prey, 2) the protective spination of various species, 

 3) the strength and sensory acuity of larger zoo- 

 plankters, and 4) the active search patterns of 

 "setting out" tentacles by the ctenophore. Each of 

 these four factors in addition to other variables, 

 which are determined by the relative abundance 

 and movement of species in nature, have some 

 bearing on the selection of prey by Pleurobrachia. 

 The first consideration is time-space co- 

 occurence of prey with the ctenophore. Since 

 the ctenophores are neritic and mostly live close to 

 shore in the upper 50-60 m, they will occur with 

 surface-living holoplanktonic and meroplank- 

 tonic species, only coexisting with deeper-living, 

 migratory species at night. Secondly, the 

 ctenophores will most frequently encounter the 

 most abundant organisms in numbers per unit 

 volume. Size and swimming activity of the prey 

 are also important to determine the chance of en- 

 counter with the tentacles. Bodily length deter- 

 mines the likelihood of retention of a given or- 

 ganism by the tentacle net, and swimming activ- 

 ity determines how often the prey will encounter a 

 given ctenophore if swimming in a random man- 

 ner. Rowe (1971) has shown, using Artemia 

 nauplii, that the instantaneous feeding rate of P. 

 pileus follows the form for effusion of an ideal gas; 

 this requires the assumption that prey move about 

 randomly. However, I have seen P. bachei make 

 at least three different types of settings of its ten- 

 tacles in apparent attempts to alter the pattern of 

 search for prey: 1) a double helix set like two 

 interwoven corkscrews perpendicular to a level 

 surface with the body at the uppermost end, 2) a 

 pair of spirals parallel to a level surface with the 

 ctenophore body at the outer end of the spiral, and 

 3) linear and curved sets which are placed at dif- 

 ferent angles with respect to the vertical and with 

 the ctenophore body either heading up or down. 

 The types of tentacle settings may be adaptive 

 responses to the nonrandom swimming patterns of 

 different zooplankton species, some of which move 

 more in a horizonal or a vertical plane. It is at this 

 point that animal behavior becomes very impor- 

 tant. Species which co-occur with Pleurobrachia 

 and are relatively abundant (up to several 

 hundred per cubic meter) are not necessarily 

 eaten by this ctenophore, because these potential 

 prey probably use their sensory acuity and 

 locomotive power to avoid danger. One outstand- 

 ing example is S. euneritica, a species which is 

 very fast and difficult to catch compared to most 

 zooplankton; it had a highly negative electivity 



FISHERY BULLETIN: VOL. 72. NO. 2 



index (Table 9). Assuming that a prey organism 

 has just made contact with the ctenophore tenta- 

 cles, three possible outcomes have been observed 

 in the laboratory for different species: 1) the prey 

 is too strong and breaks away from the tentacle 

 hold; 2) the prey provides a strong escape re- 

 sponse, becomes further entangled and is eaten; 

 and 3) the prey provides little or no escape re- 

 sponse, remains nearly motionless and "plays 

 dead," often being dislodged from the tentacle hold 

 and not eaten. A species which is too powerful for 

 P. bachei to capture is H. mediterranea. The 

 adults of this amphipod can break away from the 

 entanglement and also have the ability to exploit 

 the ctenophores as a predator. Prey which provide 

 a strong, "calanoid escape response" are almost 

 always further entangled by swirls of the tentacle 

 branches and are eaten. The immediate strug- 

 gling and pulling away appears to signal the 

 ctenophore of a successful prey capture, much as I 

 would expect that a spider detects the impact and 

 vibrations of the prey struggling on its web. The 

 copepods such as Acartia, Labidocera, Calanus, 

 etc., exhibit strong escape responses when stimu- 

 lated by contact or approaching danger. Two prey 

 species were observed to exhibit the motionless or 

 "play dead" response. These are C. anglicus and 

 P. auirostria. Penilia is also one of the species 

 which has a negative electivity index or is taken 

 less frequently than in proportion to abundance in 

 the water. Once the prey is brought to the mouth of 

 the ctenophore, the next limitations are the 

 configuration of the prey body and appendages 

 plus the protection from external spination. Bodi- 

 ly shapes such as those ofSagitta and zoea larvae 

 of Porcellanidae (a family of crabs) create difficul- 

 ties for their ingestion by Pleurobrachia. Large 

 Sagitta must be bent in half and ingested at the 

 middle section first (observations are from the 

 laboratory work; gut contents from field sampled 

 ctenophores show that this event is very infre- 

 quent). The long anterior and posterior spines of 

 the porcellanid zoeae prevent full ingestion and 

 digestion entirely, although the prey probably do 

 not survive the capture. Many other decapod lar- 

 vae possess stout spines and very thick exoskele- 

 tons (e.g., Emerita larvae), which prevent inges- 

 tion and would retard digestion as well. Some 

 brachyuran zoeae which have dorsal and lateral 

 spines have been observed to cut open the 

 ctenophore gut wall during ingestion. Recall that 

 brachyuran zoeae only make up 0.25% of the total 

 number of prey in ctenophore guts (Table 7). 



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