In response to disturbance, the pycnogonid Pycnogonum litorale discharges a mixture of eight ecdysteroids (ES). Repeated intensive molestation causes 99% secretion of the endogenous ES present. The concentration of the total ES in the defensive effluent is 1.0 × 10-3 mol · 1-1. 20-Hydroxyecdysone 22-acetate, the predominant ES, reaches 0.8 × 10-3 mol · 1-1. This is sufficient to deter significantly feeding by the common shore crab Carcinus maenas, a generalist predator in the habitat of the pycnogonid. There is evidence that the secreted ES of P. litorale contribute to its unpalatability. The present paper describes for the first time defensive secretion in marine arthropods.
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