Impingement Feeding

In the lophophorates the feeding current is generated by the lateral cilia on the tentacles of the lophophore. In gymnolaemate page 153 bryozoans particles impinge near the mouth and are sucked into the pharynx; in phylactolaemate bryozoans particles impinge in the trough between the rows of tentacles of the U-shaped lophophore and are transported to the mouth by cilia. In phoronids and brachiopods particles impinge on the frontal surfaces of the tentacles and are conveyed to the mouth by tracts of cilia.

Impingement feeders thus usually require a means of generating a feeding current and a means of transporting particles impinging on the food collecting surfaces to the pharynx. Certain suspension feeders as well as lophophorates also exhibit these functions.

Although not included in most reviews of suspension feeding, many ciliates, especially the sedentary Heterotrichida, Peritrichida and Chonotrichida, are suspension feeders. Heterotrichs, at least, possess the two systems of cilia typical of impingement feeders. The adoral zone of membranellae creates a vertical feeding current and particles which impinge on the peristome, are carried by cilia down the gullet to the cytostome (Silén, 1947). Food particles may be actively selected in the gullet and unwanted particles rejected (Schaeffer, 1910).

An interesting case, mentioned by Reynoldson (1957), is the specialized peritrich, Urceoloria mitra, which is epizoic on certain freshwater flatworms. In this peritrich the adoral cilia are motionless and form a funnel which directs the water over the gullet. In the gullet, cilia are active and transport food to the cytopharynx. The animal does not need to generate a feeding current. Instead, it selects a site on its host near the auricular sensory areas where the currents produced by the cilia of the host are strongest.

Judging from the photographs published by Machemer (1966) the free-living ciliate Stylonichia possesses the characteristics of an impingement feeder; particles are drawn under the animal over the cytostome from a wide field ahead. A ring-like vortex current system surrounds this main current ahead of the animal. Whether the animal can feed in this manner, away from a substrate, is not known.

Another group with which the method of feeding of bryozoans may be compared are the rotifers. Rotifers show great variation in food preference and feeding methods. Some are wholly carnivorous (Myers, 1941), others are suspension feeders and only collect particles less than 10 microns in diameter (Gossler, 1950), while others trap protistans larger than this (Donner, 1966). The wheel organ may be modified in many ways. In some suspension feeding rotifers a vertical current generated by the cingulum appears to impinge onto the head field and particles are transported to the mouth by the circum-apical band of cilia which lies ahead of the cingulum (Gossler, 1950). Gossler does not agree with Remane (1932) that fans of setae about the head filter particles from the feeding current. At the mouth particles may be grasped by the unci of the mastax (Gossler, 1950; Edmondson, 1965).

The feeding apparatus and feeding method of the cyphonautes, the only type of bryozoan larvae that feeds, have been described by Atkins (1955) who studied the cyphonautes larvae of Electra pilosa and Membranipora membranacea.

The flattened, triangular-shaped larvae has a ventral slit-like opening between the rows of locomotory cilia of the corona. The slit opens into a large cavity divided into two by a vertical ridge on either side in the midline. At the ventral opening to the cavity the ridges extend as free lobes. The cilia on these lobes are arranged in the same way as the cilia on the tentacles of the adults. There is a frontal band, a row of either single or double frontolateral cilia on either side and a lateral band on either side. Within the cavity the lateral and frontolateral rows are lost from the side of the ridge attached to the mantle. The frontal cilia beat towards the mouth and the lateral cilia generate the feeding current between the ridges. Atkins thought, but was not sure, that the tips of the frontolateral cilia beat in the direction of the current. The walls of the mantle in the dorsal part of the inhalant chamber are ciliated and converge on the oesophagus. The lateral cilia on the ridges generate the feeding current and form metachronal waves which travel in a laeoplectic fashion as in the adult; that is, the current traversing the ridges passes to the left of the direction of progression of the waves. Food particles which impinge on the frontal surfaces of the ridges are conveyed by the frontal cilia to the cilia in the buccal region which in turn convey them to the oesophagus. Atkins’ (1955) desbription gives no evidence for the presence of a sucking pharynx. Possibly the sharp turn that the feeding current must make as it passes between the ciliated ridges throws some food directly into the buccal region.

Feeding in cyphonautes thus resembles feeding in adult gymnolaemates and other lophophorates although the frontal cilia on the ridges play a more important part than the frontal cilia on the tentacles of adult gymnolaemates.

Ciliary Feeding

A variety of other suspension feeders all employ cilia to generate feeding currents but the method by which they collect particles suspended in the current differs from that employed by impingement feeders.

In entoprocts (Atkins, 1932; Mariscal, 1965) and in serpulimorph polychaetes (Nicol, 1930), lateral cilia on the tentacles or cirri respectively, generate the feeding current which moves up from beneath the tentacles or cirri—that is, it is opposite in direction to the feeding current in lophophorates. Suspended particles are thrown onto the inner or frontal surface of the tentacles or cirri from where cilia carry them towards the mouth.

Mariscal (1965) claims that in entoprocts the particles are embedded in mucus on the frontal surface of the tentacles, but he did not find gland cells. Atkins (1932) could not find gland cells associated with the tentacles or vestibular grooves leading to the mouth in these animals, however, rejected particles often appeared in a string of mucus. Similarly, in Sabella pavonia, Nicol (1930) thought only rejected particles were enmeshed in mucus.

