Seaside bone (Hypogymnia heterophylla) COSEWIC assessment and status report: chapter 6
Life cycle and reproduction
Lichens are dual organisms and reproduction in Hypogymnia heterophylla involves the necessity of pairing a fungal ascospore or fungal hypha with the compatible Trebouxia green alga. Asexual reproduction is effected by fragmentation of the long, narrow perpendicular lobules; the fungus and alga are “in place” in the lobule, which acts as a vegetative propagule. The abundant pycnidia on H. heterophylla’s upper lobe surface produce mitotic spores or conidiospores that when freed must find compatible Trebouxia alga in order to initiate growth of a lichen thallus. Apothecia are fairly common on H. heterophylla and sexual reproduction of a fungal ascospore resynthesizing with a Trebouxia alga most likely often occurs. Compatible Trebouxia alga partners appear to be readily available on the shore pine twigs through intact thalli of H. heterophylla, intact thalli of other lichens in the same habitat or as free-living algae from decaying lichens.
In this coastal habitat, nutrient enrichment associated with sea spray may favour Hypogymnia heterophylla by, for example, controlling the availability of suitable external algal partners. This suggestion seems consistent with this species’ habitual occurrence on small terminal twigs, which are presumably highly exposed to sea spray, but only moderately exposed to acidification from rain-carried leachates from the middle and upper canopy (Barkman 1958; Goward 1996).
Physiology
Specific physiological studies have not been conducted on Hypogymnia heterophylla. In Canada this species is a seaside epiphyte of exposed areas of the Pacific coast. In such exposed habitats, it is inferred that this lichen can withstand strong winds, high solar radiation, high humidity and salt spray.
Dispersal/migration
Lichen diaspores, including conidiospores, may be dispersed in three different ways: by wind, by water, and by animals (Bailey 1976). It is likely that all three mechanisms operate to different degrees in dispersing the spores of Hypogymnia heterophylla, though for long-distance dispersal, wind and especially birds (Bailey & James 1979; Jorgensen 1983) are expected to be the most important agents (Goward 1996).
Interspecific interactions
Hypogymnia heterophylla is found on the twigs and branches of young to intermediate-aged shore pine. Other lichen species found on shore pine branches in the same habitat and often with H. heterophylla include H. enteromorpha, H. inactiva, H. physodes, H. imshaugii, Melanelixia subaurifera, Parmelia sulcata, Platismatia herrei, Ramalina farinacea, R. menziesii, Tuckermannopsis orbata, Usnea cavernosa, U. ceratina and Usnea sp.
Adaptability
Given its extremely narrow habitat preferences, growing from the coast to approximately 1 km inland on early to intermediate seral shore pine, Hypogymnia heterophylla would not be considered as being adaptable. The presence of sea spray may be a critical factor in its establishment and survival.
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