Analysis of a Population of the Tropical Freshwater Shrimp, Atya scabra (Leach) Rezneat M. Darnell* Tulane University, New Orleans, La. During the course of recent investigations on the biology of Mexican streams, the writer secured two samples from a population of the little known tropical freshwater shrimp, Atya scabra (Leach). A review of the literature has revealed little information published in this country on the ecology of this crustacean. Ortmann (1894) discussed the distribution of the species, and Allee and Torvik (1927) mentioned its presence in pools of a creek in the Panama Canal Zone. Of the foreign papers, that by Villalobos (1943) is of interest. In this excellent morphological study the collecting method and the habitat of the shrimp in the Rio Necaxa near Coyutla in the state of Vera Cruz, Mexico were discussed. In view of the limited knowledge of the ecology of Atya scabra and the uncertain possibility of obtaining addi- tional specimens in the near future, the writer feels justified in publishing this brief population analysis based upon the small samples available. Acknowledgments.—The author takes this opportunity to thank Dr. Fenner A. Chace, Jr., of the U. S. National Museum for making specific determinations of the shrimp and for criticizing the manuscript. The author is grateful also to Mr. Everts W. Storms, who was his host in Mexico. Original field work for the present study was conducted under a grant from the Tozer Foundation of the University of Minnesota, and further field bbser- vations were carried out under a grant from the Tulane University Council on Research. The author wishes to express his appreciation to these supporting institutions. HABITAT Specimens analyzed in the present study were taken in two collections on April 27, 1950 from the Rio Sabinas, a headwater stream of the Rio Tamesi drainage in southern Tamaulipas, Mexico. The collecting area was Storms' ranch, 7 kilometers northeast of Gomez Farias and about 160 kilometers inland from the Gulf of Mexico at an altitude of 100 meters above sea level. The habitat of the shrimp was a turbulent riffle 0-1 meter in depth, with a rock and boulder bottom (fig. 1). Filamentous algae' (Lyngbya and Oscil- latoria) made up the only endogenous vegetation. This, however, constituted less than 5 percent of the total vegetation in the riffle, the remainder being composed of exogenous river borne branches, twigs, leaves, and seedpods. Vegetation from the two sources together amounted to 45.1 grams per square meter of riffle bottom. In addition to the shrimp, an abundant and varied gastropod and insect fauna was present. Details of this community will be reported in a later paper. Throughout six discontinuous months of field work during the winter, spring, and early summer no specimens of this shrimp were observed outside * Present address: Marquette University, Milwaukee, Wisconsin. Reprinted from THE AMERICAN MIDLAND NATURALIST Vol. 55, No. 1, pp. 131-138, January, 1956 132 THE AMERICAN MIDLAND NATURALIST 55(1) Fig. 1.—Habitat of Atya scabra in the Rio Sabinas. Rocks were piled to demonstrate the physical composition of the riffle. the riffle during the daylight hours. At night, however, they were occasion- ally noted on rocky bottoms in adjacent shallow backwaters, particularly in the vicinity of weed beds. POPULATION COMPOSITION A total of 46 specimens were taken from the above-mentioned riffle. These specimens were collected by hand between 11:00 and 12:00 a.m., and the density at the time of collection was estimated to be approximately 10 shrimp per square meter of riffle bottom. Of the total specimens, 28 (or 61%) were males and 18 (or 39%) were females. This difference appears to indicate a preponderance of males in the riffle population, although sta- tistical analysis by the chi-square test demonstrates the difference to be within the expected range of sampling error (P = .301). Villalobos (1943) encountered a preponderance of males in both of his large collections of this shrimp taken from a riffle in the state of Vera Cruz. His first collection containing 100 males and 27 females showed a decided statistical significance (P < .001) in the sex ratio discrepancy, and his second collection of approx- imately 400 specimens showed an even greater difference. From 10:00 to 11:00 p.m. of the day in which the previous collections were made from the riffle, another series was seined from the shallow rocky backwaters 20 to 50 feet upstream from the riffle. This collection contained 5 specimens of A. scabra and all were small to moderate sized females. While inconclusive in themselves, the two small samples indicate the possibility that females, and particularly young individuals, occupy marginal habitats around the edges of the riffle (in rocky backwaters) while males and larger females 1956 DARNELL: TROPICAL FRESHWATER SHRIMP 13 live principally in the riffle itself. If this is true, then considering the riffle and adjacent marginal areas, there should be a more nearly equal sex ratio than otherwise indicated. More striking than the apparent habitat discrepancy is the difference in Males ( N = 28) = 62.65± 2.25 — -- S x = 11.29 ± 1.59 1 Frequency 1 30 35 40 45 50 55 60 65 70 75 80 8 7 Females ( N=18+5) = 48.30 ± 1.25 6 Sx = 5.30 ± 0.88 2 1 —L_ 30 35 40 45 50 55 60 65 70 75 80 Body Length ( mm.) Fig. 2.