Mariscal states that Barentsia gracilis feeds on detritus supplemented with protistans and Dales (1957) found that serpulimorph polychaetes filtered colloidal graphite better than algal cells and suggested that they fed largely on inert detritus rather than on protistans.

The veliger larvae of gastropods and pelecypods are suspension feeders. Long cilia form a band along the smooth or lobed velum in these larvae, while recessed beneath this band is a groove lined with cilia which leads to the mouth (Yonge, 1926; Lebour, 1931; Werner, 1955; Thompson, 1959).

A cilium during its forward stroke is stiff and straight (Sleigh, 1962). The locomotory cilia on the velum margin apparently strike particles during their forward beat and throw them into the food groove, from whence they are conveyed to the mouth. This type of feeding resembles that already described as ciliary feeding to distinguish it from impingement feeding already discussed. However, it must be borne in mind that cilia often play a part in the other methods of feeding as well.

Another example of ciliary feeding is exhibited by the colonyforming rotifer, Conochilus unicornis. In this species, the feeding current is reversed and the long cilia of the cingulum throw particles onto the head field where tracts of short cilia transport them to the mouth (Gossler, 1950).

In those suspension feeding rotifers in which the trochus (the line of strong cilia ahead of the circumapical band of cilia) is well developed, ciliary feeding and not impingement feeding, might also be expected.

Ciliated epithelia bearing a layer of mucus are encountered in many animals, for example; in the mammalian lung, on the proboscis of enteropneusts, and on the inner surface of the mantle of brachiopods and molluscs. The combination of cilia and mucus appears to function to keep such surfaces clean, and in ciliary and impingement feeders mucus is also involved in particle rejection from the food collecting organ.

Filter Feeding

Mucus has a clear roll in trapping food particles in such animals as the polychaetes, Chaetopterus and Nereis, which form a mucous trap, and in certain prosobranch gastropods such as Crepidula, in ascidians and in cephalochordates, all of which secrete a moving page 156 sheet of mucus over the branchial organ. In these animals the mucus and the embedded particles are ingested. It is not known how much sorting takes place after the particles are trapped, but MacGinitie (1939) has suggested that there is some sorting at least.

As already mentioned, mucus appears to be confined to trapping rejected material in brachiopods, phoronids, entoprocts and serpulimorph polychaetes. In this way, such material will be prevented from being recirculated through the feeding organ.

Despite the large amounts of work on feeding in pelecypods, (reviewed by Jorgensen, 1966), there remains some doubt as to the role of mucus in the feeding organ of these animals. MacGinitie (1941) claimed that a mucous sheet lining the gills acts as a filter as in ascidians and cephalochordates. Other authors (Yonge, 1966; Jorgensen, 1966) hold that such a mucous sheet would not permit the elaborate arrangement of tracts of cilia on the gills to function and particles could not be sorted. Also, pelecypods do not possess an elaborate organ, such as the endostyle in cephalochordates and ascidians, for the continuous production of mucus. It seems most likely that, in this group as in the animals mentioned above, mucus functions mainly to remove unwanted particles, but a definitive study is needed.

If there is no mucous sheet, the gills of peleypods may employ at the same time, impingement, ciliary and filter feeding methods. Lateral cilia on the gill filaments generate the feeding current, and particles which impinge directly on the frontal surface of the filaments could be conveyed to the food grooves at the dorsal and ventral margins of the gills by the frontal cilia, while frontolateral cilia could catch particles passing between the filaments or through the ostia and throw them onto the frontal surfaces of the filaments (Tammes and Dral, 1955).

Rudwick (1962) pointed out that, unlike the filamentous tentacles of the brachiopod lophophore; in the more advanced pelecypods the gill lamellae are cross-connected to form a net. Such a structure is able to support a greater filtration pressure between the inhalant and exhaust chambers than could the filiform tentacles of brachiopods.

The net-like gills of pelecypods would also be expected to retain large mobile protistans too large to pass between the filaments. Thus pelecypods may also be classed as filter feeders since their gills act as a filtration separator of the larger suspended particles. Other true filter feeders include the mucous trap formers mentioned earlier, sponges (whose choanocytes filter water through their

Fig. 1: Examples of the different methods of suspension feeding.
A. Bryozoan, Zoobotryon verticillatum (from Bullivant, 1968).
B. Nudibranch veliger, Archidoris psuedoargus (from Thompson, 1959).
C. Barnacle, Balanus sp., (from photograph by William H. Amos).
D. Vermetid gastropod, mucus trap extended (after Morton, 1955).

page 158 collars), the many crustacean suspension feeders which sweep or filter particles from the water with their appendages, and probably larvaceans which secrete an elaborate filter house.

Collision Feeding

Three groups of suspension feeders have already been mentioned. For the sake of completeness a fourth group should be included. These may be described as collision feeders. Some vermetid gastropods secrete strings or sheets of mucus which hang passively in the water collecting particles which collide with them. Planktonic Foraminifera and Radiolaria presumably collect particles by collision. Crinoids, some ophiuroids and dendrochirote holothurians are suspension feeders and appear to depend on food particles drifting to them unassisted by a feeding current.

Plankton feeding coelenterates, hydroids, gorgonaceans, etc., may be included under this heading. In these animals collision of particles with the tentacles is followed by capture by the nematocysts.

The classification of suspension feeders put forward in this discussion is summarised below.