Copepoda
Feeding and feeding modes

Copepods are recognized as indiscriminate or selective suspension or particle feeders. They may be herbivorous, predatory feeders (carnivores), mixed-mode feeders (omnivores) (Arashkevich, 1969; Davis, 1984; Schnack, 1989), saphrophages or coprophages (Ohtsuka et al., 1996). However some copepods are apparently able to switch from one feeding mode to another according to circumstances (Lowndes, 1935; Kleppel, 1993). The type of feeding in general is usually reflected in the structure of oral parts and the type of locomotion. The main food items for suspension-feeders are phytoplankton and microzooplankters (ciliates, tintinnids, copepod nauplii). Suspension feeders usually move slowly and smoothly when feeding producing water streams around the body by beating of the oral parts. In general suspension-feeding calanoids have A2, Mdp, Exp, and Enp as well as Mx1 Le1 (coxal epipodite), Exp and Enp supplied with long, strong, densely plumose setae important in the current production process. High-speed photography (500 frames per second) of dye streams around feeding calanoid copepods has led to a revision of previous accounts of copepod feeding (Alcaraz et al., 1980; Koehl and Strickler, 1981; Strickler, 1982; Price et al., 1983; Turner et al., 1993). Copepods in their environment are dominated by viscous forces and the flow around them is laminar. Their appendages behave more like solid paddles than open rakes (Koehl and Strickler, 1981). They flap their A2, Mdp, Mx1, and Mxp to produce a pulsing stream of water which brings food particles near the stationary Mx2 (Fig. Co.4). The Mx2 then fly apart so that water and the food particles flow into the basket formed by the Mx2 which close over the algae and water. The endites of Mx1 then pushes the algal cells into the mouth. Algal cells are usually redirected without actually being touched by the feeding appendages. Similarly, particles can be rejected by movements of the Mx2. The capture of small cells produce a different behaviour. The Mx2 move continuously with a low amplitude interrupted by "combing" of the appendages by scraping their setae against the endites of Mx1. In these studies it was also suggested that herbivorous and carnivorous feeding are similar processes in calanoids and that predators catch their prey by grasping it with the tips of Mx2 and Mxp setae or capture a parcel of water containing the prey.
(Fig. Co.4 , copepod feeding).

Carnivorous calanoids differ from suspension-feeders in that the Md has a strongly chitinized Gn with teeth narrow and saber-shaped on the cutting edge. Usually the ventral teeth are sharper in raptorial predators. The fewer the number of teeth, the greater the degree of carnivory (Arashkevich, 1969; Ohtsuka, 1991; Ohtsuka et al., 1996). Also the Mx1 is usually characterized by strongly developed Li1-Li3, with setae of Li1 spine- and dagger-shaped, and the setae of Enp strong, claw-like, and serrated along their whole length. The setae of Le1 are usually plumose. The Mx2 has the majority of Li bearing stout, spine-like setae, and sometimes those on the distal Li are highly chitinized and claw-like, and often serrated. The Mxp is massive with spine- and claw-like setae. Many setae on the oral parts lack setules. The Mx2 and Mxp may be developed in different ways: for example in Candacia the Mx2 is more powerfully developed than the Mxp. On the other hand, in Euchaetidae and Pseudeuchaeta (Aetideidae), the Mxp is more powerfully developed. However in both cases they are both prehensile appendages used for grasping. The recent study of the Md of carnivorous heterorhabdids showed that an anaesthetic is injected into a groove or tubular lumen of the ventralmost tooth of the mandible and released into prey from a hypodermic needle-like tip of the tooth (Nishida and Ohtsuka, 1996), confirming the earlier proposal that the heterorhabdids are sucking predators based on the structure of their oral parts (Arashkevich, 1969).

The morphology of omnivore oral parts is, in general, intermediate between that of suspension-feeders and predators. Scolecitrichids, with oral parts close in structure to those of mixed-mode feeders, possess sensory setae on the distal part of Mx2 and feed on detritus, discarded appendicularian houses, and exuviae (Ferrari and Steinberg, 1993; Ohtsuka et al., 1996). The mechanism of detecting food particles or prey is speculated to be chemosensory.

In harpacticoids direct carnivory of metazoan organisms is relatively unknown, some are known to be suspension- and mucus trap-feeders, but the majority of harpacticoids are regarded as surface feeders (Huys et al., 1996). Cyclopoids more probably are carnivorous, capturing food actively, but some (Oithona similis) are mainly herbivores (Davis, 1984). A recent study of sapphirinid copepods leads to the proposal that these copepods are free living during daytime, but attach at night to other animals and feed on their tissues (Nishida and Chae, 1996); and oncaeaid copepods were observed to be active predators on larger zooplankters, such as Sagitta and Oikopleura (Davis, 1984), or have a surface mode of feeding (Boxshall, 1981) having been observed grazing on appendicularian houses.