—Sex and size composition of the population of Atya scabra. Clear blocks represent specimens from the riffle; shaded blocks, specimens from the backwater collec- tion. Statistics were computed on the basis of the former only. 134 THE AMERICAN MIDLAND NATURALIST 55 (1) mean size of the sexes. As noted in fig. 2, the males ranged considerably larger than the females, and in fact, the mean size of the males was greater than the upper limit of the females taken from the riffle. The standard error of the difference of the means is 1.87 which, on the basis of the "T-test" for small samples, is highly significant (P < .001). This size difference can not be explained on the basis of different habitats since the females taken from the backwaters averaged about 5 mm shorter than those taken from the riffle. A comparison of the sizes of the Tamaulipan specimens under discussion with those collected by Villalobos from Vera Cruz indicates that the latter were larger, viz, average length of males, 68.5 mm, of females, 50.0 mm, maximum length of males 94.5 mm. GROWTH No observations were made on the rate of growth of individuals in the population or of expansion of the population itself. Proportional growth, however, was investigated (see fig. 3). Comparison of the regression of cephalothorax length on total body length reveals a slight sexual difference which gradually becomes more pronounced as the individuals grow larger. At the larger sizes the females possess a relatively shorter cephalothorax than the males. In other words, the females possess a relatively longer abdomen, which may be correlated with production of large numbers of eggs and the ovigerous habit. As indicated in the section under reproduction, there appears to be considerable mortality in the crowded developing embryos which in turn might act as a selective agent to increase the surface available for egg attachment. FOOD HABITS The stomach contents of 20 of the specimens collected from the riffle and all 5 of the specimens obtained from the backwaters were examined under the 35 ;30 -J g 25 8 1- 20 0 15 Males Y. 1. 481.0.37X Females Y.2.57+ 0.31X 10 30 35 40 45 50 55 60 65 70 75 80 85 Total Body Length (mm.) Fig. 3.—Comparison of the relationship between cephalothorax length and total body length in males and females. Solid symbols represent riffle collected specimens; open sym- bols, backwater specimens. Computations were made on the basis of all specimens. 1956 DARNELL: TROPICAL FRESHWATER SHRIMP 135 TABLE 1.—Occurrence of food items in stomachs of 24 specimens of Atya scabra containing food. One additional stomach was empty. Percentage of stomachs con- Percentage of mining food total food Food Item item volume Unident. detritus 100.0 91.3 Plant remains 100.0 6.9 Arthropod remains 45.9 1.0 Filamentous algae Oscillatoria sp. 4.2 All others < 1.0 Mougeotia sp. 95.8 Spirogyra sp. 4.2 Diatoms Meridian (?) sp. 25.0 Rhoicosphenia sp. 20.8 Cocconeis sp 4.2 Navicula spp. 100.0 Pinnularia sp. 12.5 Gyrosigma sp. 4.2 Cymbella sp. 62.5 Surirella sp. 4.2 Unident. diatoms 37.5 Scenedesmids Scenedermus sp. 4.2 Desmids Cosmarium sp. 83.3 Micrasterias sp. 8.3 Hyalotheca sp. 4.2 Dermidium sp. 66.7 Protozoa Euglypha sp. 4.2 binocular microscope (45X). Slides were prepared from each of the stomach contents and examined under the compound microscope (645X). The per- centage of each of the major food items encountered was estimated, and all identifiable items listed. This information is summarized in Table I. Only one of the stomachs was devoid of food material. Of the remaining 24, each contained a large quantity of unidentifiable detritus and some plant remains, mostly bits of leaves. About half of the stomachs also contained arthropod remains, including antennae and other appendages of Atya scabra and heads, antennae and appendages of insect larvae and naiads. In most cases these appeared' to be bits of cast-off exoskeletons, but in at least one case the insects probably were eaten alive. A variety of algae was encountered, and these were undoubtedly from three sources, upstream (represented particu- larly by Navicula spp.