FISHERY BULLETIN: VOL. 71, NO. 1 



megalopae therefore show in this situation the 

 behavior of adults. 



Further development along the lines already 

 established by the megalopa is shown by the suc- 

 ceeding juvenile stages. Respiratory currents 

 become much stronger and well defined and are 

 now aided by the tandem motion of the two sets 

 of maxillipedal exopodites. Feeding movements 

 are the same as those described here for the 

 megalopa and for the adult by Nicol (1932). 

 In adults, food preferences vary according to 

 species. Knudsen (1964) reports that pelagic 

 diatoms are the preferred food for Petrolisthes 

 eriomerus but does not state the preferences of 

 Pachycheles rudis. In the laboratory, adults of 

 all species readily fed in unfiltered seawater, but 

 only Petrolisthes females accept and ingest frag- 

 ments of mantle and adductor muscle of Mytihis. 

 The preferred adult locomotion is pereipodal 

 walking, but swimming by abdominal clapping 

 is also used, especially in Petrolisthes. 



DISCUSSION 



PREZOEA 



Adults of both Petrolisthes species live where 

 the larvae are released into turbulence caused by 

 waves and currents. These conditions favor the 

 presence of a short-lived, rounded initial larva 

 with few body projections, that would more 

 readily escape entangling algae and debris in the 

 intertidal zone. 



The habitats of the two Pachycheles species 

 (Haig, 1960) more strongly favor a compact, 

 rounded, initially spineless larval form. In many 

 cases adults become so large that they are unable 

 to pass out through the openings of the burrows 

 or crevices they inhabit. Larvae are released 

 within the adult burrow and must escape this 

 confinement to survive. The prezoeal cuticle 

 covering the telson in P. piibescens larvae is well 

 modified for swimming. This was described by 

 Gurney (1942) for other decapods and by Le- 

 bour (1943), Wear (1965), and Greenwood 

 (1965) for other porcellanids. 



These observations suggest that in the four 

 species considered here, and probably in the 

 family as a whole, the prezoea is a short-lived 

 natural stage and is not a laboratory artifact 



as has often been suggested. Its existence as 

 a transport stage is ecologically consistent with 

 the natural habitat of the adults. Similar argu- 

 ments have been put forth by Gore (1968) in 

 defense of the interpretation of the prezoea as 

 a natural stage in the commensal porcellanid 

 Polyonyx gihhesi, which releases larvae from in- 

 side the tube of the polychaete Chaetopterus. 

 Another observation supporting this argument 

 is that, under laboratory conditions, true zoeae, 

 with their long spines, respond very unfavorably 

 to collisions, spine breakage, and collection of 

 detrital material on the spines, all of which 

 would be likely to occur in nature if full zoeae 

 emerged from the eggs and were released into 

 the adult environment. Photopositive swim- 

 ming behavior would also prove useful to pre- 

 zoeae released in burrows and crevices, and the 

 larvae studied showed a photopositive response. 

 This response was, however, weak under lab- 

 oratory conditions. 



ZOEA 



With the passage of the larva through the 

 prezoeal molt, the first true zoea emerges and 

 becomes an actively swimming planktonic car- 

 nivore. Despite the good swimming ability and 

 well-developed eyes of the zoea, no evidence of 

 true hunting behavior was found. Instead, the 

 larvae appear to rely entirely on chance en- 

 counters with prey, with capture behavior ini- 

 tiated by direct contact or by vibrations stimu- 

 lating the maxillipedal endopodites and setae and 

 the ventral surface of the abdomen. Similar 

 stimulation of other parts of the body elicits an 

 escape response by the zoea. Survival of these 

 zoea in the plankton is probably highly depen- 

 dent upon suitable prey density. The method of 

 prey capture used by these larvae, involving the 

 use of the telson to scoop up the prey and hold 

 it from below, appears to be a feeding method 

 used by zoea throughout the Decapoda. Knudsen 

 (1960), for example, describes this method of 

 feeding in xanthid Brachyura. 



MEGALOPA 



Many of the adult behavioral features de- 

 scribed by Nicol (1932) appear in the megalopa 



232 



