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Simple Summary: This study addressed the pressing issue of declining giant clam populations due to environmental changes in their habitats. Our findings underscore the critical role of environmental factors in shaping clam populations and emphasize the importance of understanding these dynamics for effective conservation strategies. This study contributed valuable insights to the intricacies of giant clam ecology, offering a scientific foundation for informed conservation efforts aimed at preserving these vital marine species and their ecosystems. The habitats of giant clams are undergoing environmental changes, and giant clam populations are declining. The present study was conducted to facilitate clam conservation. We conducted three 18-week trials to investigate the effects of nutrient, temperature, and salinity on the growth performance and survival rates (SRs) of juvenile Tridacna noae, adult Tridacna crocea, and subadult Tridacna derasa, respectively. Regarding nutrient sources, no significant differences were observed in shell length gain, specific growth rate, or SR between clams fed with Chaetoceros muelleri or commercial feed (hw nanotip) and those in a control group (juvenile phototrophs). Regarding temperature, clams cultivated at 27 °C exhibited significantly better growth performance and SR than did those cultivated at 19 °C or 31 °C (p < 0.05). By week 6, all clams in the 19 °C and 31 °C groups had died, indicating that suboptimal growth temperatures have severe adverse effects. Regarding salinity, clams cultivated at 34‰ exhibited significantly higher length gains and specific growth rates than did those cultivated at 20‰ or 25‰ (p < 0.05). SR was not significantly affected by salinity. Understanding how environmental factors affect giant clam populations may help researchers devise effective clam conservation strategies. [ABSTRACT FROM AUTHOR]

Despite the ecological importance of giant clams (Tridacninae), their effective management and conservation is challenging due to their widespread distribution and labour-intensive monitoring methods. In this study, we present an alternative approach to detecting and mapping clam density at Pioneer Bay on Goolboddi (Orpheus) Island on the Great Barrier Reef using drone data with a combination of deep learning tools and a geographic information system (GIS). We trained and evaluated 11 models using YOLOv5 (You Only Look Once, version 5) with varying numbers of input image tiles and augmentations (mean average precision—mAP: 63–83%). We incorporated the Slicing Aided Hyper Inference (SAHI) library to detect clams across orthomosaics, eliminating duplicate counts of clams straddling multiple tiles, and further, applied our models in three other geographic locations on the Great Barrier Reef, demonstrating transferability. Finally, by linking detections with their original geographic coordinates, we illustrate the workflow required to quantify animal densities, mapping up to seven clams per square meter in Pioneer Bay. Our workflow brings together several otherwise disparate steps to create an end-to-end approach for detecting and mapping animals with aerial drones. This provides ecologists and conservationists with actionable and clear quantitative and visual insights from drone mapping data.

We present a chromosomal-level genome assembly from an individual Tridacna gigas (the giant clam; Mollusca; Bivalvia; Veneroida; Cardiidae). The genome sequence is 1,175.9 megabases in span. Most of the assembly is scaffolded into 17 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 25.34 kilobases in length. Gene annotation of this assembly on Ensembl identified 18,177 protein coding genes.

Giant clams are common across a broad geographic range and contribute important ecological functions within coral reef environments. However, giant clams are subject to considerable harvest pressure and require careful management that is underpinned by accurate data collection. The taxonomy of giant clams has undergone many changes, and recently, Tridacna noae (Röding 1798) has been resurrected as a valid species, distinct from the morphologically similar Tridacna maxima (Röding 1798). Using genetic analysis, this research confirms the presence of T. noae for the first time in the Cook Islands, extending the currently known distribution of the species by 1340 km south-east. This confirmation highlights that T. noae was possibly previously misidentified, causing overestimations of the abundance of other giant clam species. This new record improves the accuracy of identification and stock assessments, and ongoing management in the Cook Islands. [ABSTRACT FROM AUTHOR]

To properly assess risk, an animal must focus its attention on relevant external stimuli; however, attention can be reallocated when distracting stimuli are present. This reallocation of attention may interfere with an individual's ability to effectively assess risk and may impede its response. Multiple stimuli presented together can have additive effects as distractors, and these include stimuli in different modalities. Although changes in noise and water flow are detectable by some bivalves, this has not been studied in the context of risk assessment or distraction. We experimentally exposed giant clams (Tridacna maxima) to changes in water particle movement through underwater sound (motorboat noise) and increased water flow to determine whether these stimuli, individually or together, modified risk assessment or caused distraction. We found that clams responded to sound, flow, and their combination by increasing frequency of mantle retractions (a potential anti-predator response) when exposed to a stimulus. Sound alone did not change risk assessment in either the latency to close or to reemerge following closure. However, when exposed to both stimuli simultaneously, clams increased their latency to close. We suggest that clams perceive sound and flow in an additive way, and are thus distracted. Interestingly, and uniquely, clams discriminate these multimodal stimuli through a single sensory modality. For sessile clams, anthropogenic noise is detectable, yet unavoidable, suggesting that they be especially vulnerable to marine noise pollution.

