PALAEMONETES KADIAKENSIS RATHBUN: POST EMBRYONIC GROWTH IN THE LABORATORY (DECAPODA, PALAEMONIDAE) BY JERRY H. HUBSCHMAN and JO ANN ROSE Department of Biology, Wright State University, Dayton, Ohio 45431, U.S.A. and Franz Theodore Stone Laboratory, Put In Bay, Ohio, U.S.A. INTRODUCTION The study of larval development in decapod Crustacea has become continually refined during the past decade. Historically, the descriptive phases of larval development of marine decapods has been based upon the study of series collected in the plankton. In time, a wide range of species representing a number of decapod orders have been reared from egg to metamorphosis in the laboratory. As a result of this work, much of the variation in size and form observed in the plankton material has also been demonstrable in the laboratory. Five species of Eastern U.S. Palaemonete.r have been reared successfully through metamorphosis in the laboratory. These represent three marine forms: Palaemonete.r vulgaris (Say) and P. pugio Holthuis by Broad (1957a, b) and P. intermedius Holthuis by Broad & Hubschman (1962); and two freshwater species: Palaemonete.r k.adiaken.ri.r Rath- bun by Broad & Hubschman (1960, 1963) and P. paludo.ru.r (Gibbes) by Dobkin (1963). Broad ( 1957b) has demonstrated variation in molting frequency and duration of larval life as a function of diet. It is apparent that the sequence of morpholo- gical and physiological changes leading to metamorphosis bears no direct relation- ship to molting history. Indeed, the control mechanisms involved in both larval processes are not known. In adult shrimp, the initiation of molting is mediated by eyestalk hormones. This is not the case in Palaemonete.r larvae. The X-organ sinus gland system important to adult physiology is not operative during larval life. Consequently, eyestalk removal had no effect on molting frequency during larval development (Hubschman, 1963). The factors influencing molting and development in larval shrimp remain to be discovered although some important information on the effects of variations in temperature and salinity on the larval development of marine crabs has been reported by Costlow, Bookhout & Monroe ( 1960, 1962). There are no such data available on shrimp. In our own work on growth and development in Palaemonete.r, it soon became clear that there was no basis for determining the norm in terms of larval growth and molting frequency in the laboratory. The descriptions of larval development mentioned above, while based upon laboratory reared stages, do not account for variations reflecting 82 fluctuations in photoperiod and temperature. The work on larval development in P. kadiaken.ri.r included data on molting frequency, but the summary of molting and development was based upon the records of sixty individuals completing metamorphosis in the laboratory (Broad & Hubschman, 1963). There are no data available on growth or molting frequency in the other freshwater species, P. paludo.ru.r. This paper presents an analysis of larval growth and molting fre- quency of the larvae of P. kadiaken.ri.r reared under regular periods of light and darkness and controlled temperature. The account is based upon the rearing, through metamorphosis, of over one thousand shrimp in the laboratory. METHODS This work was conducted at the Franz Theodore Stone Laboratory at Put In Bay, Ohio, during the summers of 1966 and 1967. The larvae of the freshwater shrimp Palaemonetes kadiaken.ri.r Rathbun were obtained by holding ovigerous female shrimp in individual aquaria until the brood hatched. The rearing proce- dure was essentially the same as that outlined in the original paper on P. kadia- ken.ri.r (cf. Broad & Hubschman, 1963). Certain conditions were changed for this work. The larvae were reared in groups of fifty in polycarbonate trays containing two liters of filtered Lake Erie water. Rearing was conducted in incubators at 25° C under incandescent illumination controlled to provide 14 hours of light in each period of 24 hours. The periods of light and dark were approximately coincident with the diurnal cycle outside the laboratory. The larvae were inspected daily. At this time, the larvae were fed newly hatched Artemia salina (L.) nauplii. The nauplii were harvested, centrifuged, and washed in fresh filtered lake water. The temperature of the culture water of the original descriptive work varied from 18.5° to 24° C over the summer (Broad & Hubschman, 1963). Measurement of the water temperature at the sites of collection in lagoons on the south shore of Sandusky Bay, indicated that 27° C was not unusual. Surface temperature of the lagoons in summer often reached 30° C. Based upon this information, 25° C was established as the basis for this work. The staging of larval Form I through VI is based upon a combination of mor- phological features. In the past, the only difficulty in staging occurred when a distinction had to be made between intergrades of Forms V and VI. These stages are distinguished by the relative degree of development of pleopods, the number of rostral teeth and the form of the telson. Analysis of the forms occurring in this work revealed that the form of the telson was a dependable diagnostic feature and could be relied upon for staging the developmental phases. We used this feature alone to characterize the larval forms discussed. Form I larvae are quite naturally the first form hatched from the egg. The telson in this stage is charact- erized by no articulation with the last abdominal segment (fig. 1 A). Form II larvae retain the general morphology of the previous zoea. Now, however, the articulation between the telson and the last abdominal segment is distinct (fig. .