Can the Movement of the Deep-Sea Bivalve Acesta excavata Lead to a Dynamic Habitat?

1 of 7 Marine Ecology, 2025; 46:e70018 https://doi.org/10.1111/maec.70018 Marine Ecology SHORT COMMUNICATION OPEN ACCESS Can the Movement of the DeepSea Bivalve Acesta excavata Lead to a Dynamic Habitat? D.Sacco1,2 | P.Cardinale1 | S.P.Canese2,3 | F.Cardone2,3 | S.Greco2,4 | R.Danovaro1,3 1Department of Life and Environmental Sciences, Polytechnic University of Marche, Ancona, Italy | 2Stazione Zoologica Anton Dohrn, Naples, Italy | 3National Biodiversity Future Centre, Palermo, Italy | 4Università di Scienze Gastronomiche, Pollenzo,Italy Correspondence: D. Sacco ([email protected]) Received: 17 December 2024 | Revised: 7 March 2025 | Accepted: 23 March 2025 Funding: This work was supported by European Union—NextGenerationEU, Award Number: project code (CN_0000033). Keywords: Acesta excavata| bivalve movement| deepsea bivalves| Mediterranean Sea ABSTRACT Acesta excavata is one of the largest and ecologically relevant bivalves along continental margins and is often associated with coldwater coral assemblages of the upper bathyal zone. Like other habitatforming species, A. excavata contributes to increasing the secondary substrata and provides opportunities for the colonization and feeding of other sessile and mobile organisms. Despite most of the bivalves producing byssus being thought to be sessile or sedentary throughout their adult life stages, some species are known to be able to displace. Here we investigated, in mesocosm conditions, the ability of this deepsea species to move/displace and compared its mobility with that of other shallowwater species. We report here for the first time that A. excavata moves almost continuously, with a maximum speed of 6.5 cm day−1 (maximum weekly displacement of ca 28 cm), with average speeds of approximately 0.3–1.3 cm per day. This speed is the highest value reported so far for byssusattached bivalves (including Mytilus spp. and Pictada imbricata radiata). The movement of these bivalves, apparently due to the search for optimal feeding and substratum characteristics, can displace the habitat they create in response to changes in environmental and ecological conditions. These findings offer new opportunities for using this species in restoration protocols of deepsea habitats and change our view of deepsea hard bottoms from static to dynamic entities. 1 | Introduction The deep sea represents the largest biome on Earth, accounting for more than 65% of the Earth's surface, encompassing more than 95% of the global biosphere (Danovaro etal.2014). Deepsea habitats provide key ecosystem goods and services (Danovaro etal.2020) and hold a critical role in the functioning and buffering capability of the planet (Levin and Bris2015), but the current knowledge of the biology of deepsea species remains extremely limited (Snelgrove1999). Bivalves producing byssus both in shallow and deep ecosystems can generate aggregates that represent additional habitats characterized by biodiversity hotspots and rich in new substrata, protection, and food (Grabowski and Peterson 2007; Guillon etal.2017; HallSpencer and Moore2000; Johnson etal.2013). They allow the settlement of the larvae of several sessile and mobile epifaunal species (Grabowski and Peterson2007; Gutiérrez etal.2003), increase habitat complexity through the creation of interstices between their valves and byssal threads, where mobile fauna can seek refuge (Gutiérrez etal. 2003), and entrap This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2025 The Author(s). Marine Ecology published by Wiley-VCH GmbH. Cardinale P. contributed equally to this study. 2 of 7 Marine Ecology, 2025 sediments and organic matter that offer additional opportunities for species colonization (Calcagno etal. 2012). Investigating biological processes, such as growth, movement, physiology, and reproduction, often requires the combination of several approaches, including ex situ experimentation. The collection of live specimens and their ex situ maintenance could represent a challenge for deepsea organisms (Orejas and Jiménez 2019), but several species adapt easily to captivity (Lartaud etal.2014; Lunden etal.2014; Orejas etal.2011; Shillito etal.2015). This is also the case of the bivalve Acesta excavata (Fabricius 1779) which was able to live and grow for years in controlled conditions (pers. observation). The longterm survival of this bivalve in mesocosms allows the collection of data on the ecology and behavior of this species that are difficult to gather insitu. A. excavata is one of the most ecologically relevant deepsea bivalves inhabiting hard bottoms typically at bathyal depths (Correa etal.2005; Taviani etal.2019; Zibrowius and Taviani 2005). It is often associated with the coldwater corals Desmophyllum pertusum, Desmophyllum dianthus, and Madrepora oculata, contributing to the formation of vulnerable habitats (Taviani etal.2019; Zibrowius and Taviani2005). Deepsea cliffs are generally characterized by the presence of a high density of A. excavata and the oyster Neopycnodonte zibrowii (Johnson etal. 2013), which provide surface area for other benthic organisms. In particular, the shells of A. excavata, reaching a total length of around 20 cm, are often covered with serpulids, bryozoans, sponges, scleractinians, and oysters (Correa etal.2005). The biology, anatomy, and life strategies of bivalves