Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: A collaborative effort of scientists and citizens

Abstract

A mass mortality event is devastating the populations of the endemic bivalve Pinna nobilis in the Mediterranean Sea from early autumn 2016. A newly described Haplosporidian endoparasite (Haplosporidium pinnae) is the most probable cause of this ecological catastrophe placing one of the largest bivalves of the world on the brink of extinction. As a pivotal step towards Pinna nobilis conservation, this contribution combines scientists and citizens' data to address the fast- and vast-dispersion and prevalence outbreaks of the pathogen. Therefore, the potential role of currents on parasite expansion was addressed by means of drift simulations of virtual particles in a high-resolution regional currents model. A generalized additive model was implemented to test if environmental factors could modulate the infection of Pinna nobilis populations. The results strongly suggest that the parasite has probably dispersed regionally by surface currents, and that the disease expression seems to be closely related to temperatures above 13.5 °C and to a salinity range between 36.5-39.7 psu. The most likely spread of the disease along the Mediterranean basin associated with scattered survival spots and very few survivors (potentially resistant individuals), point to a challenging scenario for conservation of the emblematic Pinna nobilis, which will require fast and strategic management measures and should make use of the essential role citizen science projects can play.

Figure 1

Observations of P. nobilis health conducted by citizens (yellow circles) vs. scientists (blue circles), with zoomed in areas where most observations were done.

Figure 2

Disease observations based on the health status of P. nobilis (Dead/ill vs. alive, represented by red diamonds and green circles respectively) for the whole period from September 2016 to April 2018 (A). Spatio-temporal zooms of such observations are represented in panels B–F. Note that the crossed yellow point indicates the zone where the mortality was observed for the first time (Mass Mortality Event First Observation; MME_FOb), blue empty circles surround Healthy Populations (HP) and orange arrows denote Disease Observations Sequence (DOS).

Figure 3

Drift simulations: backward simulations in the Balearic Islands area (orange path), forward simulations at L’Ametlla de Mar area (blue path) and, forward simulations at the north Catalonia area (purple path). Note that points at the end of the paths indicate the particle final position. Simulations were inferred between point 1 (p₁; infected point) and point 2 (p₂; no signs of infection at the beginning of the simulation and infection at a known time interval of the simulation). The rectangle indicates 650 km² buffer zone around a given point.

Figure 4

Partial smoothed effect of Temperature (A) and Salinity (B) on the disease expression (note that zero marks the shift on the response; >0 corresponds to higher probability of disease absence, while <0 to higher probability of disease expression). Fitted lines (solid line), 95% confidence intervals (shaded area) and partial residuals (hollow points) are shown for univariate effects. Top and bottom tick marks on x-axis represent values of the observed covariable for the absence and presence of signs of infection respectively.