Seagrass detritus as a food source for Pinna nobilis: implications for species ecology and ex-situ conservation

Technical Summary: Seagrass Detritus as a Food Source for Pinna nobilis

Problem Statement
Pinna nobilis, the largest marine bivalve in the Mediterranean, is currently classified as Critically Endangered following a mass mortality event (MME) that began in 2016. While restorative aquaculture and ex-situ conservation represent critical strategies to prevent extinction, these efforts are hindered by a lack of understanding regarding the species' trophic ecology. Previous captive breeding trials have consistently yielded growth rates significantly lower than those observed in the wild, suggesting that standard phytoplankton-based diets are nutritionally inadequate. Furthermore, the collection of mature broodstock from the wild poses risks to remaining fragile populations, necessitating the use of juveniles for stock establishment. However, the specific dietary requirements to sustain juvenile growth and development in captivity remain poorly defined, creating a bottleneck for effective conservation programs.

Methodology
The study employed a controlled feeding experiment within a Recirculating Aquaculture System (RAS) at the University of Rome Tor Vergata. Juvenile P. nobilis were collected as biofouling from mussel farms and divided into two size classes: small (150–190 mm) and large (190–220 mm). Specimens were housed individually in perforated pots with volcanic lapillus substrate to facilitate byssus attachment.

Three distinct dietary treatments were tested over a three-month period:

1. Live Microalgae (LM): A mixed culture of Isochrysis galbana, Phaeodactylum tricornutum, and Tetraselmis suecica.
2. Commercial Microalgae (CM): A concentrated commercial mix (Shellfish Diet 1800®) containing five marine microalgae species.
3. Seagrass Detritus (SD): Conditioned detritus derived from Posidonia oceanica leaves. The leaves were dried, ground, and sieved (<500 µm). Crucially, the detritus was suspended in seawater and enriched with urea to maintain a C:N ratio of 14–20, allowing for 15 days of conditioning to promote the colonization of heterotrophic bacteria before feeding.

All diets were administered to achieve a standardized target optical density (OD720 = 0.05 ± 0.01) to ensure ad libitum feeding conditions. Water quality parameters (temperature, salinity, pH, and nutrient levels) were strictly maintained within non-stressful ranges. Growth was monitored monthly by measuring shell width and estimating total shell length based on the unburied portion and a calculated buried length. Survivorship and reactivity were assessed daily.

Key Results

• Survivorship: The Live Microalgae (LM) diet resulted in generalized debilitation and mortality (3 deaths) within 20 days, leading to the discontinuation of this treatment. The Commercial Microalgae (CM) and Seagrass Detritus (SD) groups maintained 100% survivorship throughout the experiment.
• Growth Performance:
  • Small Size Class (150–190 mm): Specimens fed the SD diet achieved the highest mean monthly growth rate (8.00 ± 1.74 mm), which was significantly higher than the LM diet (1.59 ± 0.92 mm). The SD diet also showed a higher, though not statistically distinct, growth rate compared to the CM diet (6.67 ± 1.89 mm).
  • Large Size Class (190–220 mm): The SD diet again produced the highest growth rate (6.01 ± 3.73 mm), significantly outperforming both the LM (0.98 ± 0.67 mm) and CM (2.44 ± 2.15 mm) diets.
• Statistical Significance: A two-way ANOVA using the Aligned Rank Transform (ART) revealed significant main effects for both Diet and Size class, as well as a significant interaction between the two, indicating that the efficacy of the diet varied by size.

Key Contributions
This study provides the first experimental evidence supporting the hypothesis that seagrass detritus, specifically when conditioned by heterotrophic bacteria, constitutes a primary and highly effective food source for P. nobilis. The findings challenge the long-held assumption that this species relies primarily on phytoplankton. By demonstrating that conditioned detritus supports growth rates superior to optimized commercial microalgae diets, the research validates a direct trophic link mediated by microbial processing in seagrass meadows. Additionally, the study confirms the viability of using juveniles recovered from aquaculture facilities as a sustainable source for broodstock establishment, avoiding the ecological risks associated with wild collection.

Significance and Claims
The authors assert that these findings bridge the gap between ecological theory and applied conservation. The results suggest that P. nobilis derives a substantial portion of its energy from heterotrophic bacteria associated with detrital particles rather than the plant material itself. This has immediate practical implications for ex-situ conservation:

1. Optimized Rearing: The study offers a refined framework for formulating optimal diets for captive rearing, potentially enabling the successful maintenance of P. nobilis for multiple generations.
2. Ecological Understanding: The data supports the role of seagrass detritus transport and accumulation in sustaining populations in oligotrophic environments, explaining the species' persistence in specific habitats despite low phytoplankton productivity.
3. Conservation Strategy: By establishing a diet that mimics natural trophic pathways, restorative aquaculture programs can enhance growth performance and reduce dependence on wild populations, thereby supporting the long-term recovery of this iconic Mediterranean species.

The paper concludes that while phytoplankton may serve as a complementary resource during periods of detrital scarcity (e.g., summer), seagrass detritus is the superior dietary component for maximizing juvenile growth in captivity.