PREZANT: ANTIPREDATION MECHANISM OF PHYLLODOCE MUCOSA 



consumed, the predator may set up a "specific 

 searching image" iTinbergen 1960), which would 

 increase its chance of locating additional speci- 

 mens provided more of the same prey species can 

 be found while remforcement is still fresh. Thus, 

 this pattern is only relevant when prey species 

 occur in relatively high densities. Phyllodoce mu- 

 cosa is found in moderately high densities in Na- 

 hant Bay with many other polychaetes such as 

 Prionospw malmgreni, Scoloplos armiger, and 

 Nephtys spp. If a small fish encounters and at- 

 tempts to eat a Phyllodoce mucosa but is repulsed 

 by the worm's defenses several times, the fish may 

 eventually set up a negative searching image and 

 thus avoid further "discomfort" caused by at- 

 tempted ingestion. While simultaneous choices of 

 food may be a rare event in nature (Beukema 

 1968), when it does occur between a phyllodocid 

 and another tvpe of prey of similar size, a fish with 

 a negative image may "select" the nonphyllodocid 

 prey. This is indicated in the present data by the 

 relationship between ingestive attempts and in- 

 vestigations of Gasterosteus aculeatus and the 

 "loss of interest" shown by the smaller Menidia 

 menidia. 



Kneib and Sti ven ( 1978 ) recently found that the 

 diet of F. heteroclitus in a IJorth Carolina salt 

 marsh varied with the size of ;;he fish (smaller fish 

 were carnivorous while laiger individuals were 

 omnivorous). In this case, alteration in diet 

 seemed to reflect a physiological and morphologi- 

 cal change in the fish with growth. This conversion 

 of food habits may be based upon the ability of the 

 fish to eat different food items because of its pro- 

 portionally larger size or it might indicate a change 

 in the "ability" of the fish to consume less appeal- 

 ing food items if the "need" arises. Data presented 

 here indicate that larger M. menidia might be 

 more effective in consuming phyllodocids than 

 smaller M. menidia. Smaller fish may not be able 

 to "handle" a phyllodocid of a size that a larger fish 

 might readily consume. This is based solely upon 

 the reaction of the fish to the mucoid secretion 

 since smaller fish were able to consume compara- 

 tively large nonphyllodocid polychaetes. 



The largest fish used in the present study, a 

 juvenile Lophopsetta maculata, about 9 cm long, 

 consistently rejected P. mucosa. Lophopsetta 

 maculata is an active predator of mobile prey (Ta- 

 ble 1 ). The large buccal chamber and distensibility 

 of the esophagus of this flounder preclude the pos- 

 sibility that the phyllodocid mucus acts as a physi- 

 cal barrier to ingestion (i.e., an occlusive plug) but 



instead indicate that the mucus contains some 

 irritating or obnoxious substance which repels the 

 fish. 



The high sensitivity and secretory nature of the 

 parapodial cirri is reflected in the complex ultra- 

 structure shown in Figure 2, however, P. mucosa 

 does not seem able to continually produce an 

 adequate supply of protective mucus. This is indi- 

 cated by ingestion of the worm by G. aculeatus 

 following numerous rejections from this fish's 

 small, sharply toothed buccal cavity which may 

 have removed the protective cover. Similar results 

 are obtainable with a smallmouthed pipette, 

 which simulates this. The large, empty vacuoles in 

 the dorsal cirri surrounded by immature muco- 

 cytes indicate a lag between total loss of available 

 secretion and maturation of additional, functional 

 mucocytes. 



Beukema (1968) suggested that G. aculeatus 

 hunts by sight only and its sense of smell plays 

 little if any role in finding food. This is supported 

 in the data presented here by the correlation be- 

 tween investigations and ingestive attempts. In- 

 vestigations involved no direct contact but only 

 close observation of the worm by the fish. 



Ejectory behavior by G. aculeatus feeding on 

 clumps ofTubifcx spp. oligochaetes was discussed 

 by Tugendhat (1960) who found that this action 

 caused a breakdown of the clumps into individual 

 worms which were easily ingested. Hynes ( 1950) 

 noted that young G. aculeatus feed on proportion- 

 ally smaller prey items and that the diet changed 

 to larger prey as the fish grew. The largest phyl- 

 lodocid fed to a G. aculeatus in the present study 

 was 24 mm long, and it was investigated but only 

 one attempt at ingestion was made. In this case, 

 the worm probably was too large for the fish to 

 deal with. 



The only species of fish tested which consis- 

 tently ate P. mucosa wasF. heteroclitus. Fundulus 

 heteroclitus, a well-known inhabitant of salt 

 marshes, is only rarely found in strictly saline 

 environments iHildebrand and Schroeder 1928). 

 Vinceet al. ( 1976) showed thatF. heteroclitus may 

 cause an impact on the abundance and distribu- 

 tion of some prey species, and Eraser ( 1973) found 

 that Fundulus spp. would consume prey items in 

 proportion to prey densities. Since P. mucosa is not 

 a normal resident of salt marshes the question 

 must be asked: does the fact that these two or- 

 ganisms occur in different environments influence 

 the predator-prey interactions between these 

 species when brought together? According to Tin- 



613 



