THE DISTRIBUTION OF 2 SPECIES OF ALPHEID SHRIMP, ALPHEUS-EDWARDSII AND A-LOBIDENS, ON A TROPICAL BEACH

Alpheus edwardsii Audouin and Alpheus lobidens de Man (Crustacea: Deca poda: Alpheidae) are two intertidal snapping shrimp that burrow in ass ociation with rocks. They are morphologically similar and are often fo und in the same intertidal zone. The competitive exclusion theory stat es that two co-existing species must partition resources. The partitio ning of food items as a resource appears unlikely, as gut content anal yses revealed that the two species have similar diets and the relation ship of shrimp condition and shrimp density indicates that food supply was not in limited supply. Instead the two species partition the reso urces of space within the intertidal zone, and shelter. The two specie s were found to exist largely in isolation from each other. Alpheus ed wardsii occurred across much of the intertidal zone. Within this range A. edwardsii were most abundant in substrata with a low proportion (< 50%) of mud. These substrata are better suited to their elaborate meth od of burrow construction. The simpler burrow construction method of A . lobidens allows this species the potential to occupy a wide range of substratum types; however, they were found only in muddy substrata wi th substantial surface rock cover at tidal elevations of 1 . 18 m abov e datum or below. The restriction of A. lobidens to this region of the intertidal zone is probably related, in part, to their physiological requirements. In regions of co-occurrence, A. lobidens were significan tly smaller than A. edwardsii, and utilized smaller rocks as shelters, thus avoiding direct interspecific competition for this resource. The absence of large A. lobidens from these areas may reflect their abili ty to procure shelter in competition with A. edwardsii, as behavioural experiments conducted in this study found that A. edwardsii were the dominant competitors for this resource.