Can we save a marine species affected by a highly infective, highly lethal, waterborne disease from extinction?

Abstract

Anthropogenic drivers and global warming are altering the occurrence of infectious marine diseases, some of which produce mass mortalities with considerable ecosystemic and economic costs. The Mediterranean Sea is considered a laboratory to examine global processes, and the fan mussel Pinna nobilis a sentinel species within it. Since September 2016, fan mussels suffer a die-off, very likely provoked by the protozoan Haplosporidium pinnae. Population dynamic surveys, rescue programmes, larvae collector installation and protection of infected adults from predators, have increased knowledge about the factors conditioning the spread of the die-off; previous model simulations indicate that water temperature and salinity seem to be related to the manifestation of the disease, which at the end are strongly influenced by climate change and anthropogenic actions. The absence of natural recruitment implies that fan mussel populations are not recovering, but the survival of populations living in paralic environments provides an opportunity to study the disease and its conditioning factors. The present situation is proposed as an example of what is to come in the global context of climate change and poses several questions: are we the witnesses of the potential extinction of a sentinel species? Can we avoid the potential extinction of this species by applying active measures, and which measures will be more effective? How many other more overlooked species might experience a massive and unnoticed die-off before it is too late to implement any preservation action? For the extinction of community structure species will provoke unpredicted ecological cascade effects with global implications.

Recruitment Disruption and the Role of Unaffected Populations for Potential Recovery After the Pinna nobilis Mass Mortality Event

“…As previously reported in other studies (Catanese et al, 2018), the congeneric species P. rudis remains unaffected by the mortality and its recruits are still being recorded in many larval collectors and in the field (this study, García-March et al, 2020).…”

“…Observation of P. nobilis recruits was undertaken with the naked eye, allowing the detection of recruits of sizes down to 0.3 cm antero-posterior length. Recruits extracted from the collectors were either installed in aquaria (García-March et al, 2020;Vicente, 2020) or in growth cages in the field following Kersting and García-March (2017). It must be noted that at small sizes distinguishing P. nobilis from P. rudis juveniles can be difficult.…”

“…Because larval collectors are installed in shallow waters, they are highly exposed to storms and intentional or accidental removal; collectors at 8 sites were lost due to these causes during at least 1 recruitment season ( Table 1). All extracted recruits from MME affected sites that were kept in growth cages died presumably because of H. pinnae during the first months, as did the ones taken to aquaria according to García-March et al (2020).…”

“…The closest unaffected site, Delta del Ebro (∼100 km north of the Columbretes Islands), hosts one of the largest P. nobilis populations in the Mediterranean Sea (Prado et al, 2014). Pinna nobilis populations in this site have survived the MME apparently due to the low salinity conditions in the area, greatly influenced by agricultural discharges from rice cultivation (Prado et al, 2019;García-March et al, 2020) and this area has been already considered as a main source of P. nobilis larvae (Wesselmann et al, 2018). Surprisingly, even though P. nobilis populations in the Delta del Ebro remain mostly unaffected, collectors installed in this location have never recorded larval settlement, and according to Prado et al (2019) the local availability of planktonic larvae is very low.…”

“…The implications of the presence of different pathogens in the development of the disease has still to be clarified. Most known and studied P. nobilis populations in the Western Mediterranean Sea have almost completely disappeared, with mortality rates exceeding 90% (Vázquez-Luis et al, 2017;Panarese et al, 2019;García-March et al, 2020) and populations in the Eastern Mediterranean and Adriatic Sea are suffering the same fate (Katsanevakis et al, 2019;Kersting et al, 2019;Cižmek et al, 2020;Öndes et al, 2020;Özalp and Kersting, 2020). There is the exception, however, of several sites with very specific environmental settings where P. nobilis populations remain unaffected to date: Fangar Bay (Delta del Ebro, Spain), Mar Menor (Spain), Rhône Delta (France), Etang de Thau (France), Diana and Urbino pools (Corsica, France), Venice lagoon (Italy), inner Kalloni Gulf (Greece) (Catanese et al, 2018;Cabanellas-Reboredo et al, 2019;Kersting et al, 2019;Simide et al, 2019;Prado et al, 2019;Foulquie et al, 2020;Zotou et al, 2020).…”