Figure 203. — Planes minutus (Linnaeus). Male in dorsal 

 view, approximately X 2 (after Chace, 1951). 



pace, decurved, usually slightly emarginate in 

 middle, edges minutely denticulate or smooth. 

 Eyes large ; orbits large, lower margin granulate, 

 tooth at inner angle equilateral, subacute, outer 

 angle spiniform, behind it a small sinus. 



Chelipeds large and heavy ; merus and ischium 

 with a thin serrate crest along inner margin ter- 

 minating in two or three spines at distal end of 

 merus; carpus with a strong blunt spine on inner 

 face; hands inflated and smooth except for small 

 sharp granules near lower margin; immovable 

 finger bent downward, especially in males, dactyl 

 curved, both with blunt teeth throughout length. 

 Walking legs long, slender, and flattened ; second 

 and third legs subequal, fourth legs shortest ; meri 

 occasionally with an inconspicuous anterior sub- 

 terminal tooth and a few posterior denticles; last 

 three articles thorny and with a dense fringe of 

 hair on anterior edge. 



Abdomen of male rather broadly triangular; 

 telson rather narrowly triangular, approximately 

 as long as basal width. 



Measurements. — Carapace: male, length, 19 

 mm., width, 19 mm. Length of a large series re- 

 ported by Chace (1951), 3.7 to 19 mm. 



Color. — Extremely variable; irregularly mot- 

 tled or blotched with light greenish yellow or 

 pale yellow on a darker olive-green ground color; 

 or reddish-fawn color, more or less blotched with 

 dark brown, and usually with a small white spot 

 on each side or one large white spot on front of 

 carapace (various authors). 



Crozier (1918) observed no color change in a 

 mahogany-colored P. minutus placed for a day 



on a lighter background. Hitchcock (1941) found 

 three types of chromatophores in P. minutus: 

 white, black, and yellow. The most prominent 

 chromatophore is white. Color adaptation to dif- 

 ferent backgrounds is slow and though the chro- 

 matophores are responsive to change in back- 

 ground, extracellular pigment in the hypodermis 

 and exoskeleton prevents the animal from effect- 

 ing an immediate change in appearance. Hitch- 

 cock concluded that the pattern of the individual 

 crab is probably genetic. Chace (1951) remarked 

 that in view of these findings, the apparent color 

 of any individual can be changed only at the time 

 of molting. 



Habitat. — These crabs are more abundant on 

 Sarga.ssum in the Sargasso Sea than elsewhere, 

 but throughout their range they depend on flot- 

 sam, or on floating or swimming organisms to 

 which they cling (Chace, 1951). 



Type locality.— On sargasso and other sub- 

 marine sea plants on the north side of Jamaica 

 (Sloane in Holthuis, 1959). 



Known range. — Atlantic Ocean south of New- 

 foundland, west of 50° W. latitude, and exclusive 

 of the Gulf of Mexico (Chace, 1951) ; Netherlands 

 coast, North Sea (van den Oord and Holthuis, 

 1959). 



Remark*. — The genus Planes has been thor- 

 oughly reviewed by Chace (1951), and the, seri- 

 ous student should consult this paper. Chace in- 

 cluded remarks on relative growth, stating 

 that . . . 



The carapace length-width relationship remains fairly 

 constant with a slight tendency toward narrowing, from 

 the smallest immature specimens examined to a carapace 

 length of about 11 mm. At this stage the carapace seems 

 to become somewhat narrower rather abruptly and con- 

 tinues to become narrower at a slightly more rapid rate 

 than during the younger stages. A similar, but even more 

 striking, trend is noticed in the relative shortening of the 

 walking legs. 



Chace conjectured that the specimens larger 

 than about 11 mm. are found on flotsam and tur- 

 tles rather than on sargassum, and that the shorter 

 legs may be more adapted to this existence than 

 to life on floating weed. It is also possible that 

 these sizes represent different forms of Planes 

 with somewhat different habits, but Chace de- 

 ferred such a designation until a time when more 

 material is available for study. 



MARINE DECAPOD CRUSTACEANS OF THE CAROLINAS 



219 



