ONSET AND MAINTENANCE OF METAMORPHIC COMPETENCE IN THE MARINE POLYCHAETE HYDROIDS-ELEGANS HASWELL IN RESPONSE TO 3 CHEMICAL CUES

This study documents the order in which larvae of the tube-dwelling po lychaete Hydroides elegans Haswell become competent to metamorphose in response to three substances: 30 mM excess K+ in seawater, the cyclic nucleotide phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (I BMX) at 10(-4) M in seawater, and a water-soluble homogenate of adults and their tubes. Four experiments were conducted, three at 25 degrees C and one at 20 degrees C, typically with 10-13 larvae per replicate and five replicates per treatment. Each assay lasted 48 h. Larvae typi cally became responsive to IBMX at least 24 h before they became respo nsive to adult homogenate and were always slowest to become responsive to excess K+, suggesting that IBMX acts downstream from the sites at which the other cues operate and that the upstream sites become operat ional later in development. Competent larvae also responded far more q uickly to IBMX - usually within 6-8 h - than to the other cues. Excess K+ and cues associated with adult homogenate may take longer (24-48 h ) to penetrate to their sites of action. Alternatively, IBMX may stimu late metamorphosis directly, possibly by increasing intracellular conc entrations of cAMP, cGMP, or Ca2+, while the other tested substances m ay act instead to turn off production of an inhibitory factor, which m ust then degrade before metamorphosis can occur. In three additional e xperiments, competent larvae remained responsive to all three inducers during 3-4 days of starvation, indicating that the metamorphic pathwa y of H. elegans, once developed, is not readily degraded even under se vere nutritional stress. (C) 1998 Elsevier Science B.V.