The habitats of giant clams are undergoing environmental changes, and giant clam populations are declining. The present study was conducted to facilitate clam conservation. We conducted three 18-week trials to investigate the effects of nutrient, temperature, and salinity on the growth performance and survival rates (SRs) of juvenile Tridacna noae, adult Tridacna crocea, and subadult Tridacna derasa, respectively. Regarding nutrient sources, no significant differences were observed in shell length gain, specific growth rate, or SR between clams fed with Chaetoceros muelleri or commercial feed (hw nanotip) and those in a control group (juvenile phototrophs). Regarding temperature, clams cultivated at 27 °C exhibited significantly better growth performance and SR than did those cultivated at 19 °C or 31 °C (p < 0.05). By week 6, all clams in the 19 °C and 31 °C groups had died, indicating that suboptimal growth temperatures have severe adverse effects. Regarding salinity, clams cultivated at 34‰ exhibited significantly higher length gains and specific growth rates than did those cultivated at 20‰ or 25‰ (p < 0.05). SR was not significantly affected by salinity. Understanding how environmental factors affect giant clam populations may help researchers devise effective clam conservation strategies.

With the increasing demand for giant clams in the ornamental trade as well as habitat destruction of coral reefs, giant clam populations have been threatened across the Indo‐Pacific. This study documents the population status and genetic diversity of giant clams in Perhentian Islands Marine Park (PMP), a Marine Protected Area (MPA) on the east coast of Peninsular Malaysia, plus Rhu Island, an adjacent island outside the MPA.Of the 13 reef sites surveyed across an area of 11,200 m2, two giant clam species were recorded: Tridacna squamosa and Tridacna maxima, with average densities of 1.5 ± 2.2 and 5.2 ± 6.0 ind. 100 m−2, respectively. The size‐class survey revealed a higher number of T. maxima recruits (88 recruits) as compared to T. squamosa (only three recruits), suggesting a disparity in recruitment in the area.The genetic diversity of T. squamosa (n = 83) and T. maxima (n = 104) was explored using the mitochondrial cytochrome c oxidase I (COI) and 16S rRNA gene markers. Interestingly, a higher genetic diversity was detected in COI than 16S for both species. No significant genetic differentiation was detected between the populations of PMP and Rhu Island, while a low but significant genetic structure was detected in both species across the sites of PMP (COI datasets, AMOVA, T. squamosa, FCT = 0.14, P < 0.05; T. maxima, FCT = 0.11, P < 0.05).In general, the results of this study revealed healthy giant clam populations in PMP, but the decline warrants urgent attention to integrating conservation strategies such as restoration programmes in conjunction with a sustainable giant clam fishery. Given the relatively high genetic diversity of T. maxima at Rhu Island, expansion of the current MPA is needed for better conservation coverage. [ABSTRACT FROM AUTHOR]

Metabolite profiling of marine invertebrates, such as bivalve mollusks, may not only provide insights into the health state of an individual holobiont, but also the pollution levels of their environment Here, we combined 1H nuclear magnetic responance (NMR) spectroscopy and mass spectrometry (MS)-based metabolomics techniques to investigate the organ-specific metabolomic profiles of Tridacna maxima giant clams. Clams were collected from across-shelf gradient in the Red Sea, from inshore to off-shore. We unequivocally profiled 306 metabolites and observed that the sampling location had minimal effects on metabolite composition. However, we observed significant differences in metabolite profiles among different organs (i.e., gills, mantle organ, and digestive system). Importantly, in addition to endogenous metabolites, we detected the presence of terephthalic acid and isophthalic acid, which likely originate from marine plastic ingestion. Collectively, our study opens opportunities for a deeper understanding of Tridacna maxima physiology through metabolomics, and illustrates the power of invertebrate metabolite profiling for monitoring plastic-related aquatic pollutants.

