exceeds 5 wk. These findings tend to favor 

 the possible use of a parasite as a biological 

 shrimp "tag." 



It remained, however, to determine whether 

 significant differences existed between infec- 

 tion rates of shrimp fronn different bays. As 

 a result we have made a few comparisons 

 of rates of worm incidence in shrimp collected 

 fronn various bays. We have sampled shrimp 

 from four Texas bays- -Sabine Lake (estuary 

 of the Sabine River), the Galveston Bay sys- 

 tem, the Matagorda Bay systenn, and Corpus 

 Christi Bay. Groups of shrimp compared in 

 this study were collected on the same day and 

 were of the same specias and size. The May 

 and June samples probably represent the bulk 

 of the brown shrimp populations of these bays 

 for this year. Sabine Lake shrimp were less 

 frequently infected with tapeworms than shrimp 

 from the three bay systems to the southwest 

 (fig. 37). 



The nnovement of postlarval shrimp into 

 bays from offshore spawning areas is an ac- 

 cepted feature of the Penaeus life cycle. The 

 relatively large distances involved and the 

 relatively small size of the postlarvae have 

 caused speculation as to whether these animals 

 move actively or passively. Interest in this 

 problenn has led to laboratory measurements 

 of postlarval swimming speed. The time re- 

 quired for individual postlarvae of brown 

 shrimp to swim a known distance was used to 

 estimate speed. The mean value of 40 deter- 

 minations (made at 23° C. (73° F.) and 24 p.p.t. 

 salinity) was 4.7 cm./sec. (about 1-7/8 in./ 

 sec). Assuming uninterrupted movement at 

 this rate and ignoring effects of water move- 

 ment, we can extrapolate this value to 2.6 

 miles per day. Further study will be required 

 to determine whether or not postlarvae can 

 sustain these rates and to test for the possible 

 influence of environmental factors on post- 

 larval movements. From field observations 

 of R. D. Ringo (Estuarine Program), we do 

 know, however, that brown shrimp postlarvae 



g60 



o 



2 



a 



^ - NOV . 1964 - WHITE SMHIMP 

 B-MAr, l9eS-BR0WN SHRIMP 

 li-JUN. " 



Figure 37. — Incidenceof tapeworms in shrimp from Texas 

 estuaries — Sabine Lake, Galveston Bay, Matagorda Bay, 

 and Corpus Christi Bay, 1964-65. 



move through Galveston Bay at an average 

 speed of 2 miles per day. 



On the basis of field observations, earlier 

 workers have suggested a relation between 

 winter water temperature and the size dis- 

 tribution of white shrimp; the larger shrimp 

 are believed generally to occur in the highest 

 temperatures. No direct observations have 

 been made, however, of the effect of low 

 temperature on shrimp of various sizes under 

 laboratory conditions where other environ- 

 mental factors could be controlled. Four 

 experiments were conducted to supply such 

 information. In each test, 10 white shrinnp 

 from 60 mm. (2-3/8 in.) to over 100 mnn. (about 

 4 in.) total length were simultaneously exposed 

 to decreasing temperatures. Loss of equilib- 

 rium was selected as a rapid and sensitive 

 behavioral "end-point." As the experimental 

 temperatures fell to 140-12° C. (57.2°- 

 53.6° F.), the larger (100-130 mm. = about 4 

 to 5-1/8 in.) animals in each group became 

 unable to maintain equilibrium but lay on their 

 sides or backs. Greater temperature de- 

 creases of about 1° C. ( 1 .8° F.) were necessary 

 to produce similar symptoms in the smaller 

 shrimp. Low-temperature susceptibility in 

 white shrimp is, then, related to aninnal size; 

 suggesting that there is a physiological mecha- 

 nism through which temperature governs the 

 winter distribution of white shrimp. 



The growing body of evidence indicating im- 

 portant effects of temperature on growth and 

 survival of shrimp has intensified our need for 

 nnore complete temperature data from the 

 field. The fact that shrimp can burrow into 

 substrates beneath the water compounds the 

 problem of measuring features of the different 

 environments inhabited by these animals. At 

 present we are approaching this problem 

 through the use of automatic, continuously 

 recording temperature equipment at our East 

 Lagoon laboratory. The temperatures recorded 

 include those of the air, surface water, bottom 

 water, and substrate, all under the laboratory, 

 where water depth is about 6 ft. The recording 

 speed and flexibility of the equipnnent have 

 allowed us to attain a level of sampling in- 

 tensity that would not be feasible with manual 

 methods. As a result, the data are giving us 

 a previously unobtainable insight into the 

 magnitude and rate of estuarine temperature 

 fluctuations. Already they have shown that 

 rate of temperature change in the bottom 

 water differs from that in the substrate. 

 Temperatures of the substrate vary less 

 rapidly than those near the bottom of the 

 overlying water column. For example, during 

 this winter's coldest period, bottom water 

 temperature fell 3° C. (5.4° F.) in 12 hr., 

 while substrate temperature was reduced 

 only 1.5° C. (2.7° F.). Thus, it is possible 

 that the burrowing habit of shrimp may at 

 times provide a means of avoiding tempo- 

 rarily unfavorable temperature conditions. 



40 



