Ecology and conservation of the Mediterranean endemic coral Cladocora caespitosa = Ecología y conservación del coral endémico del Mediterráneo Cladocora caespitosa

Abstract

Marine ecosystems are declining worldwide threatened by an increasing number of stressors. Global change-related disturbances have highlighted the need of new complementary conservation measures; for which the knowledge on the affected species, communities and impacts is essential. The species objective of this PhD, the Mediterranean endemic coral Cladocora caespitosa, may serve as a case study of those species that even if seriously threatened, are lacking essential information on key ecological processes and the responses to the rapid environmental changes that are happening globally.

Cladocora caespitosa is the only colonial and zooxanthellate scleractinian coral endemic to the Mediterranean Sea. It is a long-lived and ecosystem engineer species, being one of the rare examples of this type of organisms found in shallow Mediterranean communities, and constitutes an invaluable natural patrimony due to its extensive ancient history, its sizeable long-lasting structures and its fragility in the actual context of climate change. Currently, large C. caespitosa bioconstructions are scarce and only a few examples are known, i.e., in Mjlet National Park (Adriatic) or in the Columbretes Islands (NW Mediterranean).

The extensive field of colonies and reefs found in the Illa Grossa Bay (Columbretes Islands Marine Reserve) shows a cumulative colony cover of 2900 m2. This population displays a high degree of geographical isolation and its spatial distribution in the bay is highly aggregated. Our results showed that Cladocora caespitosa is a slow growing species (~ 2.5 mm yr-1), with low recruitment and natural mortality rates (~ 0.30 recruits m-2 yr-1 and 1 %, respectively). Strikingly, the obtained results on the reproductive traits of this species differed significantly between Western Mediterranean and Adriatic Seas. Cladocora caespitosa is gonochoric in W Mediterranean, showing a water temperature-associated gonadal cycle that culminates at the end of the summer in contrast to the findings in the Adriatic, where the coral has described as hermaphroditic with the spawning occurring at the beginning of the summer.

Global change is rapidly altering Mediterranean marine habitats, primarily through warming and the invasion of new species. The C. caespitosa population in the Illa Grossa Bay suffered mortalities after 9 summers, separated into 2 mortality periods (2003 - 2006 and 2008 - 2012).

The highest necrosis rates were recorded during the first mortality period, after the exceptionally hot summer of 2003. Over 50 % of the area covered by C. caespitosa has suffered necrosis after these recurrent mortalities, which were significantly related to warming (summer warming trend: 0.06 °C yr-1). The differences in necrosis found after summers with similar thermal anomalies pointed out to the existence of other acting factors probably related to the interannual temperature context and delayed stress after extreme summers. These results show that while Cladocora caespitosa displays great ecological plasticity, mostly in relation to changing light conditions, it is not adapted to endure the extreme changes in temperature driven by climate change, the most worrying threat for this coral.

Regarding to the impact of invasive species, the invasive algae Lophocladia lallemandii and Caulerpa racemosa successfully spread over the Illa Grossa Bay from 2006 to 2012 and overlapped their distribution in the bay with that of C. caespitosa. No lethal effects of the invasive algae were detected on the coral colonies, which showed toxic activity. This may explain the low overgrowth of living colony parts by C. racemosa and the ability of this coral to compete in an algal dominated community.

In long-lived corals such as C. caespitosa, recovery from mortalities relies mostly on recruitment, but there are two main obstacles that may compromise recovery. Firstly, the high frequency of mortalities detected during the last decade probably exceed the recovery potential of the low recruitment rates. Secondly, both warming and invasive algae may have delayed and synergetic effects on reproduction, recruitment and juvenile survival.

All the results obtained highlight the endangerment of this species facing rapid environmental changes. Cladocora caespitosa is listed in the IUCN Red List of Threatened Species as Data Deficient. However, the information obtained in this PhD points out that this species could meet the criteria to fall into a threatened category.

Los ecosistemas marinos están siendo amenazados por un número cada vez mayor de impactos. Entre ellos, los impactos derivados del cambio global han puesto en relieve la necesidad de nuevas medidas de conservación que puedan complementar a las ya existentes. Sin embargo, su desarrollo requiere un amplio conocimiento de las especies, procesos e impactos implicados. Esta tesis se centra en la adquisición de nuevos conocimientos sobre la biología y ecología del coral Cladocora caespitosa con el objetivo de aportar información útil para su conservación. Los resultados obtenidos pueden servir como guía para la conservación de aquellas especies de características parecidas sobre las que no se tiene información. Esta especie es el único coral escleractinio, colonial y zooxantelado, endémico del Mediterráneo. Se trata de uno de los pocos ejemplos de especie longeva y estructural que se encuentra en las comunidades mediterráneas de aguas someras. La Bahía de L'Illa Grossa (Reserva Marina de las Islas Columbretes) alberga una de las poblaciones más importantes de este coral, con campos de colonias y arrecifes que muestran una superficie acumulada de 2900 m2. Los resultados obtenidos muestran que se trata de una especie de lento crecimiento, con bajas tasas de reclutamiento y mortalidad natural y que muestra importantes divergencias en sus características reproductoras entre el Mediterráneo Occidental y el Adriático. La población de C. caespitosa de Columbretes ha sufrido mortalidades recurrentes asociadas al calentamiento del agua, que han afectado a más de la mitad del área ocupada por este coral en la bahía. Por otra parte, las algas invasoras Lophocladia lallemandii y Caulerpa racemosa han invadido exitosamente la Bahía de L'Illa Grossa, solapando su distribución con la de C. caespitosa. De momento, no se han observado efectos letales de estas algas sobre el coral; sin embargo, se ha podido detectar la actividad tóxica de esta especie, que podría explicar el bajo recubrimiento de sus partes vivas por C. racemosa. El potencial de recuperación de esta población es limitado dado el bajo reclutamiento y la frecuencia e intensidad de las mortalidades. Además, no se pueden descartar efectos subletales del calentamiento y las algas invasoras sobre la reproducción, reclutamiento y la supervivencia de los juveniles. A partir de esta información esta especie podría cumplir los criterios de la UICN para ser catalogada como especie en peligro.