Dynamique des populations de Tisbe holothuriae (Crustacea: Copepoda) en élevage sur trois régimes artificiels différents

The influence of three different artificial diets (Tetramin, Bioter food and Renutryl) on the life cycle of Tisbe holothuriae Humes, 1957 has been investigated under constant experimental conditions (19°C; 38 S). Females (F2) whose life history has been followed, were obtained from a common mother (F1). They were bred in 50 ml dishes and transferred into a fresh container as soon as they produced a new egg-sac. Offspring (F3) resulting from successive egg-layings were counted as soon as they moulted into adults. Larval mortality was estimated in two ways: (1) by enumeration of adults issuing from a known number of nauplii; (2) by comparison of the average number of eggs from egg-sacs of F2 with the average number of adult offspring F3. The life-span of F2 females, including the larval stages, varies between 23.6 days (Renutryl diet) and 33.4 days (Tetramin diet). The first egg-sac appears about 12 days after hatching, whatever diet is used. The first copulation immediately follows the last moult, about 8 days after hatching. The reproductive period lasts for 6.5 (Renutryl) to 10.4 days (Bioter). The average number of successive egg-sacs laid is minimal with Renutryl (3.7) and maximal with Tetramin (5.1), but actual egg production is higher with Renutryl (86 eggs/sac) than with Bioter (67) or Tetramin (58). Nevertheless, as a result of varying larval mortality with the different diets, eventual numbers of adult F3 obtained are significantly higher with Bioter (310) than with Tetramin and Renutryl (216 and 188, respectively). The offspring F3 sex-ratio varies in successive broods and with the nature of the food offered: more females are produced with Bioter or Renutryl than with the Tetramin diet. Then net reproduction rate, R ₀, is higher with Bioter, because of the low larval mortality, high fertility, and high ratio of females resulting from this diet. The intrinsic rate of natural increase (r _m) is similar with Renutryl and Bioter diets (0.298 and 0.301, respectively), a lower value (0.260) occurs with the Tetramin diet. These different values are discussed and compared with previous data for Tisbe species fed natural diets.