Herbivory on ferns is scarcely studied and ant-colonization of ferns is mostly reported from true myrmecophytes, which possess domatia or extrafloral nectaries to attract ants. However, in very few cases petiole-boring herbivores are producing galleries, which later on can be colonized by ants. We observed ants living in the petioles and the rachis of the leaves of the mangrove fern Acrostichum danaeifolium at a mangrove site on the Gulf of Mexico, Veracruz. The objective of our study was to investigate during a period of nine months the origin, frequency and seasonality of the galleries in a natural population of 30 individuals of the mangrove fern, the damage caused by this herbivore activity, and the occurrence of ants inhabiting this microhabitat.

During the entire observation period leaves of the mangrove fern were infested by larvae of an unknown species of moth (Microlepidoptera). These infested 87% of the plants and 41% of the leaves. The damage caused by the larvae did not affect plant growth, e.g. maximum leaf size. Among ten ant species found to colonize the galleries, Tapinoma sessile and Wasmannia auropunctata, two introduced tramp species, were the most common ones. The ants inhabited an average of 29% of the moth infested leaves. Some leaves were occupied by two or three ant species simultaneously. In most cases we found complete ant colonies varying from a few dozen to up to several hundred individuals. In this way the mangrove fern becomes an involuntary myrmecophyte, since it does not produce domatia or extrafloral nectaries to attract ants directly, which are opportunistic colonizers and use the available galleries as a temporary microhabitat, without any benefit for the fern.

Key words: Acrostichum danaeifolium, ants, ferns, herbivory, mangroves, myrmecophytes