Fisheries science often uses population models that assume no external recruitment, but nearshore marine populations harvested on small scales of 0.775 (equivalent to 52.85% self-recruitment under present conditions). Elasticity analysis of demographic parameters indicated that future abundance can most effectively be influenced by increasing survival of mid-sized clams (∼80–120 mm) if the population is self-sustaining, and by increasing survival of juvenile clams (∼40–70 mm) if the population is non-self-sustaining (as for an open population). Our results illustrate that management priorities can vary depending on the amount of self-recruitment in a local population.

Giant clams (Subfamily Tridacninae), are important members of Indo-Pacific coral reefs, playing multiple roles in the framework of these communities. Although they are prominent species in Red Sea reefs, data on their distribution and densities in the region are scarce. The present study provides the first large-scale survey of Red Sea Tridacna spp. densities, where we examined a large proportion of the Saudi Arabian Red Sea coast (1,300 km; from 18° to 29°N). Overall, Tridacninae were found at densities of 0.19 ± 0.43 individuals m–2 (±SD). Out of the total 4,002 observed clams, the majority (89%) were Tridacna maxima, with 0.17 ± 0.37 individuals m–2, while only 11% were Tridacna squamosa clams with 0.02 ± 0.07 individuals m–2. We also report on a few (total 6) Tridacna squamosina specimens, found at a single reef. We identified different geographical parameters (i.e., latitude and distance to shore) and local environmental factors (i.e., depth and reef zone) as the main drivers for local Tridacna spp. densities. Our results show that the drivers influencing the densities of Red Sea giant clams are complex due to their co-occurrence and that this complexity might explain the high variation in Tridacninae abundances across the Indo-Pacific, but also within a given reef. We also estimate that giant clam calcification likely contributes to an average of 0.7%, but potentially up to 9%, of the overall mean calcium carbonate budget of Red Sea coral reef communities.

To properly assess risk, an animal must focus its attention on relevant external stimuli; however, attention can be reallocated when distracting stimuli are present. This reallocation of attention may interfere with an individual’s ability to effectively assess risk and may impede its response. Multiple stimuli presented together can have additive effects as distractors, and these include stimuli in different modalities. Although changes in noise and water flow are detectable by some bivalves, this has not been studied in the context of risk assessment or distraction. We experimentally exposed giant clams (Tridacna maxima) to changes in water particle movement through underwater sound (motorboat noise) and increased water flow to determine whether these stimuli, individually or together, modified risk assessment or caused distraction. We found that clams responded to sound, flow, and their combination by increasing frequency of mantle retractions (a potential anti-predator response) when exposed to a stimulus. Sound alone did not change risk assessment in either the latency to close or to reemerge following closure. However, when exposed to both stimuli simultaneously, clams increased their latency to close. We suggest that clams perceive sound and flow in an additive way, and are thus distracted. Interestingly, and uniquely, clams discriminate these multimodal stimuli through a single sensory modality. For sessile clams, anthropogenic noise is detectable, yet unavoidable, suggesting that they be especially vulnerable to marine noise pollution.

Iridocytes, containing multiple stacks of proteinaceous platelets and crystalized guanine, alternating with thin cytoplasm sheets, are specialized cells that act as multilayer nano-reflectors. Convergence evolution led to their arising across a broad range of organisms, including giant clams of the Tridacninae subfamily – the only sessile and photosymbiotic organism, among animals known to possess iridocytes. Through the interference of light with their nanoscale architecture, iridocytes generate “structural colors,” which are reported to serve different purposes, from intra-species communication to camouflage. In giant clams, iridocytes were previously reported to promote a lateral- and forward scattering of photosynthetically productive radiation (PAR) into the clam tissue, as well as the back reflection of non-productive wavelengths. Hence, they are assumed to promote an increased efficiency in the use of available solar energy, while simultaneously preventing photodamage of the algal symbionts. We report the use of guanine crystals within Tridacna maxima giant clam iridocytes as a basis for photonic cooperation between the bivalve host and their photosynthetic symbionts. Our results suggest that, in addition to the previously described scattering processes, iridocytes absorb potentially damaging UV radiation (UVR) and, through successive emission, emit light at longer wavelengths, which is then absorbed by the photosynthetic pigments of the algal symbionts. Consequently, both, host and algal symbionts are sheltered from (potentially) damaging UVR, while the available solar energy within the PAR spectrum increases, thereby potentially enhancing photosynthetic and calcification rates in this large bivalve. Further, our results suggest that this photonic cooperation could be responsible for the broad repertoire of colors that characterizes the highly diverse mantle patterns found in T. maxima.

