Adaptive Responses in Byssal Growth and Shedding: Insights from Pteria penguin Under Thread Trimming and Non-Trimming Conditions

Abstract

Bivalves use byssal threads for attachment and locomotion, periodically shedding and regenerating them. In the winged pearl oyster Pteria penguin—known for its strong byssus and its role in the pearl industry—shedding may occur when the byssal stalk reaches a critical size, although the underlying mechanism remains unclear. This study investigated whether artificial manipulation of the byssus (via trimming) could stimulate thread production and promote shedding in adult P. penguin from two size groups. Byssal threads attached to the substrate were trimmed every 3–5 days over a 30-day period and compared to untrimmed controls. Oysters with trimmed byssus produced significantly more threads, with smaller individuals outperforming larger ones in both thread count and byssal stalk diameter. Moreover, small oysters exhibited a higher frequency of complete byssal shedding. These findings suggest that trimming stimulates thread production and accelerates stalk thickening, potentially triggering shedding due to spatial constraints at the attachment site. This response appears to reflect an adaptive mechanism for maintaining effective attachment and may help explain how mechanical or environmental cues influence byssal dynamics. Understanding this process offers new insight into the behavioral and physiological plasticity of P. penguin, with potential applications in pearl oyster management and aquaculture.

1. Introduction

Bivalves that produce byssus have evolved a wide variety of byssal structures, which play a crucial role in their ability to anchor themselves to intertidal and underwater substrates. These specialized attachments enable bivalves to maintain stability in dynamic aquatic environments, contributing significantly to their survival, ecological, and evolutionary success [1,2,3]. This versatility is also manifested in pearl oysters, which depend on these strong, fibrous threads to anchor themselves to various surfaces. However, the strategies for managing these attachments can differ considerably between species [4]. For example, Pinctada margaritifera and P. fucata are known to continually produce byssus throughout their lives, ensuring they remain firmly anchored in their benthic environment [5]. In contrast, P. maxima stops secreting byssus once it grows large and heavy enough to stay in place despite the challenges posed by ocean currents [6]. The growth of byssal threads in these oysters is not just a matter of species; it is also influenced by a range of environmental factors like water temperature [7], pH levels [8,9], salinity [10,11], and the strength of local currents [6,12]. The effect of these factors on the byssogenesis in pearl oysters is evident, and as oysters age, their production of byssal threads tends to decrease considerably [6,13]. But as the byssal threads are constantly produced, they are also being released or shed, and this is a phenomenon that has not been explored in depth in other bivalves as in mussels [14]. Pearl oysters can also shed their byssi; they do this by discarding the old threads and then secreting new ones, sometimes even before the old threads have completely detached [13,15]. Byssal shedding allows oysters to engage in active displacement, which can be crucial for survival in challenging environmental conditions such as high predation risk or extreme environmental stressors, including temperature fluctuations and salinity changes [16]. Similar to other byssally attached species, the dynamics of byssal production, attachment, and release can be used as a form of locomotion by short movements [17].

The winged pearl oyster Pteria penguin belongs to the Pteriidae family, a byssally attached bivalve that is not considered a “true” oyster according to malacologists [18] and is well known for their use in the production of half pearls (mabe) [19,20]. There have been difficulties in attempts to produce round pearls due to the low nuclei retention rate. This is a problem that has been linked to a large byssus, as in other pearl oysters [21]. Excessive byssal development may interfere with the success of the grafting procedure by causing stress or limiting the available energy for pearl sac development, ultimately reducing pearl quality or retention. Understanding and potentially managing byssal production could therefore contribute to improved grafting success and overall pearl yield in aquaculture settings. After metamorphosis, P. penguin juveniles retain the byssal glands producing byssal threads throughout their adult life, as in many pterioideans [22]. Interestingly, despite possessing one of the strongest byssi among bivalves, P. penguin can also shed its byssus completely, including the byssal root, and young adult individuals raised in lantern nets are occasionally found without byssus [23]. Moreover, the length and diameter of the byssal stalk are more closely tied to the oyster’s weight than to the size of its shell [23], and as the oysters continue to gain weight, there comes a point where the diameter of their byssal stalks ceases to increase [13]. This observation suggests the presence of a physiological threshold, beyond which the byssus can no longer thicken, regardless of further growth of the oyster.

