How do you know if a clam is feeling under the weather, or stressed? What are the signs? One of the greatest challenges in ecology today is to understand how to knit together the physiological and behavioural information recorded from single animals within quantitative frameworks. When successful, these frameworks can be used to develop applications, like early warning systems of environmental changes, or even to make detect potential environmental impacts.
Enter valvometry
Oysters, clams, scallops, mussels are all examples of bivalves. Bivalves have two shells (the “valves”) that move constantly as part of their normal activities.
The original valvometer, from Marceau, F. 1909. Recherche sur la morphologie, et l’histologie, et la physiologie comparées des muscles adducteurs des mollusques acéphales. Archives de Zoologie Expérimentale et Générale, Série 5, 2:295– 469.
Biologists have been recording these movements since the early 1900s using valvometry. Valvometry is an ensemble of techniques that measures and records changes in the opening distance between the two shells. These methods offer cost-effective means to monitor continuously even extremely small changes in the valve positions at high measurement frequencies (e.g. 10 measurements per second or more), whether the animal is submerged or not.
These shell valve movements have important roles in respiration and digestion, as well as for burrowing and other movements. Many bivalve species are known to react very quickly to even small changes in environmental conditions. This feature of their behaviour, plus their ubiquitous distribution in marine and aquatic environments, as well as the relatively low cost of sensors has already inspired several commercial products that exploit valvometry data for water quality monitoring.
Knowledge gaps remain for valvometry
Some valvometry techniques have been commercialised, but there are still important knowledge gaps. In our literature review, we identified oversights concerning the calibration of sensors, for example. This led us to develop new methods for improving the quality and sensitivity of valvometry measurements (Guarini et al. 2020a, see details below).
In addition, we noted that the majority of methods rely on sophisticated statistical correlations to interpret the shell movements. However, this phenomenological approach can be problematic when working in situ, where conditions are not fully controlled. Hence, because we are interested in developing valvometry for other uses besides water quality, we chose an alternative path.
Genesis of a new biophysical model
During 2020, we developed a deterministic, mathematical model to describe how shell movements are produced by the animal. We started with building a biophysical model of the adductor muscle and hinge ligament system (Guarini et al. 2020b, see details below). Biophysical models are designed to mimic the workings of the muscle system, including how much energy is needed to make it function under different conditions. These types of models can therefore be used to evaluate the physiological state of organisms under different conditions. This also means we can capture the animal’s responses to its surroundings, in real-time.
Where to now?
With our new mechanistic model in place, new applications for valvometry can be developed. For example, we have now extended the biophysical model to show how it could track the condition of shellfish stocks in real-time and in response to environmental parameters (Guarini et al. 2021, see details below). This is something that has long been a shared goal for both producers and regulatory agencies.
Another potential application would be providing actionable evidence of an environmental impact. For example, with a mechanistic model it becomes feasible to estimate the cumulative environmental impact on bivalve species which is attributable to short and intermittent underwater sound events, such as pile driving. Patterns in valve movements can be analysed in terms of both continuous, environmental changes (seasonal changes in light, temperature, etc.) as well as very short-term events, produced during marine development projects, such as rapid changes in turbidity and dissolved oxygen.
Detailed summaries of the articles in our valvometry series
Figure 1 from the article on valvometry calibration (Guarini et al. 2020a). The image shows where the Hall effect sensor and magnet are positioned on the shell. The inter-valve opening distances are estimated from changes in the signal voltage.
Guarini, J-M. et al. (2020a) revised the calibration of the Hall Effect sensors (HES) used by many studies to detect the valve movements. A HES is preferred because it is cost-effective, non intrusive and very rapid. We found that the usual method of calibration in the valvometry literature does not take into account the electromagnetic properties of the sensors, nor the rotation of the valves when they open and when they grow for long-term experiments. Our first article therefore provides a method to make unbiased series of valve opening estimates. It quantifies at the same time uncertainties of these estimates as a function of both the data quality and the relative position of the sensor and magnet on both valves of the specimen. We found no other article reported attempts to do this in earlier studies.
Guarini, J.M., Coston-Guarini, J. and Comeau, L.A. [2020a preprint] Calibrating Hall-Effect valvometers accounting for electromagnetic properties of the sensor and dynamic geometry of the bivalves shell. bioRxiv 2020.12.20.423648. doi.org/10.1101/2020.12.20.423648
The new biophysical model describing the dynamics of the opening angle at equilibrium as a balance between the elongation of the hinge ligament and the contractions of the adductor muscle (see Equation 7 from Guarini et al. 2020b).
Guarini, J-M. et al. 2020b introduces the biophysical model that underlies the mechanistic processes used by bivalves to regulate the opening of their shell. The model we have formulated was adapted from one designed originally for human muscles. For the bivalve we allowed the adductor muscle to have two behaviours: sustained effort and the possibility to make rapid contractions to ensure physiological needs, including stress responses. This original study implements a new approach to fitting the model simulations with valvometry data series. Our new framework for valvometry makes possible new types of experiments, interpretations and usage of these data series that have not yet been explored, but only hinted at in the existing literature.
Guarini, J-M., Coston-Guarini, J. and Comeau, L.A. [2020b preprint] Interactions between discrete events and continuous dynamics in the regulation of scallops valve opening: insights from a biophysical model. bioRxiv.12.25.424408. doi.org/10.1101/2020.12.25.424408
The third article (Guarini et al. 2021) develops this framework even further for application in shellfish farming. But this tool has not been used in farming, probably because of its lack of sensitivity and inability to characterise the physiological condition of the organisms. We therefore introduced a formal definition of early warning signs of condition changes using real time estimates of the physiological state of the animals from the gaping dynamics. A new set of indicators combining valvometry measured parameters with a real-time comparison to the biophysical model trends and including environmental variability is now possible within the framework first described in the second article.
Figure 6 from the article on the early warning system for shellfish aquaculture farms. Example of an indicator of physiological condition that is developed in the article, and applied to valvometry data series recorded from four individual scallops. The condition of individuals in red and blue is degrading as they spend more time below the threshold value of 1.
Guarini, J.-M., Hinz, S., and Coston-Guarini, J. 2021. Designing the Next Generation of Condition Tracking and Early Warning Systems for Shellfish Aquaculture. J. Mar. Sci. Eng. 9, 1084. doi: 10.3390/jmse9101084