(e.g. in EUNIS (2019)), but Basso et al. (2016) pointed out that “maërl” in a strict
sense refers only to branched rhodoliths. In the Alboran Sea, the common morphology is that of rounded rhodoliths.
Mediterranean “maërl” beds were considered to be a facies of the “coastal detritic
bottoms” by Pérès and Picard (1964), a criterion followed in the classification of
benthic marine habitat types for the Mediterranean region (UNEP/MAP/RAC-SPA
2006). That is because the calcareous algae grow and accumulate on a variety of soft
bottoms mainly composed by bioclastic gravels or coarse sands (calcareous detritus
of coralline algae, molluscs, echinoderms, anthozoans, etc.) originated locally or
from the neighbouring biocenoses. “Maërl” or rhodolith beds are usually composed
of varying proportions of living and dead rhodoliths, with or without additional
sediment (Bosence 1983a, b). Fine sand or mud may fill interstices, with mud
content below 20% (Picard 1965), since these bottoms are under moderate to high
hydrodynamism. The nature of the underlying sediment is crucial because suitably
sized grains for spore settlement must be present. Once established, the rhodoliths
themselves provide this suitable substrate.
The best and most extensive representation of this habitat in the Alboran Sea is
found on the Alboran insular platform between 25 and at least 100 m (Fig. 9.20). The
INDEMARES Alboran project (Gofas et al. 2014a) mapped 76 km
2 , covering more
than one-third of the Alboran platform. The summit platform of Seco de los Olivos,
situated off Almería in the northeastern part of the Alboran Sea, is covered by
rhodoliths between 76 and about 100 m depth (de la Torriente et al. 2014). Occurrences along the mainland coast are extremely limited and patchy along the Andalusian coast. This is consistent with expectations since the buildup of “maërl” beds
requires flat, current swept bottoms in the 20–100 m depth range, a substrate of sand
or gravel without too much mud and clear, transparent waters away from the input of
rivers. The Strait of Gibraltar meets some of these conditions but lacks flat sedimentary bottoms, and the bay of Málaga is too muddy in the appropriate depth interval.
The sole significant occurrence documented so far is off Punta de la Polacra in the
Cabo de Gata-Níjar Natural Park (Luque and Templado 2004). The continental
platform of the Moroccan and Algerian parts of the Alboran Sea could meet the
requirements in more extensive areas, but much exploration is still needed to assess
the actual extension of “maërl”/rhodolith beds, documented so far off Cap des Trois
Fourches (Bazairi et al. 2013).
Rhodolith beds are remarkable as bioconstructions and contribute to elaborate a
complex and heterogeneous habitat which supports high species richness and diversity favoured by their three-dimensional structure (Foster 2001; Gofas et al. 2014a;
de la Torriente et al. 2014). The main species that make up the rhodoliths in the
Alboran platform are Lithophyllum racemus, Lithothamnion corallioides,
Lithothamnion philippi and Phymatolithon calcareum, although up to 21 different
species of coralline algae have been identified using molecular tools, of which 11 are
still under study (Gofas et al. 2014a). Samples yielded a total of 415 benthic species
(algae other than those forming maërl and fish excluded). These bottoms harbour an
interesting fauna of sponges and anthozoans, including some rare or new species.
Most common sponges are Bubaris vermiculata of a striking red colour, Diplastrella
9 Benthic Fauna of Littoral and Deep-Sea Habitats of the Alboran Sea: A. . .
331
sense refers only to branched rhodoliths. In the Alboran Sea, the common morphology is that of rounded rhodoliths.
Mediterranean “maërl” beds were considered to be a facies of the “coastal detritic
bottoms” by Pérès and Picard (1964), a criterion followed in the classification of
benthic marine habitat types for the Mediterranean region (UNEP/MAP/RAC-SPA
2006). That is because the calcareous algae grow and accumulate on a variety of soft
bottoms mainly composed by bioclastic gravels or coarse sands (calcareous detritus
of coralline algae, molluscs, echinoderms, anthozoans, etc.) originated locally or
from the neighbouring biocenoses. “Maërl” or rhodolith beds are usually composed
of varying proportions of living and dead rhodoliths, with or without additional
sediment (Bosence 1983a, b). Fine sand or mud may fill interstices, with mud
content below 20% (Picard 1965), since these bottoms are under moderate to high
hydrodynamism. The nature of the underlying sediment is crucial because suitably
sized grains for spore settlement must be present. Once established, the rhodoliths
themselves provide this suitable substrate.
The best and most extensive representation of this habitat in the Alboran Sea is
found on the Alboran insular platform between 25 and at least 100 m (Fig. 9.20). The
INDEMARES Alboran project (Gofas et al. 2014a) mapped 76 km
2 , covering more
than one-third of the Alboran platform. The summit platform of Seco de los Olivos,
situated off Almería in the northeastern part of the Alboran Sea, is covered by
rhodoliths between 76 and about 100 m depth (de la Torriente et al. 2014). Occurrences along the mainland coast are extremely limited and patchy along the Andalusian coast. This is consistent with expectations since the buildup of “maërl” beds
requires flat, current swept bottoms in the 20–100 m depth range, a substrate of sand
or gravel without too much mud and clear, transparent waters away from the input of
rivers. The Strait of Gibraltar meets some of these conditions but lacks flat sedimentary bottoms, and the bay of Málaga is too muddy in the appropriate depth interval.
The sole significant occurrence documented so far is off Punta de la Polacra in the
Cabo de Gata-Níjar Natural Park (Luque and Templado 2004). The continental
platform of the Moroccan and Algerian parts of the Alboran Sea could meet the
requirements in more extensive areas, but much exploration is still needed to assess
the actual extension of “maërl”/rhodolith beds, documented so far off Cap des Trois
Fourches (Bazairi et al. 2013).
Rhodolith beds are remarkable as bioconstructions and contribute to elaborate a
complex and heterogeneous habitat which supports high species richness and diversity favoured by their three-dimensional structure (Foster 2001; Gofas et al. 2014a;
de la Torriente et al. 2014). The main species that make up the rhodoliths in the
Alboran platform are Lithophyllum racemus, Lithothamnion corallioides,
Lithothamnion philippi and Phymatolithon calcareum, although up to 21 different
species of coralline algae have been identified using molecular tools, of which 11 are
still under study (Gofas et al. 2014a). Samples yielded a total of 415 benthic species
(algae other than those forming maërl and fish excluded). These bottoms harbour an
interesting fauna of sponges and anthozoans, including some rare or new species.
Most common sponges are Bubaris vermiculata of a striking red colour, Diplastrella
9 Benthic Fauna of Littoral and Deep-Sea Habitats of the Alboran Sea: A. . .
331