Species rarity

Algae

Molluscs

Other invertebrates

Although no work has been compiled specifically on the rarity of species in maerl biotopes, the importance of rarity in the marine environment has recently been considered by Sanderson (1997). Some species may be deemed rare because they are sparsely distributed, others may be rare simply because they remain unrecognised by all but the most skilled observers. For the algae, in particular, if the cryptic phase only of a conspicuous species is present, giving rise occasionally to the conspicuous phase, the species may appear rare when it is actually common. In an area towards the edges of its habitat range a species generally becomes less common. In theory, therefore, if a site is known to be on the margins for a particular species, then monitoring the population of that species might be a suitable method of indicating that changes are occurring in the area.

There are algal species which are characteristically found growing in maerl beds, but these species are not, as a general rule, restricted to maerl beds as their sole habitat. Some of the species found (such as Halymenia latifolia and Scinaia turgida) are apparently restricted to calcareous habitats by their requirement for a substratum in which the shell-boring microthallus can grow. Crustose Peyssonnelia species also show a preference for settlement on calcareous surfaces. Other species found on maerl beds may be present because they are restricted to mobile substrata rather than to those with a calcareous composition. Halarachnion ligulatum and Atractophora hypnoides are probably largely confined to mobile substrata by the poor competitive ability of the crustose tetrasporophytic phase of their life cycle. However, on mobile substrata the heteromorphic life history of some species is probably a distinct advantage in that the algae can survive periods of physical disturbance as the cryptic, boring or crustose phase (Maggs & Guiry, 1987b). These phases also assist the species in withstanding grazing pressure (Lubchenco & Cubit, 1980). Crusts or boring filaments can be severely grazed without the destruction of the entire plant, which can regenerate from remaining fragments.

There are a few algae that are almost entirely confined to maerl biotopes. For example of 23 stations around the UK, Ireland and N. France from which the new species Gelidiella calcicola was described, all but 4 were on maerl (Maggs & Guiry, 1987a). Recently, a morphologically similar species, Gelidium maggsiae Rico & Guiry (1997), has been described from maerl and coralline algal pebbles in Ireland. Similarly, of 11 sites where the crustose species Cruoria cruoriaeformis was found, only one was not a maerl bed (Maggs & Guiry, 1989).

As already mentioned, large numbers of molluscan species are found on maerl (Nunn, 1992; Hall-Spencer, 1998), but the majority of these species probably reflect the nature of the substratum on which the maerl lies, rather than the maerl as a habitat in its own right. Only four species were present at most of Nunn=s sites (Tectura virginea, Gibbula cineraria, Rissoa interrupta, Modiolarca tumida), and of these, only Tectura virginea can be considered to be associated with maerl, the others being ubiquitous in lower shore and sublittoral environments. T. virginea is found most commonly on encrusting Lithothamnion spp. on the lower shore in semi-exposed sites or areas of current. Other species frequently found with maerl were Hinia incrassata, Rissoa parva, Tricolia pullus, Hiatella arctica, Lepidochitona cinereus, Onoba semicostata and Heteranomia squamula, but all of these are common in other habitats not associated with maerl.

In Ireland, two new species of amphipod crustaceans in the genera Stenothoe and Listriella were recently described from maerl (Myers & McGrath, 1980, 1983; Costello, 1987) and Costello et al. (1997) consider that it is likely that several more rare species will found to be restricted to maerl habitats. Davies & Hall-Spencer (1996) reported that most of the maerl polychaetes were characteristic of coarse sediments, and included some probable new species in the genera Sphaerosyllis and Opisthodonta. Material from Loch Ailort maerl beds included several epifaunal species also found on Irish maerl, such as Stomatoporina incurvata and an undescribed species of Monocrepidium, both of which were considered by G. Konnecker to be exclusively associated with maerl.

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