To further explore this phenomenon, this study aims to investigate the impact of stimulating byssal growth and shedding by periodically detaching the oyster from its substrate through the mechanical trimming of its byssal threads. The underlying hypothesis is that by regularly trimming the newly produced byssal threads, the oysters will respond by producing new byssal threads that ultimately will enlarge the byssus, leading to the voluntary shedding of the whole byssal organ. This approach aims to provide deeper insights into the adaptive mechanisms governing byssal production and shedding in pearl oysters.

2. Materials and Methods

2.1. Experimental Animals and Conditions

A total of 100 adult P. penguin oysters were collected from an oyster farm in Sanya, Hainan Province (China), where the oysters were cultured in semi-natural open-ocean conditions, characterized by high water quality and a suitable environment for suspended culture using lantern nets hung from rafts. The oysters were transported in wet conditions inside styrofoam boxes to the Sanya Tropical Fisheries Research Institute (Lingshui, Hainan Province). These oysters were divided into two distinct size groups: small oysters (65.11 ± 7.07 mm maximum dorsoventral length, MDL; 10.46 ± 3.61 g wet weight, WW) and large oysters (87.34 ± 8.03 mm MDL, 24.24 ± 5.66 g WW), with each group comprising 50 individuals. Although likely from the same 10-month-old cohort, natural variability in growth resulted in these two size classes, which were used to assess potential size-related responses. Each individual was tagged using waterproof paper labels (10 mm × 10 mm), which were affixed to the oyster shell with a drop of fast-drying cyanoacrylate (super glue) to ensure durable identification throughout the experiment. After the adhesive had fully dried—typically within 5 min—the oysters were thoroughly rinsed with seawater and placed into flat net cages, each capable of accommodating 25 oysters. Inside the cages, oysters were vertically arranged in a manner that allowed each oyster’s foot to reach the adjacent oyster shell, thereby promoting byssal attachment (Figure 1). The experiment was conducted over a 30-day period, allowing for consistent monitoring of byssal production and shedding behaviors across individuals.

All oysters were allowed acclimation for 13 days in 200 L tanks under constant controlled husbandry conditions during this period and throughout this study. These conditions included daily 100% water changes, ad libitum feeding with a mixed diatom diet (Skeletonema sp. and Chaetoceros muelleri), a water temperature of 28 ± 1 °C using filtered seawater (10-micron polypropylene felt filter bag), and salinity levels maintained at 32 PSU. Aeration through upwelling was employed to create a circular water flow in the tanks, ensuring optimal oxygenation and water movement.

2.2. Byssal Thread Counts, Diameter Measure, and Trimming

To observe byssal thread production and shedding, two experimental conditions were established. In the first condition, a cage containing 25 small oysters and another cage containing 25 large oysters were submerged together in one tank. No byssal trimming was performed, allowing for observation of natural byssal shedding (Figure 1A). In the second condition, another two cages—one with small oysters and the other with large oysters—were similarly prepared and submerged in a separate tank. Throughout the experimental period, byssal threads were periodically counted and trimmed using scissors to simulate byssal shedding and observe the oysters’ responses to this intervention (Figure 1B). By maintaining these two distinct conditions, the experiment aimed to provide a comprehensive understanding of how byssal production and shedding are influenced by size and external manipulation in P. penguin. Trimmed and untrimmed oysters were maintained in separate tanks without within-treatment replication, due to logistical and space constraints. While this design enabled clear monitoring of treatment effects, potential tank effects and associated bias cannot be fully excluded.

