Atlantic European rhodolith beds, also known as maerl beds, are marine coastal habitats of high diversity composed of unattached non-geniculate coralline red algae —maerl/rhodolith— mixed with gravel, shells, and pebbles overgrown by crustose coralline algae (CCA; v.gr., Cabioch 1969, 1970; Bosence 1976; Peña & Bárbara 2009; Hall-Spencer & al. 2010; Adey & al. 2015). The life-history of coralline red algae involves an alternation among three phases: a haploid gametophyte, a diploid carposporophyte —carried by the haploid female gametophyte after fertilization of carpogonium—, and a diploid tetrasporophyte (Irvine & Chamberlain 1994). In two major Atlantic European maerl species, Phymatolithon calcareum (Pallas) W.H.Adey & D.L.McKibbin and Lithothamnion corallioides (P.L.Crouan & H.M.Crouan) P.L.Crouan & H.M.Crouan, sporangial conceptacles have been occasionally reported —usually in free-living growth forms, and rarely in encrusting plants—, while gametangial conceptacles were even more rare (i.e., sexual conceptacles), and were only found as associated CCA in maerl beds from Brittany (Cabioch 1969, 1970; Adey & McKibbin 1970; Maggs 1983; Woelkerling & Irvine 1986; Irvine & Chamberlain 1994; Mendoza & Cabioch 1998; Peña & Bárbara 2004; Peña & al. 2014). Several authors (Lemoine 1910; Cabioch 1969, 1970; Freiwald 1995; Mendoza & Cabioch 1998) have pointed out the contribution of the CCA to the development of further unattached maerl plants: after germination of the carpospores produced in CCA, juvenile tetrasporophytic plants grow also as crusts, and afterwards erect branches formed by these crusts break off, and give rise to unattached maerl plants. In Breton maerl beds, this type of recruitment was reported as the dominant (Cabioch 1969, 1970; Mendoza & Cabioch 1998). Nonetheless, others authors (Bosence 1976; Woelkerling 1988; Peña & al. 2014) have reported that fragmentation of free-living maerl thalli is the main method of propagation in the maerl beds.

In recent years, several molecular studies have been focused on the diversity and systematics of maerl-forming species in Atlantic Europe (Carro & al. 2014; Hernández-Kantún & al. 2014, 2015a, 2015b; Pardo & al. 2014; Peña & al. 2015b). However, the diversity and specific composition of their associated CCA are little-known. In Brittany, two DNA barcoding works found crustose plants of Phymatolithon calcareum and Phymatolithon lamii (Me.Lemoine) Y.M.Chamb., which showed that species composition of associated CCA could share some similarities with unattached maerl plants (Peña & al. 2014, 2015b). Based on previous results obtained for maerl-forming species of two study areas from Brittany and Galicia (Pardo & al. 2014), we identified their associated CCA using a combination of DNA barcoding —COI-5P— and morpho-anatomical features. Apart from the species diversity of associated CCA, we compared the composition of both growth-forms (maerl and CCA) in order to know if the associated CCA could be involved in the recruitment of new unattached maerl plants.

In our study, nine CCA species were recorded associated with two Atlantic European maerl beds: Molène —Brittany— and Ons —Galicia— Archipelagos. Despite that these CCA collections consisted on epilithic crustose corallines growing partial or entirely over pebbles, we are aware that could be interpreted as “rhodolith” given that this term is extensively applied to unattached nodules with non-algal core (Irvine & Chamberlain 1994; v.gr. see fig. 6 in Basso & al. 2009). While the four CCA species associated with the Breton maerl bed matched with available sequences from northern Atlantic coast —Phymatolithon calcareum, Phymatolithon lamii, Lithophyllum hibernicum, and Phymatolithon sp. 3ATcrust—, the five CCA taxa from the Galician maerl bed did not obtain any record. However, according to the molecular and morphological data obtained, CCA collected in Galicia were identified up to genus level as Phymatolithon sp. 5, Lithothamnion sp. 2, Mesophyllum sp. 3, Mesophyllum sp. 4, and Lithophyllum sp. 3. The absence of any match with public databases containing more than 4,000 COI-5P sequences of coralline red algae pointed out the cryptic diversity still uncovered in coralline algae.

The nine taxa delimited corresponded to the same four maerl-forming genera in Atlantic Europe —Lithothamnion, Phymatolithon, Mesophyllum, and Lithophyllum (Irvine & Chamberlain 1994; Pardo & al. 2014; Hernández-Kantún & al. 2015a)—. Nevertheless, the number of associated CCA species recorded was higher than the number of maerl species cited in both studied areas (Carro & al. 2014; Pardo & al. 2014; Peña & al. 2014, 2015b): four CCA vs. two maerl species in Molène Archipelago —Phymatolithon calcareum and Lithothamnion corallioides—; and five CCA vs. three maerl species in Ons Archipelago — Phymatolithon calcareum, Phymatolithon lusitanicum, and Lithothamnion corallioides—. The similarity in the species composition between the both growth-forms is approximately 25% in the Breton maerl bed, but totally dissimilar —0% shared species— in the Galician bed.

