at the Canary Islands (Tuya et al. 2007) and Madeira Island (Alves et al. 2001).
This depth gradient in the vertical distribution of the three sea urchin species has
been related to their distinct morphological characteristics, and consequently, their
different resistance to natural hydrodynamic forces (Tuya et al. 2007). Arbacia
lixula and P. lividus seem to be morphologically adapted to minimize the chances
of being dislodged by high water motion at the intertidal and first few meters of the
subtidal and coexist in relatively high densities only in the shallowest depths (\5
m). Densities of D. africana, less adapted to support intense water movement,
increase with depth. The result is a general pattern in the vertical distribution of
these sea urchins in the warm-temperate rocky reefs of the eastern Atlantic Ocean
(Alves et al. 2001, Tuya et al. 2007), in which wave exposure and island slope that
affects wave energy are major environmental factors determining densities of these
key herbivorous sea urchins (Hernández et al. 2008a). Moreover, protected coasts
have calmer waters that promote retention of urchin larvae (Hernández 2006).
Larvae at exposed sites larvae tend to be carried away. The result is that areas with
higher hydrodynamics show greater urchin density in a deep band (15–20 m) just
below the algal stand, whereas along sheltered coasts algal beds the urchins occur
only at the first meters depth where more food is available (Hernández et al.
2008a).
Because of the biological and ecological features, such as size, motility, feeding
preferences or longevity, several urchin species play a key role, even more
important than other herbivores, in controlling epibenthic communities and in turn
of the overall structure of coastal communities (Lawrence and Sammarco 1982).
The main consequence of this overall phenomenon of intense grazing activity is a
shift from large areas of rocky reefs covered by complex erect macroalgal beds to
overgrazed substrates (‘urchin barrens’ or ‘barren grounds’) dominated by
encrusting algae and some sessile invertebrates.
The influence of echinoids grazing activity on rocky reefs communities of the
eastern Atlantic oceanic islands, particularly in the Canary Islands, have been well
studied (Alves et al. 2001, Tuya et al. 2004c; Hernández et al. 2008b). These are
relatively small barren grounds produced by P. lividus in tidepools, and by A.
lixula on shallow bottoms and around artificial jetties (JC Hernández pers. obs.).
However, barren grounds resulting from the intense grazing activity of D. africana
are more extensive and commonly spread throughout the entire region (Aguilera
et al. 1994; Brito et al. 2004; Tuya et al. 2004c), reaching up to 50 m depth and
covering about 75 % of the total littoral rocky bottoms of the Canary Islands
(Barquín et al. 2004). Densities of this species can reach more than 12 ind m
-2 ,
with severe consequences (Brito et al. 1984, 2004; Alves et al. 2001; Tuya et al.
2004b; Hernández 2006). Field monitoring and experimental manipulations of
urchin densities have shown its major effects on algal communities (Hernández
et al. 2008a). At moderate densities, sea urchin grazing can stimulate plant production and promote species diversity. Because of this sea urchins should be
considered an integral part of ecosystem dynamics (Sammarco 1982; Valentine
et al. 1997). However, densities of as few as 4 urchins m
-2 can remove up to
70–80 % of algal cover (Hernández et al. 2008a) and dramatically reduce
15 Echinoderms of the Canary Islands, Spain
495
This depth gradient in the vertical distribution of the three sea urchin species has
been related to their distinct morphological characteristics, and consequently, their
different resistance to natural hydrodynamic forces (Tuya et al. 2007). Arbacia
lixula and P. lividus seem to be morphologically adapted to minimize the chances
of being dislodged by high water motion at the intertidal and first few meters of the
subtidal and coexist in relatively high densities only in the shallowest depths (\5
m). Densities of D. africana, less adapted to support intense water movement,
increase with depth. The result is a general pattern in the vertical distribution of
these sea urchins in the warm-temperate rocky reefs of the eastern Atlantic Ocean
(Alves et al. 2001, Tuya et al. 2007), in which wave exposure and island slope that
affects wave energy are major environmental factors determining densities of these
key herbivorous sea urchins (Hernández et al. 2008a). Moreover, protected coasts
have calmer waters that promote retention of urchin larvae (Hernández 2006).
Larvae at exposed sites larvae tend to be carried away. The result is that areas with
higher hydrodynamics show greater urchin density in a deep band (15–20 m) just
below the algal stand, whereas along sheltered coasts algal beds the urchins occur
only at the first meters depth where more food is available (Hernández et al.
2008a).
Because of the biological and ecological features, such as size, motility, feeding
preferences or longevity, several urchin species play a key role, even more
important than other herbivores, in controlling epibenthic communities and in turn
of the overall structure of coastal communities (Lawrence and Sammarco 1982).
The main consequence of this overall phenomenon of intense grazing activity is a
shift from large areas of rocky reefs covered by complex erect macroalgal beds to
overgrazed substrates (‘urchin barrens’ or ‘barren grounds’) dominated by
encrusting algae and some sessile invertebrates.
The influence of echinoids grazing activity on rocky reefs communities of the
eastern Atlantic oceanic islands, particularly in the Canary Islands, have been well
studied (Alves et al. 2001, Tuya et al. 2004c; Hernández et al. 2008b). These are
relatively small barren grounds produced by P. lividus in tidepools, and by A.
lixula on shallow bottoms and around artificial jetties (JC Hernández pers. obs.).
However, barren grounds resulting from the intense grazing activity of D. africana
are more extensive and commonly spread throughout the entire region (Aguilera
et al. 1994; Brito et al. 2004; Tuya et al. 2004c), reaching up to 50 m depth and
covering about 75 % of the total littoral rocky bottoms of the Canary Islands
(Barquín et al. 2004). Densities of this species can reach more than 12 ind m
-2 ,
with severe consequences (Brito et al. 1984, 2004; Alves et al. 2001; Tuya et al.
2004b; Hernández 2006). Field monitoring and experimental manipulations of
urchin densities have shown its major effects on algal communities (Hernández
et al. 2008a). At moderate densities, sea urchin grazing can stimulate plant production and promote species diversity. Because of this sea urchins should be
considered an integral part of ecosystem dynamics (Sammarco 1982; Valentine
et al. 1997). However, densities of as few as 4 urchins m
-2 can remove up to
70–80 % of algal cover (Hernández et al. 2008a) and dramatically reduce
15 Echinoderms of the Canary Islands, Spain
495