The number of single byssal threads secreted by each oyster was counted every 3–5 days over 30 days. To standardize the measurements, the number of threads was counted at the distal section of the byssus because threads suffered fusion as threads got closer to the proximal section of the byssus. The absolute number of byssal threads produced by each oyster was obtained by subtracting the thread count from the immediate previous count.

The diameter of the byssus in all oysters was measured at the commencement (day 0) and the end of the experiment (day 30) by opening the oyster valves using a pair of pearl oyster openers. The measurements of the byssal diameter (BD) at the proximal section were acquired using a vernier caliper (0.1 mm). The trimming of the byssus was carried out only on oysters that were found attached and was carried out every 3–5 days by using a pair of scissors (Figure 1B). The equation (1) used for calculating the change in byssal diameter (Δ BD) was

Δ BD = (D₁ − D₀)

(1)

where Δ BD represents the change in diameter, D₀ represents the diameter at day 0, and D₁ represents the diameter at day 30.

2.3. Data Analysis

A violin plot was constructed using the R package ‘ggstatplot’ [24]. Welch’s ANOVA, followed by the Games–Howell test for pairwise comparisons, was used to compare the absolute count of byssal threads among the four experimental groups with the function ‘ggbetweenstats’. These robust parametric methods were chosen because they do not assume equal variances or balanced sample sizes, which were conditions not fully met by our data.

Boxplots were constructed using the package ‘ggplot2’ [25]. To assess the differences between the accumulated number of byssal threads produced for each oyster at any given sampled day, the ‘lm()’ function was used to fit a linear model with the absolute number of threads as the dependent variable and day as the independent variable. The ‘emmeans()’ function was then applied to this linear model object to compute estimated marginal means and conduct a Tukey post hoc test.

The association degree between the variables ΔBD and the total number of threads produced by the oysters whose byssi were trimmed was calculated by the determination coefficient (R²) after the data underwent handling of outliers and log transformation, and its level of significance was determined by fitting the data using the ‘lm()’ function. To assess the goodness of fit and assumptions of the linear regression model, a diagnostic plot was generated using the ‘fitPlot()’ function from the ‘FSAmisc’ package [26]. All differences were considered significant at p < 0.05. Plotting, curve fitting, and statistical analyses were carried out using R [27].

3. Results

3.1. Survival Rate and Thread Counts in Oysters with Trimmed and Untrimmed Byssus

The survival rate of oysters in most experimental groups was 100%, except for the group of large oysters whose byssus remained intact, which showed a survival rate of 90%. The absolute number of byssal threads consistently revealed that byssal thread production was significantly higher in oysters subjected to periodic trimming, regardless of their shell size, than in oysters with untrimmed byssus (p < 0.05, Figure 2). Oysters with untrimmed byssal threads displayed an average of 1.24 ± 2.48 threads in small individuals and 0.68 ± 2.74 threads in large ones. In contrast, oysters subjected to regular byssal trimming showed significantly higher averages, with small oysters producing 6.09 ± 3.68 threads and large oysters producing 3.96 ± 3.54 threads.

The mean cumulative number of byssal threads produced by pearl oysters with trimmed and untrimmed byssal threads across size groups is shown in Figure 3. Oysters subjected to byssal trimming exhibited a consistently increasing trend in thread production over the 30-day observation period. Small and large oysters produced an average of 67.08 ± 23.31 and 43.63 ± 25.15 byssal threads, respectively, after their byssus was trimmed up to 10 times at regular intervals. The mean cumulative thread counts increased significantly from day 0 to day 30 in both size groups. By the end of the experiment, oysters with trimmed byssus displayed a wide range of cumulative thread counts, with the median value approaching the upper quartile, indicating robust thread production. Thread production was consistently high and significantly different from earlier time points, particularly after day 10 (Tukey HSD test, p < 0.05).

In contrast, oysters with untrimmed byssal threads produced far fewer threads. Small and large oysters in this group generated an average of 9.2 ± 5.2 and 5.28 ± 6.7 byssal threads, respectively. The variability in thread counts was lower in the small-oyster-size group, and thread production plateaued much earlier compared to the trimmed group (Figure 3). For the large-oyster-size group, there was no significant change in thread production throughout the experiment (Figure 3).

