EXPERIMENTAL BIOLOGY PROGRAM 



We need to determine which natural condi- 

 tions are biologically important to shrimp in 

 the offshore spawning grounds, the inshore 

 nursery grounds, aind the offshore fishing 

 grounds. Studies of growth, survival, metab- 

 olism, and behavior of shrimp under known 

 conditions can provide valuable leads to a 

 clearer understanding of the environmental 

 requirements of these decapod crustaceans, 



David V. Aldrich, Program Leader 



BEHAVIOR AND ECOLOGICAL 

 PARASITOLOGY 



Observations under laboratory-controlled 

 conditions have provided new information on 

 behavioral responses to low temperature in the 

 two species of shrimp commiercially nnost 

 importsint in the northwestern Gulf of Mexico. 

 Postlarval brown shrimp (8 to 12 mm. long) 

 burrowed when water ten-iperatures were ex- 

 perimentally lowered to 12°-l6.50 C. (fig. 31) 

 if the substrate was sufficiently soft and the 

 rate of temperature change was not too rapid. 

 (Changes of 1° C. per 5 or more minutes in- 

 duced burrowing.) Postlarval white shrimp 

 collected with the postlarval brown shrimp 

 did not burrow under our experimental con- 

 ditions. 



The absence of the burrowing response to 

 low tennperature in white shrimp led us to 

 consider known seasonal aspects of these 

 species' natural histories. The season at 

 which postlarvae of the two species reach the 



TEMPERiTURE CC ) 



Figure 31. — Frequency distribution of burrowing and 

 emerging of postlarval brown shrimp at various tem- 

 peratures. 



bays differ markedly. The white shrimp arrives 

 at Galveston Bay when water temperatures 

 are consistently warm (about 24° to 32° C.), 

 whereas most brown shrimp appear in March 

 or April when the bay is not only cool (average, 

 15° to 24° C.) but also subject to drastic tem- 

 perature reductions (often to 1 3° C.) by atmos- 

 pheric cold fronts. The ability to burrow in 

 response to low temperature may have special 

 survival significance for brown shrimp during 

 this period of its natural history. Burrowing 

 can attenuate temperature changes to which 

 the animal is exposed and protect it from 

 predatory attack when temperatures are low 

 enough to slow avoidance or escape move- 

 ments. On the other hand, a burrowing response 

 to low temperature would have no obvious 

 adaptive significance for postlarval white 

 shrin-ip because the seasonal characteristics 

 of this species' life cycle are such that nnost 

 postlarvae do not encounter cold tennperatures 

 in the northwestern Gulf of Mexico. 



Results in the laboratory further revealed 

 that most brown shrimp postlarvae that have 

 burrowed in response to low temperature 

 leave their burrows as the temperature in- 

 creases to 18°-21.5° C. (fig. 31). Field data 

 of other workers in the Texas -Louisiana area 

 (K. N. Baxter, L. T. St. Amant, and their 

 respective coworkers) indicate that the major 

 influx of postlarval brown shrimp is in the 

 early spring when water temperatures have 

 reached this range. This suggests the possi- 

 bility that postlarval brown shrimp are bur- 

 rowed part of the winter before entering the 

 bays in early spring. 



The fact that soft substrate is required for 

 successful burrowing indicates that physical 

 characteristics of natural substrates may be 

 an important deternninant of the survival of 

 postlarval brown shrimp at late -winter or 

 early-spring water temperatures. 



Observations of postlarval brown shrimp 

 during previous growth and survival studies 

 suggested that temperature influences the 

 behavior of these animals. The shrimp seemed 

 most active at warm temperatures (25° to 

 32° C.), somewhat less active at 18° C, and 

 immobile at 11° C. A behavioral study was 

 carried out, therefore, on a group of 50 post- 

 larvae exposed to known temperatures in the 

 laboratory. The number of animals swimming 

 was visually determined with the aid of low 

 levels of red light. No substrate was added to 

 the floor of the 45-liter glass aquarium. The 

 data clearly substantiate the previous findings 

 and indicate a marked direct effect of tem- 

 perature on activity (fig. 32). 



To gain information on shrimp ecology 

 through studies of their parasites, we have 

 continued to examine shrimp from several 

 Texas bays. Latest results indicate that inci- 

 dence of metacercariae of the trematode. 



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