In Molène Archipelago the dominant genus in the CCA studied was Phymatolithon, while in Galicia was Mesophyllum, this latter having a southern distribution along the Atlantic European coasts (Guiry & Guiry 2016). This gradual replacement of both species with latitude has been also observed in the composition of major maerl-forming species in the OSPAR maritime area, including Brittany and Galicia (Carro & al. 2014; Pardo & al. 2014).

According to the literature, Phymatolithon calcareum is considered as a major maerl-forming species in Atlantic Europe (Irvine & Chamberlain 1994; Carro & al. 2014; Pardo & al. 2014). Our record of CCA with uniporate sexual conceptacles in the Breton maerl bed agrees with previous reports of gametophytes —including the carposporophyte— under crustose forms, all of them from this region (Mendoza & Cabioch 1998; Peña & al. 2014). This observation would corroborate that, at least in Brittany, the gametangial stage of Phymatolithon calcareum occurs as associated crusts —i.e. attached—, while unattached maerl plants of this species are sporophytes (Cabioch 1969, 1970; Mendoza & Cabioch 1998). However, in adjacent areas such as the British Isles or Atlantic Iberian Peninsula where maerl beds have been intensively studied, fertile gametophytes —crusts or unattached— have been never recorded yet (Irvine & Chamberlain 1994; Peña & al. 2014, 2015b). Given that all Atlantic European records of gametangial stages of Phymatolithon calcareum are still restricted to Brittany (Mendoza & Cabioch 1998; Peña & al. 2014), we presume that this Atlantic region plays an important role in the life-history of this major maerl-forming species.

Phymatolithon lamii is a crustose species occurring from the intertidal to subtidal rocky shores of the Atlantic coasts from Arctic Norway to South Atlantic Iberian Peninsula (Chamberlain 1991; Irvine & Chamberlain 1994; Peña & al. 2015b). Our collection in the Breton maerl bed as associated CCA corroborated recent records of this species from the same maerl bed —Molène archipelago (Peña & al. 2015b).

A recent molecular study (Hernández-Kantún & al. 2015a) confirmed that Lithophyllum hibernicum is a common intertidal crust that has been repeatedly misidentified in the Atlantic European literature as Lithophyllum incrustans, which also occurs as both encrusting and maerl forms —i.e., Pardo & al. (2014), as Lithophyllum fasciculatum (Lam.) Foslie and Lithophyllum dentatum (Kütz.) Foslie—. In our study, Lithophyllum hibernicum was recorded subtidally in the Breton maerl bed as associated CCA.

A non-common morpho-anatomical observation was detected in the Galician specimen identified as Mesophyllum sp. 4 (CPVP-769): both types of conceptacles —uniporate sexual and multiporate asexual— occurred in the same crust surface. A similar observation was found in the Galician maerl species Mesophyllum sphaericum V.Peña & al. in Galicia (see fig. 2a in Peña & al. 2011).

Regarding the development of new maerl plants in the Atlantic European maerl beds, two main mechanisms have been suggested in the literature: i) vegetative fragmentation of own free-living maerl plants (Bosence 1976; Woelkerling 1988; Peña & al. 2014); ii) involvement of associated CCA in the recruitment of new maerl plants —i.e., detached branches of crusts (Lemoine 1910; Cabioch 1969, 1970; Freiwald 1995; Mendoza & Cabioch 1998)—. Our study pointed out that the latter hypothesis could be applied, but only for Phymatolithon calcareum in the Breton maerl beds, where gametangial —plus carposporangial— and sporangial stages occur under different growth-forms —CCA and maerl, respectively (Cabioch 1969, 1970; Mendoza & Cabioch 1998; Peña & al. 2014)—. However, in the Galician maerl beds the species composition of associated CCA in comparison with the main maerl-forming taxa —Phymatolithon calcareum, Phymatolithon lusitanicum, and Lithothamnion corallioides— appeared to be different. This together with the absence of maerl species records bearing gametangial —and carposporangial— structures in this region —attached or unattached—, and the occasional records of sporangial conceptacles observations (Peña & Bárbara 2004), the fragmentation of own free-living maerl plants should be considered as the main propagation method in the Galician maerl beds.

The present study carried out in two protected Atlantic European maerl beds pointed out that maerl beds harbours a high species diversity of associated CCA, which may not share the same species composition that maerl-forming algae. Also a high cryptic diversity was found in the associated CCA, as occurs in the maerl-forming species (Carro & al. 2014; Pardo & al. 2014) and in geniculate coralline algae (Walker & al. 2009; Pardo & al. 2015). Further extensive studies in other European maerl beds are necessary to understand this unexpected diversity of associated CCA. Nonetheless, according to our results, the crustose growth-forms should be taken into account in management actions of Atlantic European maerl beds.