3.2. Byssal Diameter in Small and Large Oysters

The byssal diameters of both small and large oysters, with trimmed and untrimmed threads, are shown in Figure 4. Measurements were taken at two time points—day 0 and day 30—across all experimental groups. For small oysters, the initial mean byssal diameter on day 0 was 2.29 ± 1.17 mm for the untrimmed group and 2.75 ± 1.26 mm for the trimmed group. By day 30, the mean diameter increased to 3.62 ± 1.10 mm in the untrimmed group and 5.14 ± 1.81 mm in the trimmed group.

Similarly, for large oysters, the mean byssal diameter at day 0 was 3.77 ± 1.35 mm for the trimmed group and 3.76 ± 1.54 mm for the untrimmed group. By day 30, the mean diameter increased to 4.18 ± 1.57 mm for the untrimmed group and 5.29 ± 1.79 mm for the trimmed group. These findings indicate that both small and large oysters experienced an increase in byssal diameter over the 30-day experimental period. Furthermore, trimming the byssus appeared to stimulate greater byssal growth compared to the untrimmed byssus in both size categories.

A positive and statistically significant correlation was found between the total number of byssal threads and the change in byssal stalk diameter (ΔBD, Figure 5), with a coefficient of determination of R² = 0.19 (p = 0.01, F = 6.55).

3.3. Byssal Shedding in Small and Large Oysters

Small oysters with trimmed byssal threads exhibited a higher propensity for byssal shedding than those with intact byssus. By day 30, 36.0% of the small oysters subjected to byssal trimming had expelled their byssal threads, whereas only 4.1% of those with intact byssus exhibited such behavior (Figure 6A). In the large-oyster cohort, 22.7% showed byssal shedding when the threads were trimmed, while 14.3% did so when their byssus remained intact (Figure 6B). When oysters expelled (shed) their byssus, they did it completely, including the byssal root (Figure 6C).

4. Discussion

4.1. Thread Production in Oysters with Trimmed and Untrimmed Byssus

Periodic trimming of byssal threads in P. penguin oysters provides valuable insight into byssal production and shedding behavior. Oysters that underwent trimming produced significantly more byssal threads than untrimmed individuals, indicating that trimming stimulates new thread formation. This effect was observed in both small and large oysters (Figure 2), highlighting the consistent impact of trimming across size groups.

The increase in thread counts could be attributed to several factors. Trimming might remove older, less efficient threads, prompting the pearl oyster to produce new ones to maintain its attachment to the substrate. Self-healing of the byssus is not uncommon in bivalve species. P. nobilis byssal threads exhibit self-healing behavior through a time-dependent process where hidden length refolds and bonds reform [28]. The Mytilus byssal threads demonstrate the capability to disperse as much as 70% of mechanical energy and showcase the potential for self-repair after experiencing pseudoplastic mechanical injury [29,30]. Additionally, the sole action of trimming the byssus in P. penguin could potentially stimulate the oyster’s physiological processes involved in byssal production, leading to enhanced thread growth. The byssus is a proteinaceous structure that byssate bivalves use for attachment, and its production is influenced by various environmental and physiological factors. Research has shown that environmental stressors, such as heat waves, can impair byssal production in mussels. For instance, exposure to elevated temperatures has been linked to decreased production and quality of byssal threads, affecting their length, diameter, and overall strength [31,32]. Low salinity levels also have been shown to reduce the mussels’ byssal production [33]. Research indicates that Arca boucardi can form a new byssus within 9–10 h after detachment, producing a structure similar to the original, which highlights the rapid regeneration capability of this species [34]. Similarly, in M. coruscus, while high microalgae availability can disrupt byssal secretion, the mussels still exhibit increased byssal production during recovery periods, suggesting resilience in their regenerative processes [35]. Furthermore, P. penguin maintains byssal properties under varying temperatures, indicating that environmental factors do not hinder its ability to regenerate [7]. All these findings underscore the regenerative potential of byssus in bivalves, although the efficiency and speed of regeneration may vary based on species and environmental conditions [29,36].