largely depend on their life traits (Itchell Stanley1968). For instance, the locomotion strategies divide bivalves into several ecological groups: (i) byssusattached (i.e., bivalves producing byssal filaments for the anchoring to the substratum), (ii) borers producing holes in carbonate rocks (e.g., Lithophaga lithopahga) or wood materials; (iii) nesting (within a preexisting cavity), (iv) cemented (e.g., oysters that are attached by secreted shell material to a hard substratum), (v) swimming (e.g., Limidae such as Lima lima free living with selfpropulsion) (Itchell Stanley 1968). While some bivalves are known to swim (although for limited distances) and borers/cemented/nested are permanently sessile, byssusattached bivalves can show some degrees of movement (e.g., families Mytilidae and Margaritidae), although this is expected to occur almost exclusively during their early development (Martel and Chia 1991). Here we investigated the movement ecology of the deepsea bivalve A. excavata to explore an unexplored aspect of the biology of this ecologically important habitatforming species. 2 | Material and Methods 2.1 | Study Area and Sample Collection To conduct this study, specimens of A. excavata were collected in Dohrn Canyon from a vertical cliff at about 400 m depth, corresponding to a biodiversity hotspot where A. excavata coexists with the corals Madrepora oculata, Desmophyllum pertusum, Desmophyllum dianthus, and the deepsea oyster Neopycnodonte zibrowii. Approximately ten bivalves (see a picture of their distribution insitu; Figure1a) were collected (including both juveniles and adult specimens) by using a remotely operated vehicle (ROV) with a soft basket attached to the manipulator arm (Figure1b). The number of bivalves collected was kept low to minimize the impact on the insitu population. The sampling device consisted of a cylindrical stainlesssteel structure holding a 2 cm soft net, which allowed it to avoid any potential damage caused by the limited sensitivity of the arm itself. Once on board, the organisms were immediately placed in a thermally insulated container, connected to a Teco TK 1000 water chiller (Figure1c) to maintain a constant temperature of 14.5°C, and transported to the aquarium facility for acclimatization. 2.2 | Mesocosm Setup and Maintenance The mesocosm system consisted of a 700 L Martini cargo pallet tank and a collection tank containing the filtration systems. The filtration system consisted of a 50μ sock for mechanical filtration, 200 L of biological media, and a UV sterilizer. A Teco TK 3000 chiller was installed to maintain a constant temperature. The water cycle was as follows: the pump drew water from the reservoir into the chiller and then through the UV sterilizer before reaching the main tank outlets. An overflow drain returned the water to the collection tank for mechanical and biological filtration. The FIGURE 1 | Phases of the collection of the bivalve of A. excavata. Reported are: (a) a picture of Acesta insitu (Dohrn Canyon, Tyrrhenian Sea, Mediterranean); (b) ROV equipped with the collection basket; (c) refrigerated box for the transport of the specimens. 14390485, 2025, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maec.70018 by CochraneItalia, Wiley Online Library on [18/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 3 of 7 system received continuous water exchange for 10 h a day at a flow rate of 2 L per minute. Seawater was changed weekly changed and ensuring the supply directly from the sea. This Life Support System (LSS) allowed the highest standards of animal welfare to be achieved, whilst providing a dark environment that was easily accessible for feeding, cleaning, and monitoring operations. To provide vertical surfaces and attachment points for the organisms, a plastic box was placed in the center of the tank. Debris siphoning was carried out once a week in parallel with the monitoring of key water parameters. Insitu temperature (14.5°C), oxygen (8 mg/L) and salinity (38 PSU) conditions were replicated in the controlled environment and measured three times a week. Two adult bivalves of the same size class (10.8 ± 0.5 cm) were used for the experiment. Bivalves were fed daily with a broadspectrum experimental diet (Table1), including both cultured and prepared foods. The cultured component included microalgae (Tetraselmis), rotifers, and Artemia salina nauplii. The noncultured components included a 500μm filtered mixture of marine sauce (mixtures of mussels, shrimps and fish) and Shellfish Diet 1800 (prepared mixture of Isochrysis spp., Pavlova spp., Tetraselmis spp., Thalassiosira weissflogii and Thalassiosira pseudonana). The experiment lasted approximately 7 months (225 days) from September 2020 to April 2021, and data on bivalve movements were collected randomly with a ca. weekly frequency. 2.3 | Observations and Analysis of Bivalve Movements The individuals of A. excavata were carefully placed on the bottom of the aquarium on 1 September 2020. The first detectable movements were observed after one week, followed by weekly monitoring for 6 months, from September 2020 to April 2021. HD images of the moving bivalves were collected using a Nikon COOLPIX AW130 digital camera and a ruler to set the scale. The size and position of the serpulid worms occurring on the right valves were used to identify each specimen (Figure 2). Images were processed using ImageJ to track displacement and direction of movement using the umbo as a reference point; graphical analyses were performed using Prism Graph Pad 8. 