Decapoda, Paguridea)
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PREVENTION OF SHELL BURIAL AS A BENEFIT HERMIT CRABS PROVIDE TO THEIR SYMBIONTS (DECAPODA, PAGURIDEA) BY MICHAEL R. CONOVER Department of Zoology, Washington State University, Pullman, Washington 99163, U.S.A. There has been considerable discussion in the literature as to what benefits symbionts incur through their association with hermit crabs. However, as Ross & Sutton (1961) have pointed out, most of the proposed benefits have been inferred and not demonstrated. Furthermore, these particular benefits will vary depending on the species of hermit crab and symbiont. Shelter or protection from predators (Orton, 1922) and greater ease in acquir- ing food are the most commonly proposed benefits. Associating with a hermit crab could help the symbionts acquire food in various ways depending on the species involved. The sea anemone, Adamsia palliata (Bohadsch), is actually fed by the hermit crab, Pagurus prideauxi Leach (cf. Fox, 1965). The polychaete, N ereÙ fucata (Savigny), is able to steal food from the hermit crab by placing its pharynx between the third maxillipeds of the crab (Brightwell, 1951). Other symbionts, such as the hydroid, H ydractiniä echinata (Fleming) (cf. Schijfsma, 1935; Wright, 1973), and various sea anemones (Balss, 1924) share in the crab's meal by passively waiting for pieces of food to come into contact with them. While there has been some question as to how much food these symbionts actually acquire in this manner, the hermit crabs do provide mobility that presumably enhances the symbionts' chance of finding food (Balss, 1924; Ross, 1960; Ross & Sutton, 1961; Jensen, 1970; Wright, 1973). Still other benefits have been suggested besides protection and food. From observations made on aquarium specimens, Wright (1973) concluded that the hermit crab's movement and respiratory current would help keep Hydractinia echinata free of sediments. Balss (1924) suggests that one benefit of the hermit crab's movement is in providing the symbionts with oxygen-rich water. Although specific benefits to particular organisms undoubtedly occur, the study reported here suggests that at least in the temperate sand littoral zone, a general benefit provided by hermit crabs is to keep the shells and associated organisms from being buried by sand. The study was conducted from October, 1972 through March, 1973 in Tampa Bay at St. Petersburg, Florida. To obtain a measure of the incidence of the burial of gastropod shells un- occupied by hermit crabs, the openings of some shells were sealed to prevent entry by hermit crabs. The shells were placed in small groups at various locations within 312 an area extending 150 m out from a kilometer stretch of shoreline. The 5-10 shells of each group were tied in common to a monofilament line which was buried and attached to a marker stick so that buried shells could later be easily located and retrieved. After 21 days, the shells were checked and again placed on the surface for repeated checks at 21 day intervals. The data gathered from this procedure showed that 85% of the 100 shells comprising the sample were buried by sand within a 21 day period. Shell burial occurred even as far as 150 m from shore at a water depth of 4 m. On two occasions, high winds arose after placing out 60 shells (not included in the above sample) and a check of the shells three days later revealed that 52 (87%) of them had become buried. Shell burial appeared to result mainly from agitation by water of the sandy substrate. Water movement for such agitation probably is produced by tidal cur- rents or surface-wave action. The latter, especially, would appear to be particu- larly effective in burying shells during windy weather. Another way in which shells could become buried is through the activity of some polychaete worms that collect food by sifting sand through their tubes. This practice creates a hole at one end of the tube and a mound of sand at the other end. Shells lying too close to either end could become buried by either falling into the hole or being covered by expelled sand. Such polychaete activity may not be a primary factor in shell burial, but it could well have a facilitory effect. After being buried, the shells generally resurface with further sifting of sand, although most are probably buried for a longer period than the symbionts can survive. Thus, the organisms that are relatively immobile probably perish. This account would explain why many of the shells from the observed area have a number of large dead barnacles in addition to a new generation of small live barnacles. Conceivably these shells were buried at some time in the past, killing the existing barnacles; then after the shell resurfaced, a new crop of barnacles started growing. To test directly whether shell occupancy by hermit crabs results in a reduction of shell burial, an experiment was performed comparing the incidence of burial of unoccupied shells with that of shells inhabited by crabs. Two circular pens 3 m in diameter and 10 cm high were constructed with chicken wire. The pens were placed next to each other about 30 m from shore in 1 m of water. Into one pen were placed gastropod shells (3-10 cm in length) occupied by either Pagurus pollicari.r Say or P. impresstlJ (Benedict) and into the other were placed un- occupied shells comparable in size and gastropod species. Each pen received 10 shells and was checked 24 hours later to ascertain the number of buried shells. All shells were then exchanged between pens and a second check made after another 24 hours. This two-trial procedure was repeated with new sets of shells and crabs until adequate sample sizes were attained. Of 100 unoccupied shells, 12 became completely buried within the one day test period and most of the others partially buried. In contrast, not one of the 100 .