Seasonal patterns of settlement of introduced and native ascidians in bivalve cultures in the Ebro Delta (NE Iberian Peninsula)
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1
Centre d'Estudis Avançats de Blanes (CEAB), Spain
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2
University of Barcelona, Department of Genetics, Microbiology and Statistics, and IRBio, Spain
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3
Institute of Agriculture and Food Research and Technology (IRTA), Spain
Ascidians constitute a major group of introduced species around the world. They proliferate particularly in harbours, marinas, aquaculture facilities and other man-made structures. With some prominent exceptions, they remain restricted to these habitats, thus causing little harm to natural ecosystems. However, they can have an important economic impact, because they foul infrastructures and vessels, and because they reduce aquaculture yield.
The shellfish aquaculture industry is heavily affected by introduced ascidians worldwide. Nonindigenous ascidians overgrow bivalves, increasing weight and restricting access to food, and thus decrease shellfish productivity and increase handling times. Therefore, knowledge of the biological cycles of introduced species is crucial for their management, and particularly so in aquaculture settings where the seasonality of the activity can interact with the seasonality of the fouling species. Slight adjustments in seeding or harvesting times can greatly reduce the probability of getting heavy fouling on developing bivalves.
The Ebro Delta is one of the major centres of bivalve production in the W Mediterranean, producing over 4000 tons of mussels (Mytilus galloprovincialis) and over 200 tons of oysters (Crassostrea gigas). Recent proliferations of newly introduced ascidians species (Clavelina oblonga, Didemnum vexillum) have added to the presence of a suite of introduced and native species resulting in heavy overgrown of the bivalves. We have performed a monitoring of the settlement and growth cycles of the main ascidian species recruiting at the oyster cultures of the northern bay (Fangar Bay) of the Ebro Delta, by deploying settlement plates at different periods of the main reproductive season for most ascidians (spring-summer) and analysing the recruitment and growth of the main ascidian species settling on them.
The deployment was conducted from mid-May to mid-September in 2015. Each month, three ropes with three roughened PVC plates 20*20 cm in size were hung from the beams of oyster rafts. The plates were located at 0.2, 1, and 2 m of depth. The plates were subsequently monitored monthly, photographed at both sides, and the main ascidian species quantified in terms of surface cover by image analysis. Additional morphological and genetic information was gathered to verify the identification of the ascidian species. Our final goal was to provide meaningful advice for minimising losses due to ascidian overgrowth of bivalves.
The species that could be quantified on the pictures were the introduced forms Clavelina oblonga, Styela plicata, Ecteinascida turbinata and Clavelina lepadiformis (Atlantic form), and the native Trididemnum cereum, which nevertheless constitute a pest in oyster cultures.
The results showed in most species a significant effect of the date of deployment of the ropes, an effect that can last for long time periods. Thus, homogenization of the fouling communities is not achieved in the short to middle-term that is relevant for bivalve culture schedules. An effect of depth is also apparent, with some species preferring the deeper plates (e.g., E. turbinata), while others are more abundant in the shallower plates (e.g., S. plicata). Overall, there is a high seasonality in the abundance of the species, albeit with different patterns. The most significant fouler in the plates was Styela plicata, that could cover up to 100% of the plates’ surface. The succession of abundance peaks of different species makes it difficult to devise a seeding and harvesting schedule that minimises fouling, but seeding just after the time of settlement of main foulers can effectively avoid their proliferation in the oysters.
Acknowledgements
This research has been funded by project CHALLENGEN (CTM2013-48163) of the Spanish Government.
Keywords:
ascidians,
Introduced Species,
settlement,
Growth,
Bivalve culture
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Poster Presentation
Topic:
2. GLOBAL CHANGES, INVASIVE SPECIES AND CONSERVATION
Citation:
Casso
M,
Navarro
M,
Ordóñez
V,
Fernández-Tejedor
M,
Pascual
M and
Turon
X
(2016). Seasonal patterns of settlement of introduced and native ascidians in bivalve cultures in the Ebro Delta (NE Iberian Peninsula).
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00184
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Received:
29 Apr 2016;
Published Online:
03 Sep 2016.
*
Correspondence:
Dr. Xavier Turon, Centre d'Estudis Avançats de Blanes (CEAB), Blanes, Spain, xturon@ceab.csic.es