Rhodothamniella floridula in upper littoral fringe soft rock caves
Ecological and functional relationships
Chalk habitats, especially in south east England, are intrinsically low in species richness due to the unusual friable and easily eroded nature of chalk (Anon, 1999e). Caves offer some protection from the environment outside. This biotope is predominantly algae which dominate the rock walls and ceiling of the cave. Dominance of one algal species over another and zonation within the cave depends on light intensity, moisture and temperature (Anand, 1937c). Hard flint intrusions in softer chalk are found at North Landing, where Hildenbrandia rubra is present (George et al., 1988).
Characteristic fauna of soft chalk habitats include soft rock-boring invertebrates such as Polydora sp. and Pholas dactylus (Anon, 1999e).
Grazers such as Patella vulgata and Littorina saxatilis may also be present.
Seasonal and longer term change
The growth and abundance of the fine green filamentous algae, Pseudendoclonium submarinum (previously described by Anand as Endoderma perforans) and red algae is favoured in winter (Anand, 1937b, c; Tittley & Shaw, 1980; Burrows, 1991).
Habitat structure and complexity
Zonation of algae inside the cave is dependent on light intensity and is therefore vertical (Anand, 1937c). The chalk rock surfaces of caves are mainly unbroken but with shelves, fissures and flint nodules. Characteristic communities in inner parts of caves at <10% full daylight are Rhodothamniella floridula
sp.) and chalk-boring algae (Anand, 1937c). The shape of the cave also determines how much light reaches the walls and therefore which species will occur. For instance, on surfaces facing away from the cave entrance red algae may predominate, and on surfaces facing towards the cave entrance there is a dominance of Ulva
sp. at a greater distance from the opening (Anand, 1937c).
Spray created by wave motion and lack of direct sunlight means that the cave walls and ceilings are kept sufficiently moist for algal growth (Anand, 1937c) and rock crevices may offer protection for small marine invertebrates.
Macroalgae produce considerable amounts of dissolved organic carbon which is taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local ecosystems, or be exported further offshore.
Annual variation in recruitment success of algae can have a significant impact on the patchiness of the shore. The propagules of most macroalgae tend to settle near the parent plant (Schiel & Foster, 1986; Norton, 1992; Holt et al.
, 1997). Red algal spores and gametes are immotile. Norton (1992) noted that algal spore dispersal is probably determined by currents and turbulent deposition (zygotes or spores being thrown against the substratum). The reach of the furthest propagule and useful dispersal range are not the same thing and recruitment usually occurs on a local scale, typically within 10m of the parent plant (Norton, 1992). Vadas et al.
(1992) noted that post-settlement mortality of algal propagules and early germlings was high, primarily due to grazing, canopy and turf effects, water movement and desiccation and concluded that algal recruitment was highly variable and sporadic. However, macroalgae are highly fecund so that recruitment may still be rapid.
Many species of epifauna, such as Patella vulgata
, Littorina saxatilis
and polychaetes that may be associated with rock crevices, have long lived pelagic larvae and/or are highly motile as adults.
Time for community to reach maturity
No information was found concerning time for community to reach maturity. However, as algae are generally highly fecund, it is likely that maturity will be reached rapidly.
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This review can be cited as follows:
Rhodothamniella floridula in upper littoral fringe soft rock caves.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=39&code=1997>