Trimming the byssus in P. penguin may induce a stress response that could trigger the pearl oyster’s physiological processes to regenerate the byssus. This regeneration process could involve increased metabolic activity and energy allocation towards byssal thread synthesis. Byssal properties are maintained through biochemical processes like polyphenol oxidase (PPO) activity, which is key for byssal formation under stress [7]. As oysters adapt to environmental changes, they increase metabolic activity, focusing energy on byssal thread synthesis. This is supported by transcriptome analysis, showing the role of genes related to protein quality control and cytoskeletal reorganization [7]. The results showed that regardless of the oyster’s shell size, periodic trimming significantly increases byssal thread production. This suggests a strong relationship between trimming and enhanced byssal regeneration, indicating that pearl oysters can effectively reattach by producing new byssal threads, regardless of the size of the individual.

Nevertheless, smaller pearl oysters produce significantly more byssal threads than larger ones. Larger oysters may prioritize energy towards maintaining and reinforcing their existing byssal stalks rather than producing new threads. Since heavier oysters already have wider byssal stalks, they might allocate their resources to sustaining the strength and structural integrity of these stalks rather than producing additional threads. In contrast, smaller oysters, which have less massive byssal stalks, may invest more in producing new threads as a way to enhance their attachment strength. Small pearl oysters prioritize byssal development, especially under environmental challenges, while larger ones are less prone to byssal shedding, suggesting limited regeneration as they age [13,23]. The smaller oysters also may be in a different growth phase compared to the larger ones. Small individuals may be in a phase where they naturally produce more byssal threads as part of their growth and development. On the other hand, larger oysters may have passed this phase and instead focus on maintaining their established attachment structures. The larger byssal stalk in large oysters might reach a threshold beyond which the addition of more threads results in diminishing returns, making further increases in byssal diameter less efficient or necessary. This mechanical limitation could explain why larger oysters do not produce as many new threads as small ones, even when their existing threads are trimmed.

The cumulative count of the number of threads produced by each oyster over the observation period reveals a marked difference between the trimmed and untrimmed ones (Figure 3). Pearl oysters subjected to byssal trimming exhibited a consistent and significant increase in byssal thread production over time, with small oysters producing an average of 67.08 ± 23.31 threads and large oysters producing an average of 43.63 ± 25.15 threads after up to 10 trimming sessions over 30 days. In contrast, oysters whose byssi were not trimmed displayed less byssal thread production, with small oysters averaging 9.2 ± 5.2 threads and large oysters averaging 5.28 ± 6.7 threads. This reinforces the earlier discussed observation that periodic trimming stimulates byssal thread production in pearl oysters. There is evidence that by cutting byssal threads daily, the thread production increased substantially in the zebra mussel Dreissena polymorpha. After the byssi were cut, the animals still exhibited the ability to establish reattachment to a substratum [37]. Periodic byssal thread removal also increased thread production in the mussels M. trossulus [38] and M. edulis [39]. The mussels initially produce about two threads per day during a period of acclimation (7 days). Following this acclimation, they produce between 6 and 12 threads per day. This response indicates that detaching the threads stimulates the mussels to increase their attachment efforts significantly. In the result presented in this investigation, the pearl oysters started to produce more threads after 10 days and continued this trend until the end of the experiment. It is unknown how long they can maintain this trend, particularly because byssal regeneration energy costs usually result in decreased animal growth [38].

