SERFLING and FORD: ECOLOGICAL STUDIES OF PANULIRUS INTERRUPTUS 



localities and work there was discontinued. 

 Night-lighting observations during 1969 at the 

 Scripps Institution pier provided a more complete 

 picture of the apparent seasonal occurrence of 

 pueruli, which first appeared in late May and were 

 not observed after 4 September 1969. 



These initial night-light studies allowed several 

 interesting new observations on puerulus 

 behavior. Previous investigators who reported 

 that pueruli were occasionally attracted to night- 

 lights (Harada 1957; Johnson 1960) presumed that 

 the pueruli were lured off the bottom from rocky 

 habitats where they had settled. However, this 

 apparently was not the situation at the Scripps 

 Institution pier for the following reasons. 



The natural bottom substrate adjacent to the 

 Scripps Institution pier for a radius of at least 500 

 m is entirely surf-washed sand. There is a small 

 intertidal and subtidal rocky area approximately 

 500 m to the north, and an extensive rocky 

 shoreline begins approximately 2 km to the south. 

 While the pier pilings might afford a suitable 

 habitat, we observed no pueruli or juveniles in 

 several careful daytime and nighttime examina- 

 tion of the pilings, using scuba, during periods 

 when there was active puerulus settlement in the 

 habitat traps. In addition, direct observations of 

 the free-swimming pueruli indicated that many 

 individuals approached the light from a direction 

 opposite that of the pilings. Perhaps most sig- 

 nificant was the observation that the pueruli were 

 seen swimming in only the top few centimeters of 

 water; no individuals were ever seen approaching 

 the light from a greater depth. 



This opportunity to observe the free-swimming 

 puerulus stage of P. interruptus thus demon- 

 strated that, at least under these conditions, it is a 

 surface-dwelling organism which occurs only in 

 the top few centimeters of water. Consequently, 

 the inconsistency or failure of standard plankton 

 net tows to collect pueruli of this and other species 

 probably is not due to the presumed benthic habits 

 of this stage, but rather to improper sampling 

 techniques. Only nets extending above the sur- 

 face, and streamed parallel to the vessel's course 

 with an unobstructed path, appear to be suitable. 

 The likelihood of net avoidance accounting for 

 previous failures seems minimal, as the pueruli of 

 this species are relatively slow moving and easily 

 captured in a hand held dip net, at least near a 

 night-light. 



Puerulus swimming speed in nature was 

 measured by timing the passage of an individual 



between two lines suspended from a 2 m long, 

 horizontally oriented pole, as it approached the 

 underwater light. The mean swimming speed of 

 six different pueruli under these conditions was 8 

 cm/s (range 6 to 9 cm/s). Individuals appeared to 

 maintain this speed continuously, unless dis- 

 turbed. In response to disturbance, they would 

 spread their antennae and legs, sink slowly 5-20 

 cm, and then resume normal surface cruising with 

 antennae held together and legs withdrawn. 

 Illustrations of these swimming and sinking pos- 

 tures are shown in Figure 5. 



SINKING POSTURE 



SWIMMING POSTURE 



Figure 5.— Sinking and swimming postures of the puerulus lar- 

 val stage of Panulirus interruptus as observed under a night- 

 light. 



Puerulus Pigmentation in Relation 

 to Date of Settlement 



Newly settled pueruli, or those collected during 

 night-lighting, were always completely trans- 

 parent. Pueruli held in open and closed system 

 aquaria provided with Phyllospadix or other in- 

 tertidal flora and fauna almost immediately began 

 acquiring pigmentation, and this continued at a 

 rapid rate. At temperatures of 18-20°C, pueruli 

 would change from complete transparency to 

 nearly complete pigmentation within 9-10 days, 

 and would then moult into the first postpuerulus 

 stage. 



This occurred consistently among over 50 in- 

 dividuals we held in aquaria and observed 

 systematically. Thus, the degree of pigmentation 

 provided a reliable method of determining, within 

 1-2 days, the date of settlement. This technique 



365 



