Pygmy pocket moss (Fissidens exilis) COSEWIC assessment and status report: chapter 6

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Biology

Very little has been published on the biology of Fissidens exilis. As an acrocarpous, autoicous, soil-dwelling moss, F. exilis possesses the characteristics summarized below.

General

The moss life cycle has four main stages, each of which is characterized by different ecological requirements:

1. Dispersal -- Mosses such as F. exilis disperse as spores, which sift into the air through specialized teeth surrounding a hole, or “peristome,” at the end of the capsule. Upon contact with a favourable substrate in a suitable microhabitat, spores germinate to produce protonemata.
2. Establishment -- At the protonemal stage, mosses may be very sensitive to desiccation. Leafy plants grow from the protonemata.
3. Growth -- Gametophytes of Fissidens exilis proliferate vegetatively by outward expansion of filamentous protonemata from existing plants. Protonemata, in turn, initiate the growth of new leafy plants. The expansion of acrocarpous moss colonies relies on the addition of new plants to the colony. Moisture is required for photosynthetic activity and growth.
4. Reproduction -- Gametophytes produce sessile eggs and flagellate sperm, and free water is required for the two to unite. A fertilized egg, still enclosed within the gametophyte, grows into a sporophyte consisting of a spore-filled capsule at the end of a stalk, or ‘seta.’

Reproduction

As discussed in the species description, F. exilis is autoicous, meaning that male perigonia, containing the antheridia and female perichaetia, containing the archegonia, both occur on each gametophyte. This makes self-fertilization possible, eliminating the need for male and female plants to occur close together to ensure spore production. At least five of the seven Canadian collections of F. exilis bear sporophytes (one of the two remaining collections could not be located) (Appendix 1). Spores mature in the winter (Steere 1950).

Survival

The persistent protonemata of F. exilis may influence the species’ survival. Embedded in the substrate, thereby avoiding water loss and minor surface disturbance, the protonemata may form an important ‘diaspore bank’ to maintain populations through periods during which environmental conditions prohibit growth and reproduction of mature gametophytes. They may also facilitate the expansion of colonies by invading nearby substrates.

Fissidens exilis relies on bare, clayey mineral soil, making it vulnerable to successional changes in its habitat. In places where disturbance returns periodically (e.g. stream and river banks, floodplains), soil may be kept bare, or new patches may open as old ones are covered with vegetation.  In other cases where the disturbance is not repeated, however, F. exilis may be eliminated over time. Populations of species of patchy, temporary substrates may be more difficult to track than more perennial species, as they may move frequently.

Movements/dispersal

As noted above, F. exilis is autoicous, making self-fertilization possible and spore production likely. Spore production is very important to ‘shuttle’ type species reliant upon patchy, temporary, but locally recurring substrates (e.g. During 1979). Fragmentation of the embedded protonemata is also possible with repeated soil disturbance, which is characteristic of the riparian habitats preferred by F. exilis. If this form of dispersal is important, riparian corridors would also provide an important dispersal route. However, demographic evidence demonstrating dispersal rates/modes and establishment ability are not available.