FISHERY BULLETIN: VOL. 70. NO. 1 



DESCRIPTION OF THE STUDY AREA 



The main study site was located approximately 

 1.5 km offshore from Pt. Loma, California (lat 

 32°42'N; long 117°16'W). The area is within 

 a stand of giant kelp, Macrocijstis jnjrifera, and 

 the bottom in this vicinity is from 13 m to 15 m 

 deep. The primary observation site encom- 

 passed an area about 300 m by 150 m. The sea 

 floor within this area is relatively heterogeneous 

 and is composed primarily of siltstone pavement 

 rocks, rocky outcrops, ledges, and intermittent 

 patches of coarse sand. Vertical relief is gen- 

 erally less than about 2 m. Much of the hard 

 substratum was occupied either by the holdfasts 

 of M. pyrifera or by low standing, brown algae 

 such as Pterygophora californica, Cystoseira 

 osmundacea, and Laminaria farlowii. 



METHODS 



Emphasis was directed towards observing the 

 organisms under natural conditions in the sub- 

 littoral zone; however, laboratory observations 

 were used in addition to those made in the field. 

 Experiments in the laboratory were designed to 

 supplement our field observations, since it al- 

 lowed us to observe behavioral interactions be- 

 tween D. imbricata and S. purpumtus over a 

 more continuous period of time. The laboratory 

 portion of the study was conducted at the NMFS 

 Fishery-Oceanography Center, La Jolla, Cali- 

 fornia. Experimental animals were maintained 

 from February through April 1970, in fiber 

 glass water tables which contained circulating 

 seawater. Water temperatures in the aquaria 

 varied from 12° to 17°C during the period of 

 observation. 



The field studies are the result of approxi- 

 mately 150 hours of underwater observations 

 made while scuba diving from November 1969 

 through November 1970. All of the leather stars 

 encountered underwater within the perimeter of 

 the study area were included in the feeding be- 

 havior observations. In each case we recorded 

 the size (center of the aboral area to the tip of 

 the longest arm) of the leather star, and noted 

 whether or not it was feeding. It was usually 

 necessary to turn the leather star over in order 



to make the feeding observation and identify the 

 prey. In situations where the prey was ingested 

 whole by the leather star, we forced the food item 

 out of the sea star's mouth by applying pressure 

 to the aboral and oral surfaces as described in 

 Mauzey et al. ( 1968) . Measurements were made 

 with plastic calipers or rules, and the data were 

 recorded underwater on plastic slates. 



Estimates of leather star density were deter- 

 mined by swimming belt transects 4 m wide and 

 25 m long. Initially, these transect lines were 

 placed haphazardly along the bottom, running 

 either perpendicular or parallel to the shoreline. 

 However, we also placed lines in predetermined 

 locations where leather stars were believed to be 

 more abundant. Size distributions and density 

 estimates of the S. purpuratus, which inhabited 

 the study area, were determined by removing 

 all of the visible purple urchins from 16 ran- 

 domly selected m- quadrats. The quadrats were 

 chosen randomly along two 50-m transect lines. 

 One of the transect lines followed a siltstone 

 ledge, and the other was placed perpendicular 

 to the shoreline through a dense stand of giant 

 kelp, M. pyrifera. After removal from the 

 quadrats the S. purpuratus were placed into 

 plastic bags and carried to the surface for mea- 

 surement and enumeration. 



Three methods were used in an attempt to 

 determine the rate of feeding and the time ne- 

 cessary for a leather star to digest a purple 

 urchin: 



(1) A wire mesh cage which covered an area 

 of 4 m- was placed over a natural population of 

 at least 50 purple urchins, and four leather stars 

 were introduced as predators into the cage. 



(2) Leather stars were marked with num- 

 bered disc tags which enabled us to identify and 

 observe individuals over extended periods of 

 time. The tags were attached to the aboral sur- 

 face of the leather star with a loop of monofila- 

 ment line. The line was passed through the 

 leather star's epidermis and under the calcareous 

 ossicles using a heavy duty needle. All leather 

 stars were tagged at the sea surface and imme- 

 diately returned to the bottom. 



(3) In the laboratory we placed seven leather 

 stars and 76 purple urchins into a 0.915 X 4.27 m 

 (3 ft X 14 ft) fiber glass water table. The water 



206 



