Sargassum hornschuchii, C. Agardh

Sargassum hornschuchii

is usually reported as a deep-water species at least in the Western Mediterranean and in the Adriatic. It is mostly reported between 15-30 m in the Albères coast ( Feldmann 1937), 30 to 60 m depth in most French territories including Corsica ( Thibaut et al. 2016), 30 to 64 m in Italy ( Giaccone 1967, 1972) and 10 to 60 meters in the Adriatic Sea ( Špan 2005), with very rare collections from the northern Adriatic at 5 m ( Munda et al. 2006). Reports from Tunis ( La Galite) come from 25-50m depth ( Feldmann 1961). In Menorca it has been reported byRodríguez-Femenías (1889) between 110 and 150 m depth and in the Adriatic Sea down to 250 m depth ( Ercegović 1960), although these ancient records whether belong to drift specimens or depths were measured inaccurately. Excluding these non-reliable depths, our collection at 65 m is, thus, at the lowest reported depth range for the species. The clear waters surrounding the Balearic Islands ( Ballesteros & Zabala 1993) are probably responsible for it. Another issue is that most reports from the Levantine Sea come from very shallow water ( Shams El-Din & El-Sherif 2012; Fakhry et al. 2013; Noaman et al. 2013a, 2013b; El-Zabalawy & El-Kenany 2015; Fathy et al. 2017; Abdel-Kareem et al. 2020; Ghallab et al. 2022), which casts some doubt about their correct identification.

The collection of S. hornschuchii in Menorca was accompanied by L. rodriguezii and E. zosteroides . The same three species were also found growing together in Banco Appolo (Ustica, Tyrrhenian Sea) between 42 and 64 m depth ( Giaccone 1967) and in La Galite ( Feldmann 1961). According to Giaccone & Bruni (1973), S. hornschuchii characterizes erect macroalgal assemblages from the lower infralittoral and upper circalittoral zones subjected to a high unidirectional water movement and light intensity ranging from 20 to 0.01% of the surface light. These include assemblages dominated either by G. montagnei (as Cystoseira spinosa ), by E. zosteroides (as Cystoseira zosteroides ), by L. rodriguezii and, only in the Alboran Sea and the straits of Messina, by Gongolaria usneoides (Linnaeus) Molinari-Novoa & Guiry (as C. usneoides ), Laminaria ochroleuca Bachelot Pylaie and Saccorhiza polyschides (Lightfoot) Batters. Feldmann (1937) reported S. hornschuchii from the assemblage dominated by E. zosteroides and G. montagnei at the Albères coast, the same kind of assemblage where it appears in some islands of the Tyrrhenian Sea ( Piazzi et al. 2009; Piazzi & Ceccherelli 2020). Sargassum hornschuchii may be restricted to rocky bottoms since it has never been reported from Balearic Islands’ coastal detritic assemblages, where L. rodriguezii , E. zosteroides and G. montagnei are usually common ( Joher et al. 2012, 2015, 2016, 2023).

Sargassum hornschuchii has suffered an important regression in the western Mediterranean ( Gómez-Garreta et al. 2000; Thibaut et al. 2005, 2015, 2016; Ould-Ahmed et al. 2013; Moussa et al. 2018) and it seems to be also disappearing in areas of the Ionian ( Cecere et al. 1996) and the Adriatic seas ( Sidari & Bressan 1992; Cormaci & Furnari 1999; Munda 1993, 2000; Cecere et al. 2001; Curiel et al. 2009; Falace et al. 2010; Perkol-Finkel & Airoldi 2010). There are no experimentally tested factors accounting for this decrease although increased turbidity, hyper-sedimentation, trawling, anchoring and uprooting by fishing nets have been blamed by Thibaut et al. (2015) and Verlaque et al. (2019). In fact, our samples come from trammel net fishing, pointing to fishing gear as a real cause of harm. Because of the current rate of decrease, S. hornschuchii is considered as “Endangered” at the Mediterranean level according to the IUCN classification ( Verlaque et al. 2019). It was also included in the first internationally recognized and accepted list provided under the umbrella of the Barcelona Convention in 1996 in the Annex to the protocol concerning Specially Protected Areas and Biological Diversity in the Mediterranean that was lately updated in 2009 ( Verlaque et al. 2019). This list has been roughly transcribed into some national legislations (e.g. France: Decree no. 2002-1454, December 2002 amended by Decree no. 2014-1195 of 16 October 2014; Spain: Law 42/2007 amended by Order AAA/75/2012) or has been used as a base to produce more accurate local checklists (e.g. Catalonia: Decree 172/2008 of 26 August 2023).

Considering everything stated regarding S. hornschuchii ’s rarity and decrease, this new record from Menorca acquires great relevance. This collection is the only one in the last 35 years from the westernmost part of the Western Mediterranean sea and the first one after more than a century in Spain. We agree with Verlaque et al. (2019) that it is almost impossible to give the IUCN “Extinct” category to deep-water algal species such as S. hornschuchii , since they are naturally scarce and it is extremely difficult to look for new populations. In fact, unequivocally detecting the extinction of a species is not an easy task ( Diamond 1987; Akçakaya et al. 2017; Lee et al. 2017) and specialists debate about the criteria to be considered ( Butchart et al. 2006; Roberts et al. 2023). Uncertainty increases if extinction is local, since recolonisation coming from individuals from other distant populations is possible and random in space and time. But we do not expect that recolonisation could be the case for S. hornschuchii in Menorca as we are very confident that it has never disappeared from the location we collected it. Remote, non-accessible areas are a good place to search for potentially extinct species as local anthropogenic impacts ( i.e., fishing and associated impacts of fishing gear) are presumed to be lower and species persistence to be higher. Thus, protection to fishing of these areas is highly recommended. In fact, the area we collected S. hornschuchii is inside a Site of Community Importance (SCI) within the Natura 2000 network since 2014, and it was designated as a trawl-fishing protected zone in 2016 ( Farriols et al. 2022). Moreover, all the area affecting the rocky bottoms known as “Fora de Sa Barra” has been proposed as a no-take area ( Ballesteros 2022), which would greatly increase the chances of survival of the remaining specimens of S. hornschuchii as well as many more endangered species that are known to thrive in the area ( Barberá et al. 2012; Joher et al. 2012, 2015; Farriols et al. 2022).