Observations on the life cycle and nutrition of Dinophilus Taeniatus harmer 1889 (Annelida: Polychaeta)

Abstract

The annual life cycle and nutrition of the monomorphic dinophilid polychaete Dinophilus taeniatus on the Yorkshire coast have been studied from samples taken every 14 days over three years. Freeswimming worms occur October-June in sheltered mid-tide pools rich in diatom masses, Enteromorpha spp. and Ulva lactuca; their numbers increase throughout winter, peak in March-April and then rapidly decline as pool temperatures increase and diatom growths decrease. Decline is due to death of breeding adults and obligatory encystment of the immature progeny. Partial dedifferentiation within the cysts is followed by progressive redifferentiation and excystment October-January as pool temperatures decrease and diatoms flourish. Excysted worms feed, mature and reproduce sexually until May, the females producing a maximum of four cocoons in sequence over 2-3 months with up to 16 eggs per cocoon. Temperature has no effect on initiation of encystment, which is probably related to the age of the immature worm and occurs in April-June even in worms held at winter temperatures. The rate of encystment, though, increases with temperature. Excystment, in contrast, is entirely temperature- dependent and is inhibited by summer temperatures. The polychaete feeds principally on diatoms; digestion and absorption are mediated by l3-glucuronidase, non-specific esterases, arylarnidases, and phosphatases; lipids form the main food reserve. These observations support an earlier hypothesis, based on studies of a Swedish population, that D. taeniatus evolved in cold northern waters and that encystment is primarily an adaptation for surviving freezing of the tide pool habitat in the far north of the range. Its persistence in southern latitudes has the secondary, and here more important, advantage that post-encystment growth and sexual reproduction occur in winter months when food is plentiful due to seasonal absence of macroherbivore competitors. An ability to vary relative durations of different phases in the life cycle allows the species to enjoy an extensive geographic range.