Giant clams (subfamily Tridacninae) are prevalent members of coral reef communities and engage in symbioses with algal photosymbionts of the family Symbiodiniaceae, similar to their scleractinian coral counterparts. However, we know little about their associated bacterial microbiome members. Here, we explored bacterial community diversity of digestive system, gill, and mantle tissues associated with the giant clam Tridacna maxima across a cross-shelf gradient (inshore, midshore, and offshore reef sites) in the central Red Sea using 16S rRNA gene amplicon sequencing. Different tissues harbor spatially stable and distinct microbial communities. Notably, diverse assemblages of bacteria affiliated to the family Endozoicomonadaceae were prevalent in all tissues, but particularly abundant in gills and to a lesser extent in digestive tissues. Besides Endozoicomonadaceae, bacteria in the families Pasteurellaceae, Alteromonadaceae, and Comamonadaceae were common associates, depending on the tissue queried. Taxonomy-based functional inference identified processes related to nitrogen cycling (among others) to be enriched in giant clam tissues and contributed by Endozoicomonadaceae. Our study highlights the tissue-specificity and broad taxonomic range of Endozoicomonadaceae associates, similar to other marine invertebrates, and suggests their contribution to nitrogen-related pathways. The investigation of bivalve-associated microbiome communities provides an important addition to the pathogen-focused studies for commercially important bivalves (e.g., oysters).

We present a chromosomal-level genome assembly from an individual Tridacna gigas (the giant clam; Mollusca; Bivalvia; Veneroida; Cardiidae). The genome sequence is 1,175.9 megabases in span. Most of the assembly is scaffolded into 17 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 25.34 kilobases in length. Gene annotation of this assembly on Ensembl identified 18,177 protein coding genes., Competing Interests: No competing interests were disclosed., (Copyright: © 2024 Li R et al.)

Giant clams (subfamily Tridacninae) are prevalent members of coral reef communities and engage in symbioses with algal photosymbionts of the family Symbiodiniaceae, similar to their scleractinian coral counterparts. However, we know little about their associated bacterial microbiome members. Here, we explored bacterial community diversity of digestive system, gill, and mantle tissues associated with the giant clam Tridacna maxima across a cross-shelf gradient (inshore, midshore, and offshore reef sites) in the central Red Sea using 16S rRNA gene amplicon sequencing. Different tissues harbor spatially stable and distinct microbial communities. Notably, diverse assemblages of bacteria affiliated to the family Endozoicomonadaceae were prevalent in all tissues, but particularly abundant in gills and to a lesser extent in digestive tissues. Besides Endozoicomonadaceae, bacteria in the families Pasteurellaceae, Alteromonadaceae, and Comamonadaceae were common associates, depending on the tissue queried. Taxonomy-based functional inference identified processes related to nitrogen cycling (among others) to be enriched in giant clam tissues and contributed by Endozoicomonadaceae. Our study highlights the tissue-specificity and broad taxonomic range of Endozoicomonadaceae associates, similar to other marine invertebrates, and suggests their contribution to nitrogen-related pathways. The investigation of bivalve-associated microbiome communities provides an important addition to the pathogen-focused studies for commercially important bivalves (e.g., oysters). [ABSTRACT FROM AUTHOR]

This study assesses for the first time the ingestion of microplastics by giant clams and evaluates their importance as a sink for this pollutant. A total of 24 individuals of two size classes were collected from the Red Sea and then exposed for 12 days to 4 concentrations of polyethylene microbeads ranging from 53 to 500 μm. Experiments revealed that clams actively take up microplastic from the water column and the average of beads retained inside the animal was ∼7.55 ± 1.89 beads individual −1 day −1 (5.76 ± 1.16 MPs/g dw). However, the digestive tract itself cannot be considered the only sink of microbeads in Tridacnids. Indeed, shells play a key role as well. The abundance of microplastic adhering to the shells, which was estimated directly, was positively correlated to the concentration of beads found in the surrounding seawater. Therefore, clams' shells contribute to the removal of 66.03 ± 2.50% of the microplastic present in the water column. Furthermore, stress responses to the exposure to polyethylene were investigated. Gross Primary Production:Respiration (GPP:R) ratio decreased throughout of the experiment, but no significant difference was found between treatments and controls. Image 1 • Removal of plastic from the water by giant clams is assessed for the first time. • We identified two removal processes: active (ingestion) and passive (adhesion). • Passive removal is 35-fold higher that active removal. • Adhesion contributes to remove > 60% of plastic from the water column. • Removal rate depends on the concentration of plastic. Giant clams remove almost 70% of plastic from the water column mainly through a passive removal process of plastic adhesion to the shell. [ABSTRACT FROM AUTHOR]

