A proline-rich peptide originating from decomposing mangrove leaves is onenatural metamorphic cue of the tropical jellyfish Cassiopea xamachana

Planula larvae of the scyphozoan Cassiopea xamachana settle and metamorphos e on degrading mangrove leaves of Rhizophosa mangle that he submerged in sh allow water mangrove ecosystems. Our prior study (Fleck & Fitt 1999; J Exp Mar Biol Ecol 234:83-94) indicated that marine bacteria are involved in the release of at least 1 peptidic compound from such leaves. The goal of our present study was to isolate and purify at least 1 natural peptidic cue ori ginating from deteriorating leaves by means of ultrafiltration, gel filtrat ion and reversed phase HPLC and subsequently obtain characteristic data of this cue. The ultrafiltrate (less than or equal to 10 kD) of the homogenate of decaying mangrove leaves was subjected to gel filtration on a Sephadex G 25 column, resulting in 3 fractions which were tested for their capacity to induce metamorphosis of planula larvae in bioassays performed in the lab oratory. Fraction I (greater than or equal to 5 kD) was most effective in i nducing metamorphosis of 75 % of planulae at 1 mg freeze-dried material ml( -1) seawater within 24 h. Fractions II and III (both less than or equal to 5 kD) resulted in metamorphosis of only 1 % of larvae or less within 72 h w hen applied at 5 mg ml(-1). Isochratic HPLC separation of Fraction I with 2 4 % methanol yielded 2 biologically active fractions. One fraction (A/B), w hich induced 47 % of the larvae to metamorphose at 0.9 mg lyophilized mater ial ml(-1) seawater within 24 h, consisted of a mixture of at least 2 subfr actions and was not further analyzed. The other fraction (C) effected metam orphosis of 85 % of larvae at a concentration of 0.5 mg ml(-1) within 24 h. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of t his fraction revealed a molecular weight of approximately 5.8 kD. Automated amino acid analysis showed. that Fraction C was rich in proline (ca 44 %) and glycine residues (ca 16 %), corresponding to characteristic proline-ric h cell wall proteins of plants. Automated sequencing of the natural inducer failed due to a blocked amino terminus. The results of our present study s uggest that metamorphic inducers for C. xamachana may emerge nonspecificall y as a byproduct of bacterial degradation of deteriorating, proteinaceous p lant tissue in their habitat.