FISHERY BULLETIN: VOL 72, NO. 4 



interactions was a compromise between a repre- 

 sentative density which might result from a large 

 stocking of//, americanus in natural habitats on 

 the southern California coast and the physical 

 limitations of the experimental tanks which 

 prevented using more P. interruptus. Both males 

 and females of each lobster species were used 

 for experiments. However, only male C. anten- 

 narius were used because all females obtained 

 were carrying eggs at the time these experiments 

 were conducted. 



Interaction experiments involving P. inter- 

 ruptus and C. antennarius were employed to 

 help in the process of assessing potential effects 

 H. americanus might have on P. interruptus. 

 That is, we wanted to establish a behavioral 

 base line by using a clawed decapod crustacean 

 which is naturally sympatric with P. inter- 

 ruptus. Thus, the 5:1 ratio used for the C. 

 antennarius and P. interruptus interactions 

 was employed for comparative purposes. 



The P. interruptus subjects used ranged in 

 weight from 209 to 911 g, with a mean of 

 436 g. Homarus americanus used weighed from 

 417 to 635 g, with a mean of 471 g. Cancer 

 antennarius used ranged in weight from 560 to 

 840 g, with a mean of 673 g. California spiny 

 lobsters were assigned to groups by a randomiza- 

 tion process. Each group usually consisted of 

 both sexes and individuals which were larger, 

 smaller, and the same size as the single 

 American lobster tested. Cancer antennarius 

 used in the experiments weighed more than most 

 of the California spiny lobster subjects with 

 which it was tested. An attempt was made to 

 use P. interruptus that weighed between 400 and 

 600 g for all experiments so that their weight 

 matched the weight of the H. americanus avail- 

 able to us. As a result of poor trap catches, 

 however, we were sometimes forced to use P. 

 interruptus that were either smaller or larger 

 than the H. americanus subjects. 



Each of the 15 separate experiments consisted 

 of three observation periods. These were called 

 the precontrol, experimental, and postcontrol 

 periods. For the social interaction experiments 

 involving P. interruptus and //. americanus, 

 precontrol observations were taken for 5 days, 

 experimental observations were taken for 10 

 days, and postcontrol observations were taken 

 for 5 days. For the experiments involving social 

 interactions between P. interruptus andC. anten- 



narius, the precontrol, experimental, and post- 

 control observation periods were all 5 days in 

 duration. For the precontrol observation period, 

 only individuals of P. interruptus were present 

 in the experimental tanks. Upon completion 

 of the precontrol period, an H. americanus or 

 C antennarius was introduced into the tank. 

 Following the experimental observation period, 

 the H. americanus or C. antennarius was re- 

 moved and the postcontrol observations of P. 

 interruptus taken. 



For P. interruptus and H. americanus experi- 

 ments, three 30-min observation periods were 

 conducted daily for each tank. These 30-min 

 observation periods were taken between 0800 and 

 1000 (4-6 h after the onset of the lights), 

 1200 and 1400 (8-10 h after the onset of the 

 lights), and 1510 and 1700 h (10 min-1 h 50 

 min after the lights went off) P.s.t. for pre- 

 control, experimental, and postcontrol periods. In 

 the experiment involving P. interruptus and 

 C. antennarius, only two 20-min observation 

 periods were taken daily. One was taken between 

 0800 and 1000 and the other between 1510 

 and 1700. The 1200-1400 observation period was 

 omitted because of the low frequency of inter- 

 actions shown by these species during initial 

 observations. Table 1 summarizes the experi- 

 mental paradigm used for this study. 



Observations taken between 1510 and 1700 

 were made using two 100-W red lights with a 

 spectral distribution between 550 and 720 nm. 

 Most of the red light (75%) fell within 600- 

 650 nm. Spectral sensitivity of H. americanus 

 ranges from 400 to 600 nm with peak sensi- 

 tivity at 520-525 nm (Kennedy and Bruno, 1961). 

 At present no information exists on the spectral 

 sensitivity of P. interruptus, but the assumption 

 was made that this species also is insensitive to 

 light within this spectral range. During the study, 

 we observed no evidence that movement and 

 social behavior of either species was inhibited by 

 the use of red lights. 



TERMINOLOGY 



The frequency and type of behavioral inter- 

 actions between P. interruptus and//, americanus 

 or C. antennarius were recorded on a data sheet 

 which was partitioned into 1-min intervals. The 

 behavior of the actor (the individual presenting 

 the stimuli or initiating the interaction) and the 

 subsequent response of the reactor (the individual 



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