3 | Results Visual observations indicate that in A. excavata the byssus can be secreted, utilized, and discarded in a sequential process to actively pull the bivalve and determine its displacement from one substratum to another. Both horizontal, vertical, and rotational displacements were observed (see Additional file1). All specimens showed a recurrent pattern for both vertical and horizontal displacement which is reported as follows: (i) the foot extends from the anterior side and directs the deposition of a new byssus; (ii) the outer part of the byssus, consisting of a few filaments (usually five), anchors the animal to the substratum; (iii) the newly deposed byssus is used to pull the shell, while the old byssus remains anchored and is stretched in the direction of movement; and finally (iv) the old byssus is detached and left behind (Figure3). Once the specimens were left on the bottom of the tank, they began to move in the horizontal plane until they encountered a vertical surface. Individuals showed two alternative approaches: (i) wall climbing and continuous movement; (ii) short distance movement to then settle for a longer time in a suitable area (e.g., the corner of a plastic box over a vertical surface). TABLE 1 | Composition of the experimental diet administered to A. excavata individuals throughout the maintenance period. Food Type Quantity (ml) [C] (ind/ ml*—cell/ ml**) Tetraselmis suecica Cultured 500 1–2 x 106** Brachionus plicatilis Cultured 500 20* Artemia salina (nauplii) Cultured 500 10* Mix marine flesh Prepared 500 ND Shellfish Diet 1800 Prepared 10 ND Note: The asterisks referred to the units of measurement related to each food. (*) stands for ind/ml and (**) stands for cell/ml. FIGURE 2 | Specimens of A. excavata used in this study and their distinguishing features (serpulid worm remains on the right valves). 14390485, 2025, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maec.70018 by CochraneItalia, Wiley Online Library on [18/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 4 of 7 Marine Ecology, 2025 Daily measurements provided evidence also of the rotational ability: A. excavata can rotate for more than 180° in approximately 4 h using exclusively its foot (Figure4). During the entire experimental period, the most dynamic specimen covered a total distance of 230 cm with an average speed of 1.3 cm per day−1. The least dynamic specimen moved for a total of 76 cm, with an average speed of 0.3 cm per day−1. Considering net displacements, the speed was 1.8 ± 1.5 cm per day−1 and 1.1 ± 1.0 cm per day−1 for the two specimens, respectively. The maximum velocity was recorded during horizontal displacements with 6.5 cm per day−1 (Figure5). 4 | Discussion Data reported in this study document, for the first time, that the adults of the large bivalve A. excavata actively move by using their byssal threads, thus revealing that the byssus of this bivalve is not used only as a tool to remain firmly attached to the substratum. Our observations also provided evidence of the mechanism through which the byssus is used: (i) the byssus is secreted and attached to various parts of the substratum, then part of the byssus is discarded, and (ii) the animal actively pulls through some attached filaments allowing its movements. Available information indicates that the oyster Pinctada imbricata radiata (family Margaritidae) can move with a maximum displacement of 3.1 cm day−1 (Giraldes etal.2019), and similar values were reported for adult mussels (Mytilus spp.) (Schneider et al. 2005). In addition, laboratory experiments have shown that mussels tend to orient to intercept the prevailing water flow, thereby exposing a smaller surface area and reducing the hydrodynamic force (Zardi etal.2006). The movements of A. excavata are almost continuous and, together with the ability to rotate on the substratum using the foot, allow this bivalve to find the best attachment position. In FIGURE 3 | The sequence of displacement of A. excavata highlights the deposition of new byssus filaments (red lines) and the stretching of old filaments (white lines) in the direction of movement. FIGURE 4 | Photographic sequence of the rotation of the A. excavata over the substratum carried out using its foot. 14390485, 2025, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maec.70018 by CochraneItalia, Wiley Online Library on [18/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 5 of 7 fact, due to the high spatial heterogeneity of hard substrata and surrounding water flows, even smallscale displacements can result in relevant changes in the environmental characteristics to which the bivalves are exposed. Such changes may play an important role in offering better feeding and growth conditions, thus ultimately regulating population fitness. Although the specific factors triggering the movement in A. excavata remain unknown, visual observations carried out in mesocosm conditions indicate that the most likely factor triggering the bivalve displacement is represented by the search for optimal water flow conditions associated with adequate protection (pers. observ.). It is known that bivalves in general have a wide range of sensory structures, including simple and complex organs (Audino etal.2015; Haszprunar1985), but