Life history

Reproduction

Growth rates

Two fundamentally different growth forms of L. corallioides, L. glaciale and P. calcareum can be found. The plants may form crusts attached to rock, pebbles or sometimes shells, or they may be free-living, growing as nodules, rhodoliths or as branched structures resembling "jacks" or caltrops. Only two crustose plants of L. corallioides have been recorded from UK waters (Dorset and Devon; Irvine & Chamberlain, 1994) and none of P. calcareum. L. glaciale on the other hand is commonly found both in the free-living and attached forms although it has frequently been misidentified as L. corallioides in the more northerly parts of the British Isles (Hall-Spencer, 1995b). Maerl beds are usually composed of one of a combination of L. corallioides and P. calcareum or, in Scotland, L. glaciale and P. calcareum. The proportions in which the species are present may vary widely spatially and temporally.

Jacquotte (1962) and Cabioch (1969, 1970) reported that juvenile plants of the maerl species grow as crusts on pebble or shell substrata. Erect branches formed by these crusts break off and give rise to maerl thalli. Thus two growth forms of Lithothamnion corallioides, L. glaciale and Phymatolithon calcareum occur: encrusting or free-living.

In Brittany, recruitment to free-living maerl populations was predominantly from branches shed from crustose plants; vegetative propagation from unattached plants was rare (Cabioch, 1969). Freiwald (1995) likewise found that free-living L. glaciale maerl in N. Norway originates from branched attached crusts. Huvé (1956), by contrast, reported that fragmentation of free-living maerl thalli was the main method of reproduction in the maerl beds near Marseilles. Propagation from branches shed from crustose plants may also occur in Madeira and Tenerife, where crustose plants are frequent, but in UK waters, as noted above, crustose plants of L. corallioides and P. calcareum are extremely rare or unknown (Irvine & Chamberlain, 1994). Unattached plants of these species must therefore be almost entirely vegetatively propagated. Lithothamnion glaciale, on the other hand, is commonly found in both the free-living and attached forms.

Most authors working on the taxonomy or ecology of maerl species comment that reproductive organs are rarely found. During a 2-year-long monthly sampling sequence in maerl beds in Galway Bay, Maggs (1983a) did not find any fertile thalli of L. corallioides. Only one fertile plant of L. corallioides has been reported for the British Isles, an epilithic plant from the south coast of England. In Galway Bay, only tetrasporangial conceptacles were found for P. calcareum. At one site, these varied between an average of 1 and 3 thalli per sample (except during May and June when none were found), representing less than 1% of thalli. At a second site nearby, an average of 1 to 14 fertile thalli were found per sample, with a mid-summer maximum, although fertile plants were found throughout the year. Many hundreds of specimens of P. calcareum and L. corallioides were collected in the Ria de Vigo (Adey & McKibbin, 1970) of which only 24 and 3 plants respectively showed evidence of conceptacles. Of these, only about 6 plants (all P. calcareum collected in March-April) had developing conceptacles, all the others being mature or degenerate.

In the baie de Morlaix, Brittany, Cabioch found P. calcareum with tetrasporangial conceptacles in the winter and L. corallioides with tetrasporangial conceptacles mainly in the winter; she suggested that phasic reproduction occured, reaching a peak perhaps once in 6-8 years (Cabioch, 1969). This may explain the observed variations in the continually changing proportions of the different maerl species forming a maerl bed (Cabioch, 1969). Depending on the length of time since the most recent reproductive event and the relative success of the settlement and colonisation, one species may become dominant within an area of maerl in terms of numbers of live plants. This dominance may decline with time as the plants die and another species becomes reproductive. Dominance cycles with periods of about 30 years have been recorded on some of the maerl beds of northern Brittany.

By contrast, Lithothamnion glaciale plants have reproductive conceptacles all year in Greenland and Sweden (Rosenvinge, 1917; Suneson, 1943). In Scotland, however, although conceptacles are common in winter, the thalli are sterile in summer (Hall-Spencer, 1994).