4.2. Byssal Diameter in Oysters with Trimmed and Untrimmed Byssus

The examination of the byssal stalk diameter further elucidates the effect of thread trimming on byssal growth (Figure 4). Both small and large oysters exhibited an increase in byssal stalk diameter over the 30-day experimental period. Interestingly, trimming the newly formed byssal threads seemed to stimulate greater byssal diameter compared to leaving its threads intact in both size groups. It seems that trimming the threads at the distal section not only encourages the production of new threads but also ‘promotes’ the thickening of existing byssal stalk, potentially enhancing the oysters’ attachment strength and stability. The merging of the trimmed threads to the byssal stalk seems to be responsible for the increase in the diameter at the proximal section. The correlation between the total number of byssal threads and byssal diameter in adult P. penguin oysters confirms this assumption with a significant but modest positive relationship (Figure 5). A potential explanation for this relationship is the merging of newly produced byssal threads with the pre-existing byssal stalk. As more threads are produced, they may fuse and reinforce the existing stalk, leading to an increase in its overall diameter. This merging process could provide structural benefits, enhancing the strength and durability of the byssus, which is crucial for the oyster’s ability to anchor securely to substrates. However, the modest correlation (R² = 0.19) suggests that while the number of threads contributes to the increase in byssal diameter, other factors are likely involved. Environmental conditions, genetic factors, and the individual health of the oysters could also influence byssal diameter. The relatively low R² value implies that the number of threads is only one of several variables affecting the diameter. These findings underscore the complexity of byssal production in P. penguin, indicating that the diameter of the byssus is influenced by multiple factors, with the number of threads produced being a significant, but not the sole, determinant.

The exact mechanism of how a single byssal thread merges with the stalk in P. penguin is still unknown. The byssus consists of a dense and highly aligned fiber inner core, surrounded by an outer cuticle that contains protein granules embedded in a protein matrix. This structural composition may facilitate the merging of the individual threads into a singular, robust byssus [40]. In various bivalves, the byssal gland in the foot secretes the proteins that form the byssal threads. Multiple threads are simultaneously secreted from the foot. As the threads emerge from the foot, they are guided by grooves and folds in the foot that direct them to converge at a central point [31,41]. The converging threads usually fuse to form a stem-like structure called the byssal stem or peduncle [42]. In P. nobilis, two types of threads or filaments are produced: the thick and the thin filaments. The thin filaments are laterally fused in pairs. Two of them form a junction where the other two fit to form the thick filament. A fully developed adult byssus can be formed by 20,000–30,000 filaments [43]. The stem acts as a common attachment point for the individual threads [41]. At the distal end of the stem, the threads splay out and form adhesive plaques that attach to the substrate. The plaques are secreted by the foot and adhere to the surface using specialized proteins. After secretion, the byssal threads and plaques undergo hardening through oxidation and cross-linking of the proteins, increasing the strength and durability of the attachment. Moreover, studies on the biomechanical properties of byssus outside the Mytilidae family have found evidence that the different mechanical properties, such as strength, differ significantly among different groups, although there are some similarities, such as byssal diameter [44].

4.3. Byssal Shedding in Oysters with Trimmed and Untrimmed Byssus

The propensity for byssal shedding provides insight into oyster behavior in response to trimming. Among small oysters with trimmed byssal threads, a higher proportion exhibited byssal shedding compared to those with intact byssus, indicating a potential response to the trimming stimulus (Figure 6A,B). A similar trend was observed in the large-oyster cohort, albeit with a lower shedding rate. Byssal shedding may represent a mechanism for oysters to adjust their attachment in response to environmental conditions or physiological changes induced by trimming. Shedding of byssal stalk or byssal filaments can be attributed to various environmental and biological factors. Excess food availability induces byssal shedding in mussels. Animals fed with high levels of microalgae experienced increased shedding of byssal filaments. High food levels altered metabolic pathways and downregulated genes related to mussel foot proteins, which may explain the increased shedding [35]. Byssal shedding also occurs when animals are stress-free. Control groups in experiments testing the effect of chemical cues from predators or damaged conspecifics of the green-lipped mussel Perna viridis exhibited a higher rate of byssal shedding, indicating that stress-free animals were more likely to lose their byssal threads [45]. Changes such as ocean acidification can reduce the mechanical performance of the byssus, resulting in byssal shedding as well [46]. The cause for the byssal shedding seems to be related to the necessity of mobility, allowing reattachment and movement in response to environmental changes [41]. However, the direct influence of extreme environmental factors, such as hyposalinity, or climate change-related factors may cause a reduction in byssal secretion, weakening, or impairment but do not promote detachment [11,33,47,48].

