FIGURE 2. — Diagram of measurements made for analysis of mor- 

 phometry. .4, carapace, length (/), width (u); B, third maxilliped, 

 length merus (m), ischium (i); C, palm of major chela, length (/), 

 height (/?). 



and on the DE C System 1 Computer 1 of the Nation- 

 al Marine Fisheries Service (NMFS), using comput- 

 er programs written and maintained by Joseph L. 

 Russo for the Systematics Laboratory, NMFS. The 

 ANCOVA procedure follows that presented by Zar 

 (1974), with the exception that the probability asso- 

 ciated with the calculated value of the F statistic, for 

 the purpose of simplification and clarity, is gener- 

 ated by the computer program instead of being calcu- 

 lated as a value of the F statistic with its associated 

 numerator and denominator degrees of freedom. 



Results 



Ecology 



Panopeus simpsoni occurs among oysters,rocks, and 

 rubble. The crabs burrow in rubble, clearing out 

 shallow depressions under pieces of cover, each 

 excavation usually having more than one opening. 

 When uncovered, the crabs again bury themselves in 

 the substrate by wedging their flat bodies between 

 loose shells. On jetties, this species occupies 

 burrows, shallow depressions beside pieces of stone, 

 and interstices among rocks and attached oysters. 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Collection of such individuals is difficult because 

 they rapidly burrow among sharp oyster shells 

 when disturbed. 



Undercut marsh and mud bank stations exclusively 

 yielded P. obesus. The burrows of this species are 

 tubular, those in mud banks being the most intricate 

 and often consisting of numerous interconnecting 

 galleries. Each burrow generally has one or more 

 openings at the surface of the marsh near the edge of 

 a bank and a lower opening near the interface be- 

 tween hard and silty substrates. From the lower open- 

 ing, a passage normally penetrates horizontally into 

 the bank. This passage divides into one more or less 

 vertical connection with the upper openings and 

 another branch which angles downward at about 45°. 

 The lower part of most burrows is < 1 m long (about 

 an arm's length), the end commonly being filled with 

 soft, silty mud. The mud bank habitat contains many 

 large adult P. obesus. 



Burrows of larger mud crabs in the undercut marsh 

 connect to both upper and lower surfaces of the 

 marsh mat. Larger burrows are all vertical or nearly 

 so, while smaller burrows are inclined or nearly 

 horizontal. Juveniles (the majority of individuals in 

 this habitat) live in the tangle of marsh roots on the 

 lower surface of the mat. 



Panopeus simpsoni occurs both inter- and subtidally 

 (under pieces of rubble); P. obesus occurs only inter- 

 tidally but does occupy habitats of P. simpsoni if suit- 

 able cover is available. For example, a large P. obesus 

 was found among concrete rubble at one station but 

 not burrowed under it as was P. simpsoni. Small P. 

 obesus (carapace length about 10 mm) were also 

 found among pebbles behind a stone jetty. The pres- 

 ence of young P. obesus in rocky P. simpsoni habitat 

 tends to contradict the assumption by Benedict and 

 Rathbun (1891) that dorsal curvature in the P. obesus 

 carapace results from development in burrows. 



Principal associated macroinvertebrates observed 

 were Sesarma cinereum (Bosc), S. reticulatum (Say), 

 Uea spp., and Littorina irrorata (Say) in the marsh; 

 Crassostrea virginica, Eurypanopeus depressus 

 (Smith), andBalanus spp. in the intertidal rubble and 

 on jetties. 



Stations north of Heron Bay (5, 10, 11, and 12, Fig. 

 1) had salinities which were too low (< 14 ppt) to sup- 

 port populations of either species. 



Panopeus obesus fed actively on oysters, snails, and 

 crabs which were offered to them in captivity; P. 

 simpsoni consumed the offered prey least actively. 

 Both species fed on small American oysters, Crassos- 

 trea virginica (up to 5 cm long). A crab would grasp 

 the oyster in its chelae and begin to chip around the 

 edges of the valves with its major chela. As soon as an 



887 



