Polystichum muricatum hybridises with both P. polyphyllum and P. speciosissimum on the Cerro de la Muerte in Costa Rica. Both hybrids are intermediate between their two parents in most of the assessed characters. True indusia, which are fully developed in P. muricatum, vestigial in P. polyphyllum, and absent in P. speciosissimum, are intermediate in mean development in both hybrids. However, one sorus is likely to have either a fully-developed indusium or a vestigial indusium. False indusia are characteristic of P. speciosissimum, but are vestigial to absent in its hybrid with P. muricatum. False-indusium development appears to behave as a recessive trait in the interaction of P. speciosissimum with P. muricatum.

Environmental disturbance has played a major role in the development of hybrids in Costa Rica. Mixed populations of the parent species are more common in disturbed terrain than in pristine areas, and hybrids are found only in disturbed terrain; this suggesting that disturbance promotes hybridisation.