MEPS 183:115-124 (1999)  -  doi:10.3354/meps183115

ABSTRACT: Planula larvae of the scyphozoan Cassiopea xamachana settle and metamorphose on degrading mangrove leaves of Rhizophora 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.

KEY WORDS: Natural metamorphic inducer · Cnidaria · Peptides · Settlement · Scyphozoa · Mangrove leaves · Proline-rich proteins