thick wall of the sponge and are linked to the paragaster by a series of canals (the leucon grade of organization).

Calcareous sponges grow skeletons composed of entirely of calcium carbonate spicules. Spicule type is very varied within the group. Demosponges, the most common modern group, usually have unmineralized skeletons, but can produce siliceous spicules, or calcareous bases to their colony. Hexactinellid sponges produce skeletons from a series of six-rayed spicules with each spine arrayed at 90° to its neighbors. These are formed into skeletal frameworks with cubic symmetry. Spicules are the most commonly preserved evidence for the presence of sponges in fossil communities. They can be identified easily in thin section.

Sponges are attached to the sea bed by a holdfast that may resemble roots or a series of fine hairs. These can attach the sponge to hard substrates, such as coral reefs and boulders, or to sand grains and abyssal muds.

(a) Direction of water flow

(a) Direction of water flow

Water Movement Through Sponge

Ascon-grade sponge -a simple cup with perforated walls

Direction of water flow

Direction of water flow

Sponge Morphology

Sycon-grade sponge -several chambers open onto a central large paragaster

Osculum: wide opening through which water is expelled

Canals: link feeding chambers to ostia and paragaster

Basal holdfast

Direction of water flow

Direction of water flow

Basal holdfast

Basal Holdfast

Paragaster: central chamber of sponge

Flagellate chamber: (d)

where cells move water and filter feed

Paragaster: central chamber of sponge

Ostia: narrow openings through which water enters the sponge

Leucon-grade sponge -complicated canals link a series of sycon chambers to the central paragaster

Fig. 3.1 The main features of sponge morphology. Sponges of each of the three main classes grow in broadly similar shapes that can be divided into three grades of internal organization. (a) Ascon-grade organization. These are usually small sponges, less than 10 cm in diameter. (b) Sycon-grade organization.

(c) Leucon-grade organization. These complicated sponges can grow much larger than simpler forms, sometimes reaching 50 cm or more in diameter.

(d) Sponge spicules, about 1 mm long. Hexactinellid spicules always have six points.

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  • elena
    How is the Morphology of sponges related to its feeding?
    8 years ago
  • christine
    What are the morphologies of sponges?
    2 years ago

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