FISHERY BULLETIN: VOL. 71, NO. 1 



simultaneous change in the pitch of the max- 

 illipeds so that the exopodites beat downward 

 and anteriorly. This rapid adjustment sends 

 the larva on its way backward with its posterior 

 spines leading. Unlike some larvae of the Gal- 

 atheidae (Foxon, 1934), which swim only back- 

 ward with the telson leading, the zoeae of por- 

 celain crabs swim almost equally well in either 

 direction. Normally, zoeae swim forward, re- 

 versing direction on contact with an object. 



Zoea larva are voracious predators and are 

 cannibalistic if not well supplied with other 

 types of food. However, true hunting behavior 

 of the kind reported for certain other predatory 

 zoeae (Knudsen, 1960) was not observed. In 

 fact, when a potential food organism touches any 

 portion of the rostrum or posterior spines, or 

 the top or sides of the carapace, the zoea imme- 

 diately moves away from the point of contact, 

 apparently in an act of avoidance. If, however, 

 the prey approaches the thoracic region of a 

 zoea closely enough to touch the setae of the 

 ventrally extended maxillipedal endopodites 

 (Figures 2, 3) or the ventral surface of the first 

 two segments of the abdomen, the zoea reacts 

 immediately. The prey is clutched between the 

 endopodites, and the telson is used to force it 

 forward and upward within reach of the func- 

 tional mouth parts. This sequence has been ob- 

 served many times, with no indication that sight 

 is involved in locating and capturing the prey. 

 The zoea also demonstrates what appears to be 

 sensitivity to water movements. If a prey or- 

 ganism passes near the thoracic or abdominal 

 region of the zoea but does not touch the sensitive 

 setae, the zoea, without the stimulus of direct 

 contact, will go through all the motions normally 

 associated with the capture of prey. In these 

 cases, the prey is usually out of reach, and the 

 attempts to capture the passing animal fail. 



The second zoeal stage is only slightly more 

 complex morphologically than the first stage, and 

 behavior patterns remain essentially the same 

 throughout both zoeal stages. Metamorphosis 

 to the megalopa (Figure 4), however, brings 

 about immediate and drastic changes in struc- 

 tures and habits of the larva. Basic behavior 

 patterns which were stable in the zoeal stages 

 undergo progressive changes throughout the 



megalopa stage, gradually becoming more adult- 

 like in nature. 



Figure 4, — Megalopa of Pachycheles pubescens, in for- 

 ward pleopodal swimming posture, except antennae not 

 extended posteriad. PI, detail view of pleopod. 



MEGALOPA 



In the megalopa respiratory currents are pro- 

 duced primarily by fanning motions of the max- 

 illary scaphognathites (Figure 5C) and are 

 aided by the outward flicking of the setose ex- 

 opodite of maxilliped II (Figure 5B). The out- 

 ward flicking causes an exhalent current by 

 "spooning out" the branchial cavity. The ex- 

 opodite of maxilliped III is nonfunctional at this 

 stage. In addition, a newly settled larva ele- 

 vates its body above the substrate and concur- 

 rently vibrates its pleopods (Figure 4), thus in- 

 creasing circulation of the surrounding water. 



Cleaning behavior becomes highly complex in 

 the megalopa. The fifth pereiopods (Figure 4), 

 which are mobile, chelate, and armed with spe- 

 cialized hooked setae and dense clusters of short 

 bristles, are carried folded along the sides of the 

 carapace when not in use. These structures 

 serve to clean all portions of the abdomen and 

 telson, the three pairs of functional walking legs, 



228 



