Growth and asexual reproduction of the sea anemone Metridium: comparative laboratory studies of three species

The rate of growth, individual size attained, and mode and frequency of asexual reproduction were investigated in three species of the sea anemone Metridium. The species differ in ecological distribution, characteristic habitat, and morphology. M. exilis Hand is characteristically intertidal and occurs in aggregations of numerous small (0.2 cm ² pedal disc area, pda) individuals. Growth in laboratory culture ceased at 1.2 cm ² pda despite copious feeding. Feeding accelerated the rate of asexual reproduction by binary fission but did not result in larger individuals. Individuals of M. senile (L.) are larger and reproduce both sexually and asexually by fragmentation. The species occurs from the mid-intertidal to the shallow subtidal in central California. Individuals from intertidal populations grew rapidly in laboratory culture when fed daily (mean pda after 5 months in culture was 45 cm ²). The regenerative ability of M. senile is excellent: 100% of excised fragments (including column and pedal disc tissue) had completely regenerated, including tentacle formation, after 3 wk with no mortality. An undescribed species of Metridium is much larger (individuals grow >/ l m in height) and apparently is exclusively subtidal in central California. Asexual reproduction is rare or absent and no pedal lacerates were produced by individuals during the period of study. The regenerative ability of Metridium sp. is poor: 53% of the excised fragments had regenerated after 19 wk with 25% mortality. The three species thus show distinct patterns of growth and asexual reproduction, paralleling their morphological, ecological, and genetic distinctness.