ALGAL PREFERENCES FOR DECORATION BY THE CALIFORNIAN KELP CRAB, PUGETTIA PRODUCTA (RANDALL) (DECAPODA, MAJIDAE)

BY

EDWIN MASTRO 87 Gaviota Way, San Francisco, California 94127, U.S.A.

INTRODUCTION

The common kelp crab Pugettia producta (Randall, 1840) is a member of the superfamily Oxyrhyncha, which as a group are known to mimic their environ- ment by attaching pieces of their surroundings on themselves. According to MacGinitie & MacGinitie (1968), these crabs actively place decorations on themselves, yet many other crabs also tolerate other organisms which passively settle on their carapaces. Adult P. producta belongs to the second group (Ricketts & Calvin, 1968). Juvenile P. producta, however, are active decorators. Juvenile crabs possess two rows of paired curved setae on their upper rostral surface which act as hooks, serving as a placement site for decorating material (fig. 1). On the transforma- tion to adulthood, these setae are lost and the crabs lose their ability to decorate (Schmitt, 1921). The purpose of this study is to determine if there are selection preferences involved in algal decoration by juvenile P. producta. Possible functions of algal decoration are also considered.

Fig. 1. Algal decoration on Pugettiaproducta (Randall). 65

METHODS AND MATERIAI,S To determine if preferences are involved in algal decoration by juvenile P. producta, three different approaches were utilized. The first was to determine if juvenile P. producta prefer a certain alga for decoration in the field. The second was to test whether there is a selection process by the quantification of algae available to the crabs. The third was to test if in the laboratory a preference for certain algae could be shown. During April and May, 1979, sixty (total) decorated and nondecorated juvenile P. producta were collected from Shell Beach and Horseshoe Cove, Sonoma County, California. Animals were collected individually, and the available algae in the immediate area around the crabs were noted. The crabs were then brought back to the laboratory, where they were sexed and the carapace widths (the distance between the postero-lateral spines) were measured. If the crabs were decorated, a record was kept as to which alga they had utilized. Crabs then were individually marked and placed in aquaria with running seawater with several types of algae until further experiments were performed. The biomass and distribution of algae available to the crabs in the field were then quantified. A pair of parallel transects, 3 m apart, were sampled at both study sites. Five 15 x 15 cm quadrats were sampled along each of these two parallel transects, every 15 cm vertically. The center quadrat was placed at roughly the same level from which the crabs were collected (approximately mean lower low water). All algae in each quadrat were collected, sorted, and weighed on a Mettler analytical balance after blotting dry with paper towels. In the laboratory juvenile P. producta were tested to determine if there was an algal preference. Crabs were given a choice of different marine plants which they could use as food and decoration. Two separate experiments were con- ducted, each with a different set of plants. The first experiment used a variety of marine plants that were found throughout the intertidal zone, viz., Bossiella plumosa, Cryptosiphonia woodii, Endocladia muricata, Egregia menziesii, Fucus distichus, Gigartina canaliculata, Gigartina papillata, Iridaea flaccida, Laminaria den- tigera, Macrocystis integrifolia, Odonthalia floccosa, Pelvetiopsis limitata, Phyllospadix scouleri, Plocamium pacificum, Rhodomela larix, and Ulva sp. The second experi- ment contained a subset of plants used in the first experiment, namely those that were found commonly in the same general area in which the crabs were collected: Cryptosi,bhonia zvoodii, Egregia menziesii, Iridaea flaccida, Odonthalia floc- cosa, Phyllospadix scouleri, Plocamium ,bacificum, and Rhodomela larix. Approx- imately equal surface areas of each species of marine plant were made available to each crab. Plants were blotted dry with paper towels and weighed before each experimental trial. The different sets of plants used in each experiment were placed in twelve 1.5 liter plastic containers with screen lids. Crabs were then stripped of their decoration and one crab was placed in each tub. In each experimental trial one tub was crabless as a control for monitoring weight changes in the plants. The