Form and Function

Sponges feed primarily by collecting suspended particles from water pumped through internal canal systems. Water enters canals through a multitude of tiny incurrent pores in the outer layer of cells, a pinacoderm. Incurrent pores, called dermal ostia, have an average diameter of 50 μm. Inside the body, water is directed past the choanocytes, where food particles are collected on the choanocyte collar. The collar comprises many finger like projections, called microvilli, spaced about 0.1 μm apart. The use of the collar as a filter is one form of suspension feeding. One may be surprised to discover that a large sponge can filter up to 1500 liters of water each day. Sponges non selectively consume food particles sized between 0.1 μm and 50 μm. The smallest particles, accounting for about 80% of the particulate organic carbon, are taken into choanocytes by phagocytosis. Choanocytes may acquire protein molecules by pinocytosis. Two other cell types, pinacocytes and archaeocytes, play a role in sponge feeding. Sponges may also absorb dissolved nutrients from the water.

The capture of food depends on the movement of water through the body. How is water flow controlled in an animal with such as simple body? There are three main designs for a sponge body, differing in placement of the choanocytes. In the simplest asconoid system, the choanocytes lie in a large chamber called the spongocoel; in the syconoid system, the choanocytes lie in canals; and, in the leuconoid system, the choanocytes occupy distinct chambers. These three designs demonstrate an increase in complexity and efficiency of the water pumping system, but they do not imply an evolutionary sequence. The leuconoid grade of construction is of clear adaptive value; it has the highest proportion of flagellated surface area for a given volume of cell tissue, so it efficiently meets food demands. This leuconoid grade has evolved independently many times in sponges.

Types of Canal Systems

1. Asconoids

Asconoid sponges have the simplest organization. The sponge draws water inside through microscopic dermal pores by the beating of large numbers of flagella on the choanocytes. These choanocytes line the internal cavity called the spongocoel. As the choanocytes filter the water and extract food particles from it, used water is expelled through a single large osculum. This design has distinct limitations because choanocytes line the spongocoel and can collect food only from water directly adjacent to the spongocoel wall. Were the spongocoel large, most of the water and food in its central cavity would lie in “dead space,” inaccessible to choanocytes. Thus, asconoid sponges are small and tube-shaped. As an example, examine Leucosolenia (Gr. leukos, white, + solen, pipe), where slender, tubular individuals grow in groups attached by a common stolon, or stem, to objects in shallow seawater. Clathrina (L. clathri, latticework), another asconoid, has bright yellow, intertwined tubes. Asconoids occur only in class Calcispongiae.

2. Syconoids

Syconoid sponges look somewhat like larger asconoids. They have a tubular body and single osculum, but the body wall, which is really the spongocoel lining, is thicker and more complex than that of asconoids. The lining has been folded outward to make choanocyte-lined canals. Folding the body wall into canals increases the surface area of the wall and thus increases the surface area covered by choanocytes. The canals are of small diameter compared with an asconoid spongocoel, so most of the water in a canal is accessible to choanocytes.

Water enters the syconoid body through dermal ostia that lead into incurrent canals. It then filters through tiny openings, or prosopyles, into the radial canals. Here food is ingested by the choanocytes. The beating of the choanocytes’ flagella forces the used water through internal pores, or apopyles, into the spongocoel. Notice that food capture does not occur in the syconoid spongocoel, so it is lined with epithelial-type cells rather than the flagellated cells present in asconoids. After the used water reaches the spongocoel, it exits the body through an osculum. As an example, examine Sycon.

During development, syconoid sponges pass through an asconoid stage, following which flagellated canals form by evagination of the body wall. This developmental pattern provides evidence that syconoid sponges were derived from an ancestor with an asconoid body plan, but the syconoid condition is not homologous among all the sponges that possess it. Syconoids occur in class Calcispongiae and in some members of class Hexactinellida.

3. Leuconoids

Leuconoid organization is the most complex of the sponge types and permits an increase in sponge size. In the leuconoid design, the surface area of the food-collecting regions with choanocytes is greatly increased; here the choanocytes line the walls of small chambers where they can effectively filter all the water present. The sponge body comprises an enormous number of these tiny chambers. Clusters of flagellated chambers are filled from incurrent canals and discharge water into excurrent canals that eventually lead to an osculum.

A sponge pumps a remarkable amount of water. Leuconia, for example, is a small leuconoid sponge about 10 cm tall and 1 cm in diameter. It is estimated that water enters through some 81,000 incurrent canals at a velocity of 0.1 cm/second in each canal. However, because water passes into flagellated chambers with a greater cross-sectional area than those of the entry canals, water flow through the chambers slows to 0.001 cm/second. Such a flow rate allows ample opportunity for food capture by choanocytes. Leuconia has more than 2 million flagellated chambers where food collection occurs.

After food is removed, the used water is pooled to form an exit stream. The exit stream, containing the entire volume of water that entered the sponge over the myriad incurrent canals, leaves the sponge through an exit pore whose cross-sectional area is many times less than the total cross-sectional area of all the incurrent canals. The relatively small size of the exit pore, together with the large volume of used water, produces a very high exit velocity. In Leuconia, all water is expelled through a single osculum at a striking velocity of 8.5 cm/second—a jet force capable of carrying used water and wastes far enough from the sponge to avoid refiltering. Some large sponges can filter 1500 liters of water a day, but unlike Leuconia, most leuconoids form large masses with numerous oscula, so that water exits from many local sites on the sponge. Most sponges are of the leuconoid type; leuconoid bodies account for most species within class Calcispongiae and are the most common types in other classes.