(1966b), on the basis of a mark-recapture, ex- 

 periment, estimated that in the Tortngas area 

 shrimp 17.9 to 20.2 mm. c.L, 84.2 to 94.1 mm. t.h, at 

 the Ijeginnino: of the experiment grew an average 

 of about 3.4 mm. t.l. per week. He also demon- 

 strated that in shrimp above 30 mm. c.l. weight 

 increases rajiidly in relation to increase in cara- 

 l^ace length. Williams (1955a), using upper ex- 

 tremes of size distribution, calculated that ju- 

 venile shrimp in Xortli Carolina grew at a mean 

 rate of 52 nmi. t.l. in July, and 7.5 mm. t.l. per 

 month in winter and early spring. 



Growth rate in adult pink shrimp has attracted 

 the attention of a number of investigators. Iversen 

 and Idyll (1960) studied the shrimp on the Tortn- 

 gas grounds and showed that females 31 mm. c.l. 

 grew an average of about 2 mm. c.l. per month, and 

 males 26 mm. c.l. greM* an average of about 1 mm. 

 c.l. per month. Growth stud'es on the same TToimds 

 by Iversen and Jones (1961) indicated, for both 

 sexes combined, an average monthly increase of 1.8 

 mm. c.l. in "small shrimp'' (25 mm. c.L), 1 mm. c.l. 

 in "medium shrimp" (33 mm. c.l.), and very little 

 (0.5 mm. c.l.) or no growth in "large shrimp" (40 

 mm. c.l.). 



SEX DIFFERENCES IN SIZE 



The females of P. d. duomrum reach a maximum 

 length of 280 mm. t.l. : a truly giant specimen of 

 this size from the Campeche fishing gi'ounds is in 

 the collection of the St. Petersburg Marine Lab- 

 oratory, Fla. The males are usually much smaller, 

 although Thomas J. Costello (personal communi- 

 cation) measured one of 269 nun. t.l. 



In North Carolina, Williams (1955a) observed 

 that the size disparity between sexes in P. d. duora- 

 rum is statistically significant at mean total length 

 slightly exceeding 100 mm. [no less than 21 mm. 

 c.l.]. In Tampa Bay, Saloman (1968) found that 

 small females with a mean carapace length of 13.7 

 mm. were larger than the juvenile males collected 

 with them. The disparity in length between larger 

 females and males has been reported for many lo- 

 calities by various authoi-s. Because the females 

 arelarger than the males, a sample containing both 

 sexes produces a bimodal distribution. On the 

 average, the large males and females are found at 

 the greater dej^ths throughout the subspecies range. 



Ecology 

 FOOD 



P. d. duorarum is mostly a bottom and a noc- 

 turnal feeder. Omnivorous like other penaeids, it 



ingests algae and fragments of higher plants, as 

 well as sand, mud, and organic debris. It also feeds 

 on a larger variety of animals, including forami- 

 niferans, hydroids, nematodes, mollusks, poly- 

 chaetes, crustaceans, tunicates, and fish lan^ae 

 (Williams, 1955a, 1958; Flint, 1956; Woodburn 

 et al., 1957; Eldred et al., 1961). 

 SUBSTRATE 



P. d. duorarum prefers rather firm bottoms of 

 mud and silt with coral sand containing a mix- 

 ture of mollusk shells, as has been shown by investi- 

 gations in the (iulf of Mexico by Springer and 

 Bullis (1954), Hildebrand (1954, 1955) and Gun- 

 ter (1956), and by laboratory experiments by 

 Williams (1958). Pink shrimp also live on hard 

 sand bottoms. The distribution of the subspecies in 

 the northern Gulf of Mexico may be explained by 

 its preference for firm bottoms. Whereas it is scarce 

 on the soft muddy bottoms from Mississippi to 

 middle Texas, it is very abundant on the finner 

 bottoms off the Tortugas Islands and the south- 

 eastern part of the Golfo de Campeche. Its pres- 

 ence north of Ciudad Campeche, Mexico, where P. 

 setiferus is absent, also could be attributed to the 

 firm bottoms there. 



DIEL CYCLE 



Adult pink shrimp are nocturnal and are fished 

 at night in all areas. They seem, however, to be 

 somewhat active on cloudy days and during days 

 when the water is turbid (Hildebrand, 1955; 

 Elldred et al., 1961). Several observations indicate 

 that they normally remain buried during daytime. 



Larvae seem to be as nocturnal as adults. Jones 

 et al. (1964) made by far the largest catches after 

 dark and concluded that larvae move vertically, 

 gathering near the surface at night and descending 

 in the water during the day. Postlarvae, however, 

 apparently are acti\-e day and night, because El- 

 dred et al. (1965) captured 54 percent of the post- 

 larvae taken over 24 hours in the daytime in 

 Tampa Bay. 



Juveniles and subadults seem to be mostly noc- 

 turnal but capable of diurnal activity. Higman 

 (1952) and Tal>b, Dubrow, and Jones (1962) col- 

 lected large nuinliers of specimens leaving estu- 

 aries in southern Florida at night but few or none 

 during the day. In Tampa Bay, Saloman (1968) 

 made larger catches during darkness, and Joyce 

 (1965) noted that liis sampling in northeast Flor- 

 ida yielded a larger percentage of yomig P. d. 

 duoranim at night, but because of the size composi- 



516 



U.S. FISH AND WILDLIFE SERVICE 