This study is a first assessment of Metallic Trace Elements (MTE) contamination of four lagoons in French Polynesia, chosen because their main anthropic uses are potential sources of MTE (military, agriculture, maritime). We used the giant clam Tridacna maxima as a biointegrator of fifteen MTE to assess the environmental footprint of human activities. The study of organotropism showed that not only the kidney but also the gonad and digestive system (GDS) are accumulator organs of interest for ETM biomonitoring. This result confirms that heterotrophic feeding is the pathway of ETM accumulation. Here, T. maxima appeared to be a super-accumulator of Ni (2700 ppm.dw), but also revealed at Tubuai (As, Fe, Mn) and Arutua (Cd, Zn) the environmental footprint of agricultural and maritime activities. The concentration of Cd recorded in Arutua (5 ppm.dw) both in KID and GDS, also underline the need for better control of effluents resulting from human activities. • T. maxima is a biointegrator of metallic trace elements contamination. • Kidney but also gonad and digestive system are accumulator organs of interest. • T. maxima is a super-accumulator of Ni. • T. maxima can be used to monitor the environmental footprint of human activities. [ABSTRACT FROM AUTHOR]

Tridacna gigas, the world's largest living bivalve, could once be found on shallow coral reefs across the Indo-Pacific Region where its presence can be traced back to around 10 million years. However, following several local extinctions throughout its geographic range, its SCS population appears to have also been the subject of unprecedented overharvesting. This article discusses the obligations of littoral States of the SCS to protect and preserve this threatened resource and its habitat, to cooperate and consult with each other to that effect and act with due diligence. Given these obligations and the risk of irreversible harm and species' extinction in the SCS, it proposes that littoral States should proceed to list the SCS population of T. gigas in the CITES Appendix I, impose an immediate ban on further harvesting pending (re-)assessment of the population, and adopt a management plan to ensure its survival and sustainable use. [ABSTRACT FROM AUTHOR]

Abstract: To validate the usability of the giant clam shell as a recorder of short‐term environmental changes such as typhoons, we collected a live Tridacna maxima from Okinotori Island, Japan, on 15 June 2006. Growth increment thickness, stable isotope ratio (δ18Oshell, δ13Cshell), and the barium/calcium ratio (Ba/Ca) in the T. maxima shell sample were measured and compared to Okinotori Island instrumental environmental data. In the outer layer of the shell sample, there were 365 ± 6 growth increments per year, as estimated by the δ18Oshell profile compared with sea surface temperature. The growth increments in the specimen were formed daily, and thus, we can determine the date of the sampling points of δ18Oshell, δ13Cshell and the Ba/Ca ratio by counting growth increments. After typhoon approach, there is a decrease in increment thickness and some disturbed growth increments. The positive peaks in the shell Ba/Ca ratio and δ18Oshell corresponded to lower sea surface temperature caused by typhoons. These results indicated that the microstructural and geochemical record in Tridacna maxima shells could be useful for detecting past typhoon events. [ABSTRACT FROM AUTHOR]

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The cyanobacteriosponge Terpios hoshinota has been reported throughout the Indo-Pacific including the Great Barrier Reef, Australia. The species encrusts live coral, giant clams, and other benthos and can be a threat to benthic communities on coral reefs. The Kimberley region of Western Australia has some of the least impacted reefs globally. We report for the first time the presence of T. hoshinota in the eastern Indian Ocean on Kimberley inshore coral reefs. Given its invasive potential, reef health surveys should include this species, and monitoring approaches developed to audit the remote Kimberley for this and other invasive species.

