Growth and reproduction of Pycnogonum litorale (Pycnogonida) under laboratory conditions

Abstract

The complete life cycle of Pycnogonum litorale Ström was observed under laboratory conditions. At 15 °C and a 16 h light:8 h dark photocycle, the duration of embryonic development from oviposition to egg-hatching varied from 32 to 111 d. The onset of egg hatching ranged from 32 to 81 d after oviposition in different egg batches, and the period between the first and the last hatch within a single egg batch varied between 16 and 57 d. The larval period, consisting of five larval instars, lasted between 66 and 113 d. Juvenile development required seven (exceptionally 8 or 9) moults and lasted between 263 and 400 d. The average interval between successive moults in juveniles increased with increasing size from 24 to 82 d. Though females did not moult more often than their male conspecifics, they reached a size from 8.5 to 11.0 mm (average 10.1 mm) while adult males measured only 7.0 to 8.5 mm (average 7.7 mm). At low temperatures (2 °C) moulting was almost completely inhibited. After raising the temperature to 6 °C, the moulting frequency increased to a rate almost as high as at a constant temperature of 15 °C. Moulting was also retarded by starvation and accelerated by subsequent feeding. Adults lived for up to 9 years without further moults, with several periods of mating and oviposition at irregular intervals. In combination with previous long-term field observations, the present results provide a more complete picture of the life cycle of P. litorale in a natural habitat. The great variation in the duration of the different developmental stages, the ability to survive periods of cold and starvation, and the longevity of the adults are important for the survival of pycnogonid populations under changing environmental conditions.