iii the samples taken. Pearson found postlarvae 

 in Louisiana in all months of the year, but in no 

 more than 11 months of any calendar year. The 

 usual recruitment period seemed to run from late 

 January to late summer. 



Gunter (1950) found young brown shrimp in 

 Texas bays most common in spring and fall with 

 low abundance in late summer and winter. 



Evidence indicates that P. aztecus has an ex- 

 tended spawning season which probably varies in 

 different parts of the range. The spawning site 

 is probably deeper and farther from shore than 

 in P. setiferus and P. duorarum. 



Young P. aztecus enter estuaries as postlarvae, 

 migrate to shallow, often low-salinity water, and 

 undergo a remarkably rapid growth in the warmer 

 months. In North Carolina, the juveniles in- 

 crease in mean length by an estimated 46 mm. 

 per month (1.5 mm. per day, Williams, 1955a). 

 As they grow, they gradually move to deeper, 

 saltier water and eventually return to sea. In 

 North Carolina, and perhaps elsewhere, P. 

 duorarum and P. aztecus use essentially the same 

 nursery grounds over large areas of the sounds; 

 however, the seasons of recruitment to and oc- 

 cupation of these areas is staggered in such a 

 manner that the two species are rarely on common 

 ground (Williams, 1955a, b). The growing bait 

 fisheries for estuarine shrimp in recent years 

 have given impetus to studies in Alabama and 

 Texas. Additional information on migration 

 (Loesch, 1957) and studies on length-weight re- 

 lationships (Chin, 1960) have resulted from this 

 work. 



Again, distribution of P. aztecus is usually 

 thought of as limited by such factors as salinity, 

 temperature, and substrate. Hildebrand (1954, 

 1955) and Springer and Bullis (1954) stated that 

 the commercial fishery for this species in the 

 Gulf of Mexico was confined largely to bottoms 

 of terrigenous silt. (Like P. duorarum, P. 

 aztecus is a burrower, and in many areas is more 

 active in open water at night than in daytime.) 

 In experimental tanks, when given a choice of 

 sand, shell-sand, loose peat, muddy sand and 

 sandy mud, P. aztecus was found to favor loose 

 peat, sandy mud and muddy sand, closely paral- 

 leling P. setiferus in choice of bottom type (Wil- 

 liams, 1958). The experiments confirm field ob- 



servations on habitat along the coast of North 

 Carolina and in the Gulf of Mexico. 



Penaeus aztecus has been found to have a less 

 efficient osmoregulatory mechanism at low tem- 

 peratures in low salinities than P. duorarum 

 (Williams, 1960). For this reason, it is probably 

 not so resistant to wintertime conditions in 

 estuaries as P. duorarum. 



Genus Parapenaeus Smith, 1886 



Burkenroad, 1934b, p. 107. 



Parapenaeus longirostris (Lucas) 



Figures 13-15 



Pencils longirostris Lucas, 1849, p. 46, pi. 4, fig. 6. 

 Parapenaeus politus: Hay and Shore, 1.918, p. 379, pi. 2. r >. fig. 7. 

 Parapenaeus longirostris: Burkenroad, 1934b, p. 108 (rev.). — 

 1939, p. 53. 



Recognition characters. — Integument smooth, 

 not setose. Carapace with a low carina extending 

 almost to posterior margin and bearing a spine 

 some distance behind rostrum. Rostrum arched; 

 distal half deflexed, tip somewhat upturned, ex- 

 tending to or bej'ond distal edge of eye; dorsal 

 margin with usually seven teeth diminishing in 

 size anteriorly; ventral margin heavily ciliated. 

 Hepatic, antennal, and branchiostegal spines well 

 developed; later spine placed a little behind 

 margin of carapace. A rectangular toothlike 

 eminence at orbital angle. A shallow groove ex- 

 tending from behind eye almost to posterior edge 

 of carapace, and another, extremely faint, run- 

 ning upward from inferior margin at base of 

 second pair of legs. Basal antennular article 

 with a spine on ventromedial! margin distally. 



Thelycum of female composed externally of a 

 triangular plate with obtuse apex on 13th somite 

 (base of 4th leg) ; 14th sternite raised anteriorly, 

 with low relief posteriorly; lateral ridges with 

 nearly straight but divergent medial borders 

 separated by nearly plane, broad medial groove; 

 lateral border of ridges ornately curved to fit 

 contour of adjacent coxae and 13th sternite. 



Figure 13. — Parapenaeus longirostris (Lucas). Anterior 

 portion of animal in lateral view, X 3. 



MARINE DECAPOD CRUSTACEANS OF THE CAROLINAS 



27 