little information is currently available on the sensory system of A. excavata and its perception of environmental stimuli (Järnegren and Altin2006). We know that this bivalve possesses structures along the mantle margin that act as mechanoreceptors, chemoreceptors, or photoreceptors (Audino etal.2015). Acesta, like other members of the Limidae family, shows long tentacles and eyes on the mantle margin (Järnegren and Altin2006). In the pectinid bivalve Limaria hians, these tentacles are thought to contain structures acting as multidirectional mechanoreceptors, vibration receptors, and/or chemoreceptors (Owen etal.1997). Another issue to consider is the fact that all measures are obtained in controlled conditions (mesocosms), which, although recreates most of the ambient conditions, do not reflect perfectly the insitu conditions (e.g., pressure). Therefore, we assume that the data collected in captivity might reflect those observed in natural conditions, which, of course, might vary from those observed experimentally. Nonetheless, the result obtained offers evidence of the speed and range of displacement that is possible for this bivalve. Moreover, these measurements allow a comparison to be carried out with other species whose movements were observed in similar controlled conditions. During the ca. 7 months of the experiment, the longest track of a single bivalve was ca. 2.3 m with an average speed of ca. 1.0 cm day−1. The maximum speed (6.5 cm day−1) was observed during horizontal displacements (see supplementary information). These values are higher than any others reported so far, including those measured for common mussels at shallow depths (< 25 cm month−1) (Nicastro etal.2008; Schneider etal.2005; Zardi etal.2006). Although the data available for other bivalve species are too limited to draw general conclusions, it might be hypothesized that bivalves in the deep sea might require important movements to increase their fitness to local conditions. The dominant direction of the movements of the bivalve A. excavata once released on the bottom of the mesocosm was vertical, thus leading to the hypothesis that the bivalve was in search of vertical substrata offering the optimal distance from the bottom to intercept the strongest currents. A decrease in speed and displacement was observed after the bivalve reached a position on a vertical wall, yet small movements and rotatory adjustments were performed even after vertical walls, suggesting a continuous search of the optimal water flow. The active displacement capacity of byssusattached species has been so far largely overlooked. The present study on A. excavata could provide important insights for understanding the functional responses of this species to abiotic and biotic drivers. This information can also be useful for the attempts to restore deepsea habitats created by A. excavata or coldwater coral habitats with which this bivalve can cooperate as observed for other bivalves in vegetated habitats (Gagnon etal.2020). These findings also indicate that we should revise our view of deepsea hard bottoms and related megafaunal habitats as static entities, as those created by this bivalve could displace significantly over time. Author Contributions Danovaro R. and Sacco D. planned the experiments. Sacco D., Cardone F. and Canese S. P. collected the organism; Sacco D. and Cardone F. arranged the experiments in the tanks; Sacco D. performed the experiment and photographic data collection; Sacco D. and Cardinale P. analyzed the data and wrote the manuscript. All authors edited the manuscript. Danovaro R. reviewed the manuscript. FIGURE 5 | Range of observed displacement (a) and speed (b) of A. excavata specimens during the monitoring period. 14390485, 2025, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maec.70018 by CochraneItalia, Wiley Online Library on [18/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 6 of 7 Marine Ecology, 2025 Acknowledgments This study was supported by the REDRESS project (Horizon Europe, Grant Agreement 101135492) and the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4—Call for tender no. 3138 of 16 December 2021, rectified by Decree no. 3175 of 18 December 2021 by the Italian Ministry of University and Research, funded by the European Union—NextGenerationEU, Award Number: project code CN_00000033, Concession Decree No. 1034, of 17 June 2022, adopted by the Italian Ministry of University and Research, Project title “National Biodiversity Future Center—NBFC”. Open access publishing facilitated by Universita Politecnica delle Marche, as part of the Wiley - CRUI-CARE agreement. 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Porri. 2006. “Hydrodynamic Stress and Habitat Partitioning Between Indigenous (Perna perna) and Invasive (Mytilus galloprovincialis) Mussels: Constraints of an Evolutionary Strategy.” Marine Biology 150: 79–88. Zibrowius, H., and M. Taviani. 2005. “Remarkable Sessile Fauna Associated With Deep Coral and Other Calcareous Substrates in the Strait of Sicily, Mediterranean Sea.” In ColdWater Corals and Ecosystems, 807–819. Springer. Supporting Information Additional supporting information can be found online in the Supporting Information section. 14390485, 2025, 3, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/maec.70018 by CochraneItalia, Wiley Online Library on [18/11/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License