The biodiversity of the Red Sea remains relatively understudied, particularly for invertebrate taxa. Documenting present patterns of biodiversity is essential for better understanding Red Sea reef ecosystems and how these ecosystems may be impacted by stressors (such as fishing and climate change). Several species of giant clams (genus Tridacna) are reported from the Red Sea, although the majority of research effort has occurred in the Gulf of Aqaba. We investigated the genetic diversity (16S rDNA) of the Tridacna species found in the central Saudi Arabian Red Sea. We also investigated the genetic diversity (ITS rDNA) of symbiotic dinoflagellates Symbiodinium associated with these clams. Samples were collected from nine reefs on a cross-shelf gradient near Thuwal, Saudi Arabia. Two species, T. squamosa and T. maxima, were recorded, with the latter being the most abundant. Tridacna squamosina, a species recently reported in the northern Red Sea, was not found, suggesting that this species is not present or is very rare in our study region. All tridacnids sampled were found to harbor Symbiodinium grouped in Clade A, considered an opportunistic, heat-tolerant symbiont group in anemones and corals. The consistent association with Clade A Symbiodinium in central Red Sea tridacnids may reflect the consequence of adaptation to the relatively extreme conditions of the Red Sea. This study contributes to an ever-growing catalog of Red Sea biodiversity and serves as important baseline information for a region experiencing dynamic pressures. [ABSTRACT FROM AUTHOR]

Giant clams are in symbiotic relationship with the photosynthesizing zooxanthellae and dwell in the shallow coral reefs of the Indo-Pacific region where they may be exposed to fluctuating salinities during high levels of precipitation. This study evaluated the effects of reduced salinities (18, 25, and 35‰ as control) on the rates of photosynthesis and respiration in juvenileTridacna gigasfor 14 days. At 18‰, total mortality was recorded after day 4, so no photosynthetic measurements were subsequently conducted for this treatment. However, results showed no significant differences in the photosynthetic rates among treatments. Respiration rates were significantly increased at both low salinities resulting in low Pg/R ratios. After 14 days of exposure, photosynthetic parameters at 25‰ were not significantly different from the control which suggests that juvenileT. gigasmay be able to acclimate to osmotic stress and adjust its photophysiology following reductions in salinity of up to 25‰. [ABSTRACT FROM PUBLISHER]

Dinoflagellates in the genus Symbiodinium (zooxanthellae) provide the photosynthesis that sustains the majority of primary production in coral reefs. They occur symbiotically with several phyla, including mollusks such as giant clams ( Tridacna spp.). This mutualistic association is obligatory for the giant clams, but the exact point in which this symbiosis is established and the main translocated photosynthate are unknown. In this study, we tracked the expression of specific genes for symbiosis and glycerol synthesis during a time course experiment. Giant clam larvae were raised until 75 h post-fertilization and then infected with cultured isolates of Symbiodinium clade A3. Expression of symbiosis-specific and housekeeping genes was monitored at four time points. The expression of H-ATPase, a symbiosis-specific gene in Symbiodinium, was observed at 24 h after symbiont acquisition by the clam larvae. The expression of an enzyme responsible for glycerol synthesis was also observed. Together, these results show that the symbiotic relationship was already in place 24 h after Symbiodinium acquisition, during veliger larval stage. This is the first report using a molecular symbiosis-specific marker that supports symbiotic activity between Symbiodinium and a metazoan larva of an organism that acquires symbionts horizontally. From the expression of the glycerol-synthesizing gene, it was qualitatively determined that Symbiodinium cells may produce glycerol regardless of whether they are free-living or in symbiosis. [ABSTRACT FROM AUTHOR]

The decision to hide from predators and the time allocated to hiding are economic decisions that integrate the benefits of escaping a predator and the costs of reduced resource acquisition. Body size is a factor that influences many antipredator decisions. Giant clams, Tridacna maxima, depend on photosynthesis as their main source of energy; thus, retracting their mantle into their shell inhibits energy acquisition and ultimately growth. We experimentally encouraged 95 individual clams to hide by touching them and found that after accounting for variation explained by observer, larger clams remained closed longer. When we looked at the response of these 95 clams to a total of four consecutive experimentally induced closings over about 10 min, we found that larger clams on average hid longer and that clams had individually consistent hiding times and generally habituated to repeated experiments. We then focused on a subset of 30 clams and continued this experiment every other day over 6 days for a total of four sessions. Over this longer duration, clams consistently habituated to repeated disturbance, but the effect of size disappeared. We also found that larger clams were bitten by predators more often than smaller clams. Large clams pay an immediate cost to hiding from benign stimuli and apparently learn to modify their behavior to repeated benign experiences. [ABSTRACT FROM AUTHOR]