length are approximately 2 years old. Most in- 

 dividuals that grow to maturity live a year or 

 longer. Adult females attain a larger size than 

 adult males (various authors). 



The young shrimp tend to seek shallower, often 

 somewhat fresher, portions of the estuaries in the 

 early part of their benthonic existence, and with 

 increasing size move gradually into deeper, saltier 

 water as they grow; finally, with approaching 

 maturity they return to sea. This general pattern 

 of movement may best be taken as a model which 

 is variously complicated in different parts of the 

 species' range. In North Carolina, and perhaps 

 elsewhere (Iversen and Idyll, 1960; Eldred, Ingle, 

 Woodburn, Hutton, and Jones, 1961) many of the 

 juveniles present in the sounds in fall do not 

 return immediately to sea but linger in the estu- 

 aries over winter. Here they are often subjected 

 to quite low temperatures (4° C.) and in severe 

 winters nearly all of this overwintering popula- 

 tion may be killed. Normally they survive the 

 winter, but because they are relatively inactive in 

 the cold, they grow little, at least in North Caro- 

 lina (November-April estimated average increase 

 in length, 7.5 mm. per month). Rising springtime 

 temperatures induce increased activity and feed- 

 ing, resulting in resumption of rapid growth and 

 an ensuing spring migration to sea in May and 

 June. In North Carolina, this migrating popula- 

 tion is large enough to create a sizable late spring 

 fishery. 



In Texas (Hildebrand, 1955) and Florida 

 (Higman, 1952) young of P. duorarum are espe- 

 cially abundant in grassy areas of estuaries where 

 salinities are 20°/ oo or more. This is true of 

 some areas in North Carolina, and the young are 

 also concentrated most heavily in areas where 

 there is a possibility of tidal transport (Williams, 

 1955b). 



Tagging experiments to determine movements 

 of this species have been restricted to waters off 

 southern Florida. Iversen and Idyll (1960) and 

 Iversen and Jones (1961) demonstrated a general 

 northwestward movement for shrimp tagged on 

 the fishing grounds west of Key West and sug- 

 gested that small shrimp move from shallow water 

 at the end of the Florida peninsula to the Tortugas 

 fishing grounds. 



Distribution of P. duorarum may be limited by 

 the same factors that influence P. setiferus; i.e., 



salinity, temperature, and substrate. Hildebrand 

 (1954, 1955) and Springer and Bullis (1954) 

 emphasized the fact that P. duorarum, fisheries I 

 are located in areas where the bottom is composed I 

 of calcareous muds and sands or mixtures of mud | 

 and sand. (This species usually burrows in the 

 substrate in the daytime and is most active at 

 night (various authors).) Laboratory experiments 

 have shown that subadult P. duorarum, when al- 

 lowed a free choice of sand, shell-sand, loose peat, 

 muddy sand or sandy mud, tend to choose shell- 

 sand with a somewhat lower preference for loose 

 peat (Williams, 1958). In shell-sand the animals 

 were often completely buried, and could be ob- 

 served only after being dug out. The experiments 

 confirmed observations on the correlation of loca- 

 tion of fisheries with bottom type in the Gulf 

 of Mexico and in North Carolina. 



The fact that P. duorarum, alone, of the three 

 commercially abundant species of shrimps, is able 

 to overwinter in estuaries in the northern part of 

 its breeding range has suggested that it is better 

 able to withstand a combination of low salinity 

 and temperatures because of superior osmoregu- 

 latory abilities at low temperature. Preliminary 

 studies have shown that P. duorarum is better able 

 to regulate its internal fluids at low temperatures 

 than its close relative P. aztecus, but regulatory 

 ability of both species is impaired when tempera- 

 tures are lowered to about 8° C, and survival 

 of both species is better in moderate to high 

 salinities at low temperatures (Williams, 1960). 

 Eldred, Ingle, Woodburn, Hutton, and Jones 

 (1961) considered that the depth to which the 

 species may burrow in cold weather (6 inches) 

 may protect it from sudden cold snaps. 



Penaeus aztecus Ives. Brown or grooved shrimp 



Figure 12 



Penaeus trasiliensis aztecus Ives [In part], 1S91, p. 190. 

 Penaeus brasiliensis: Hay and Shore [In part], 1918, p. 377. 

 Penaeus aztecus: Burkenroad, 1939. p. 34, figs. 20, 21, 24, 

 28-33 (rev.). 



Recognition characters. — Integument thin, pol- 

 ished, translucent. Carapace with a median ca- 

 rina continuous anteriorly with rostum and 

 extending nearly to posterior border of carapace, 

 flanked on each side by a broad, rounded groove ; 

 posterior half of carina with a median longi- 

 tudinal groove; anterior half arcuate, highest 

 above orbit and with 9 or 10 sharp teeth; pos- 



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FISH AND WILDLIFE SERVICE 