Very few experiments to measure the growth rates of coralline algae have been attempted, due to the technical difficulties of working on these organisms, particularly the maerl morphologies. Results reported to date suggest that there are wide variations (between species, between geographical areas, and seasonally) in growth rates of maerl whether measured as gross calcium carbonate production, or as apical extension of maerl branches. Further work is currently ongoing to determine >typical= growth rates for maerl (Fazakerley, 1997; Fazakerley & Guiry, 1998; Hall-Spencer, pers. comm.).

Gross measurements of calcium carbonate accumulation have been made for some maerl beds; these show a high degree of variation. Lithothamnion corallioides and Phymatolithon calcareum accumulated over 400 g CaCO₃ m^-2 yr^-1 in Ireland (Bosence, 1980). On the basis of buoyant density measurements of baskets of live maerl, L. corallioides was estimated to produce 876"292 g CaCO₃ m^-2 yr^-1 at a shallow site in the rade de Brest (Potin et al., 1990). On the Mallorca-Menorca shelf most of the modern algal carbonate production occurs at depths of less than 85-90 m, which is the lower limit of the coralligenous and maerl communities (Canals & Ballesteros, 1997). Maerl beds in moderately deep waters (40-85 m) formed 210 g CaCO₃ m^-2 yr^-1. These growth rates are similar to those for the temperate crustose species Lithophyllum incrustans of 379 g m^-2 yr^-1 (Edyvean & Ford, 1987) but an order of magnitude lower than that of the tropical reef coralline genus Porolithon (3120 g CaCO₃ m^-2 yr^-1) measured by Johansen (1981). Much lower estimates of only 16-41 g CaCO₃ m^-2 yr^-1 were made by Cucci (1979) for maerl deposits in the Sound of Iona, perhaps indicating less than optimal environmental conditions.

Field growth rate measurements were made by Adey & McKibbin (1970) on numerous individual rhodoliths of P. calcareum and L. corallioides in the Ria de Vigo (Summary table). The study was conducted on maerl beds at a depth of 5-6 m below low water. Using repeated photographs as well as physical measurements, they determined that the growth rates of branch tips on the rhodoliths were very slow. Little or no growth was recorded during the winter months (less than 1mm d^-1 between October and March) with maximum growth occurring in June and July. In total their estimates indicate an annual growth rate of 0.55 mm yr^-1 for branch tips of P. calcareum and 0.10 mm yr^-1 for L. corallioides. According to their calculations based on ambient temperatures and irradiance, mean yearly growth in the south-western British Isles would be less than 1 mm per year. Böhm et al. (1978), however, calculated apical branch elongation of Baltic plants of P. calcareum as 0.5-2.7 mm per year. Potin et al. (1990) likewise found the maximum growth rate of L. corallioides in Brittany (0.26% per day) to occur in July, and the minimum in February, but the rates cannot be compared directly with those recorded by Adey & McKibbin (1970) due to different methodology.

More recently, Fazakerley (1997) and Fazakerley & Guiry (1998) have carried out pilot studies on the growth rates of Lithophyllum dentatum, Lithophyllum fasciculatum and Lithothamnion corallioides in Kingstown Bay, Connemara, by tagging 20 individuals of each in very shallow water. Mean growth of the three species over the 30-week period was 5.93 mm d^-1, 5.14 mm d^-1, and 2.57 mm d^-1 respectively. These were increases in the diameter of the thalli, so represent approximately double the apical growth rate. The overall increase in the diameter of L. corallioides thalli over 7 months (July to January) was 0.96 mm, or approximately 1 mm tip growth per year. This figure, although very low by comparison with other algae, is nevertheless an order of magnitude higher than Adey & McKibbin's (1970) figures for Spanish maerl. The differences recorded may be related to different methodology (the Spanish thalli were tied to a line while the Irish ones were free) and highlight the difficulties of extrapolating from the results of single studies.

A comparison with the encrusting species Lithophyllum incrustans, for which extension of the margins was a mean of 2.9 mm yr^-1 (Edyvean & Ford, 1987), shows that maerl growth rate appears to be of the same order as that of temperate encrusting corallines.

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