The shedding of the whole byssal stalk, including the byssal root (Figure 6C), in response to periodic trimming of the byssal threads in P. penguin may be driven by several factors. One possible reason for shedding the whole byssus is the oysters’ ability to recalibrate their attachment strength in response to the trimming of their anchoring threads. Byssal shedding may somehow facilitate the renewal of damaged threads (self-healing), allowing oysters to maintain a robust and functional attachment apparatus over time, as already described in mussels [49,50]. By removing excess threads that are no longer necessary for secure attachment, oysters can also optimize their energy expenditure and adapt to fluctuations in their surroundings. Byssal production is highly energy-intensive, accounting for around 8% of total energy expenditure in M. edulis, with significant demands for both carbon and nitrogen [51], accounting for nearly double the rate of oxygen consumption in P. canaliculus during byssal attachment [52]. In M. trossulus, byssal production significantly impacts energy reserves. In control groups with low byssal thread output, production consumed 2–8% of their energy budget, but in mussels induced to produce threads daily, this increased up to 47% [38]. Despite this demand, energy allocation is prioritized towards byssal thread production over growth [53]. This change in priorities may explain why pearl oysters shed their byssi once the stalk reaches a certain size, as increasing the diameter further becomes less efficient. This idea aligns with previous observations suggesting that byssal growth in these species eventually hits a threshold [13]. However, a clear explanation of how the oyster’s byssus is shed after reaching this point is still unclear. A mechanical limitation of space could also trigger a physiological response that explains why oysters promptly exhibit shedding of their byssus after reaching the size threshold. The voluntary and relatively fast release of the entire byssus has recently been described in mussels [14]. Under specific conditions, mussels can shed their byssus cleanly without damaging the stem root, a phenomenon also seen in P. penguin (Figure 6C). This clean release is essential for preserving the integrity of the attachment system while enabling mobility when necessary. Serotonin facilitates this release, while dopamine increases the force required for release, highlighting the important role these chemicals play in regulating the organism’s ability to detach from surfaces [14].

5. Conclusions

Periodic trimming of byssal threads in P. penguin significantly enhances byssal thread production, especially in small-sized oysters, which produce more new threads than their larger counterparts. Trimming stimulates the pearl oyster’s regenerative mechanism to maintain substrate attachment, with smaller oysters investing more in thread production, while larger oysters focus on reinforcing their existing byssal stalks. Trimming also increases byssal stalk diameter, likely due to the merging of new threads with existing ones, enhancing attachment strength and stability. Periodic trimming increased both thread production and byssal stalk diameter, suggesting that new threads merge with existing ones to improve structural integrity and attachment strength. Additionally, trimming stimulated byssal shedding, an adaptive response that allows pearl oysters to ‘recalibrate’ their attachment apparatus. This behavior may optimize energy use, prevent damage to overgrown byssal stalks, and facilitate locomotion in dynamic environments. These findings provide new evidence for the plasticity of attachment strategies in sessile marine invertebrates, with potential applications in aquaculture management and biomaterials research. The byssal shedding behavior in P. penguin underscores the dynamic nature of pearl oyster attachment biology and highlights the complex interplay between physiological processes, environmental cues, and adaptive responses. Despite the recent efforts in examining byssal origin, structure, and formation, further research into the molecular mechanisms underlying byssal shedding could provide valuable insights into the regulatory pathways involved and enhance our understanding of pearl oyster ecology and behavior. Such knowledge could inform strategies for improving oyster husbandry practices and resilience in changing marine environments.