tion of sample catches he concluded that the very 

 small shrimp were mainly nocturnal whereas the 

 larger individuals were more diurnal. 



In the laboratory, Williams (1958) noted that 

 several groups of shrimp, with mean total length 

 of 50 to 60 mm., were active at night, and, during 

 the day, remained almost or completely hidden 

 in the substrate. In contrast, Eldred et al. (1!)61) 

 i-e}X)rted that in aquariums, individuals smaller 

 than 55 mm. t.l. were active during the day or 

 when exposed to bright light, whereas large speci- 

 mens remained buried in the sand. Aaron and 

 Wisby (1964) stated that in their laboratory ex- 

 periments, more than half of the shrimp, 55 to 105 

 mm. t.l. ''between the leading edge of the blades 

 and the spine of the telson,"' showed positive photo- 

 taxis when exposed to 3.23 lumens per 1 m.=, those 

 75 mm. showed the greatest. Studies by Fuss and 

 Ogren (1966) indicated that larger shrimp are 

 more sensitive to light than smaller shrimp, al- 

 though most individuals within the size ranges (80 

 to 180 mm. t.l.) observed showed a negative photic 

 response: burrowed in the presence of daylight 

 or ai-tificial light. They are of the opinion tha^t light 

 is apparently the most important single factor in 

 shrimp diel activity. 



Availability of pink shrimp oifshore seems to 

 vary with the phases of the moon. Fishermen 

 say, and many others have observed, that offshore 

 catches show a sharp decrease during the full moon. 

 The activity of shrimp inshore through the lunar 

 cycle has not been clearly determined. Eldred et al. 

 (1965) recorded a higher percentage of post- 

 larvae during the full-moon spring tides. Idyll 

 et al. (1965) are of the opinion that moon phase 

 and speed of ebbing current are two of the more 

 imj^ortant factors responsible for the variation in 

 the size of the catch of shrimp at the mouth of 

 the estuaries. At Buttonwood Canal, Fla., they 

 consistently obtained higher numbers of shrimp 

 mo\'ing on the ebb tide during or near the new 

 and full moon than during the other moon 

 phases. Copeland (1965) reported peak seaward 

 migrations at the time of the full moon, whereas 

 Saloman (1968) caught gi-eater numbers of shrimp 

 during the dark phases of the lunar cycle than 

 during the full moon. 



In the. laboratory, Aaron and Wisby (r.»()4) 

 found evidence that the moon phase has a sig- 

 nificant effect on the activity of shrimp; they 

 observed that maximum photoactivation occurred 

 during the full moon, and the minimum during 



the new moon. Fuss and Ogren (1966), in turn, 

 reported that in aquariums, direct correlations be- 

 tween lunar cycles per se and pink shrimp noc- 

 turnal activity were not well defined. Size of ani- 

 mals and light intensity— as Fuss and Ogren 

 have suggested — could be responsible for the dif- 

 ferences in behavior in relation to lunar changes. 



MOVEMENTS 



The larvae of P. d. duorarvm move from the 

 spawning sites toward inshore waters and, thus, 

 have the same migratory pattern as the sym- 

 patric Penaeus. The larvae are thought to be 

 carried by currents, but, as stated earlier, the lar- 

 \ae probably are not entirely passive during their 

 onshore movement. 



Larvae develop at sea and the young shrimp 

 arrive in the nursery grounds usually as postlarvae. 

 Postlarval shrimp are predominantly transported 

 inshore by flood tides (Tabb, Dubrow, and Jones, 

 1962; Copeland and Truitt, 1966; Hughes, 1966). 

 Hughes (1966) stated that the movement of the 

 postlarvae into the nursery areas is apparently 

 effected largely by their passive displacement by 

 the tide. Although most postlarvae reach estua- 

 rine waters, some pink shrimp may complete their 

 entire life cycle in oceanic waters. Ingle et al. 

 (1959) and Eldred et al. (1961) have advanced the 

 view that the extensive shallows from Key West. 

 Fla.. west through the Marquesas Keys to Re- 

 becca Shoal, are nursery grounds for pink shrimp. 

 The studies by Williams (1959) in North Caro- 

 lina, Bearden (1961) in South Carolina, and 

 Tabb, Dubrow, and Jones (1962) in Florida Bay 

 showed that most postlarvae enter tlie inshore 

 waters at the six-to-seven-rostral-tooth stages, al- 

 though in Florida Bay a few were at the four- 

 rostral-tooth stage. In Tampa Bay, Eldred et al. 

 (1965) found that the postlarvae arrive when 

 younger — when the rostrum possesses a miuinuun 

 of two and a mode of four teeth. 



The period of postlarval movement of pink 

 shrimp varies with the range of the subspecies. In 

 Xorth Carolina, Williams (1955a, 1965) reported 

 that influx takes place from late May to Novem- 

 ber. Bearden (1961) showed similar movement in 

 South Carolina. Joyce (1965) on the basis of the 

 presence of individuals 40 mm. t.l. and smaller, 

 concluded that in northeast Florida major in- 

 shore movement appears to take place from June 

 through December. 



The extensive sampling data from south- 

 west Florida suggest that postlarvae enter in- 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



517 



