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

Planula larvae of the scyphozoan Cassiopea xamachana settle and metamorphose on degrading mangrove leaves of Rhizophoia mangle that lie submerged in shallow 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 originating from deteriorating leaves by means of ultrafiltration, gel filtration and reversed phase HPLC and subsequently obtain characteristic data of this cue. The ultrafiltrate (≤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 laboratory. Fraction I (≥5 kD) was most effective in inducing metamorphosis of 75% of planulae at 1 mg freeze-dried material ml–1 seawater within 24 h. Fractions II and III (both ≤5 kD) resulted in metamorphosis of only 1% of larvae or less within 72 h when applied at 5 mg ml–1. Isochratic HPLC separation of Fraction I with 24% methanol yielded 2 biologically active fractions. One fraction (A/B), which induced 47% of the larvae to metamorphose at 0.9 mg lyophilized material ml–1 seawater within 24 h, consisted of a mixture of at least 2 subfractions and was not further analyzed. The other fraction (C) effected metamorphosis 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 this 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-rich cell wall proteins of plants. Automated sequencing of the natural inducer failed due to a blocked amino terminus. The results of our present study suggest that metamorphic inducers for C. xamachana may emerge nonspecifically as a byproduct of bacterial degradation of deteriorating, proteinaceous plant tissue in their habitat.