Exploring the allometric relationship between resting metabolic rates and body mass in herbivorous, omnivorous, and carnivorous invertebrates within New Zealand's marine ecosystems
Exploring the allometric relationship between resting metabolic rates and body mass in herbivorous, omnivorous, and carnivorous invertebrates within New Zealand's marine ecosystems
Abstract
This report investigates the allometric relationship (m-w) between resting metabolic rates (MO2) and body mass in herbivorous, omnivorous, and carnivorous marine invertebrates. The findings contribute insights into morphological and physiological differences based on ecological variables. Data collected on January 31st, 2024, from 16 different species demonstrated an allometric relationship between metabolism and mass, with a decrease in MO2 as body mass increased. The study revealed approximately 45.61% variability in the allometric relationship, with a p-value of <0.05, proving statistical significance. Herbivores displayed lower average mass than the trend line, potentially due to the complexity of plant structures leading to digestion challenges. Omnivores exhibited higher activity levels and a positive average residual in the m-w relationship, while carnivores, especially the Japanese mantis shrimp, displayed intriguing variations. Potential discrepancies, such as stress induced by handling and the impact of shell weight, were considered. Despite potential sources of error, the model's statistical significance aligned with our hypothesis that diet and activity levels impact metabolic rates in invertebrates.
1. Introduction
Metabolic rates in invertebrates are a crucial indicator of energy utilisation, influencing morphology, physiology and species interactions. This study examines the relationship between resting metabolic rates (MO2) and body mass across herbivorous, omnivorous, and carnivorous invertebrates. On February 1st, 2024, students of Marine 305 from the University of Auckland measured the MO2 of 16 different invertebrate species. We hypothesised that there was an allometric relationship between metabolism and mass, and as body mass increased, MO2 decreased. We aimed to understand this relationship and consider potential contributing factors, such as dietary preferences and activity levels. Additionally, we worked to contribute insights into the morphological and physiological dynamics of invertebrates and how they influenced MO2.
2. Methods and Materials
2.1. Resting metabolic rate and mass measurement
Screw-top containers matching the invertebrates' size were filled with oxygenated seawater, ensuring no air bubbles were present to prevent misreadings. The selected containers had a volume no more than 200 times the organism's volume. Each container's initial O2 saturation level was recorded using an O2 meter before sealing. The containers were placed in a thermal bath of 18°C and maintained a constant temperature.After 20 minutes, O2 saturation levels were re-recorded, and if a 20% drop was not achieved, containers were resealed for an additional 15 minutes, repeating until a significant drop was visible. The exact time to reach a viable MO2 reading was noted for calculation. Organisms were removed from their containers and patted dry with a paper towel. Their body weight was measured in grams (g) using scales.
2.2. Calculation of resting MO2
The change in % O2 saturation per unit time in hours was calculated using Excel (slope, Δ O2 sat Δ t^-1). This slope was required to calculate the mass-specific O2 consumption (MO2) rate. The resting MO2 was calculated in units of mg O2 g^-1 h^-1 using the formula: MO2 = a Vresp ɑ W^-1, where ‘a’ is the slope, ‘Vresp’ is the chamber volume, ‘ɑ’ is the solubility of O2 in water, and ‘W’ is the weight of invertebrates. Individual MO2 and body weight values data were compiled into a class dataset and plotted on a log scale for easier visualisation and interpretation.
3. Results
The allometric relationship between log resting metabolic rates and log mass showed a linear regression with some variability (Fig. 1a). From the trendline, approximately 45.61% (R²=0.4561) of the variability in the dependent variable (MO2) is explained by the independent variables (mass). The p-value of the m-w relationship was <0.05 (1.61776E-07).
Herbivores exhibited a lower average m-w than the trend line, while omnivores showed a higher average (Fig. 1a). Carnivores displayed an average slightly below the trend line. However, a notable outlier observed was the Oratosquilla oratoria (Japanese mantis shrimp), being generally heavier with a higher metabolism than other tested carnivores.Regarding mass (non-log), omnivores exhibited the lowest average mass, weighing 3.240g. In comparison, herbivores displayed the highest average mass at 37.198g, with carnivores slightly less at 21.468g (Fig. 1b). The heaviest recorded invertebrate was a herbivore weighing 119.45g (Evechinus chloroticus), while the lightest was an omnivore weighing 0.014g (Melita inaequistylis).Regarding the residuals (observed vs. predicted outcome) of m-w, herbivores had an average residual of -0.329, while omnivores had an average residual of 0.332 (Fig. 2). Carnivores exhibited an average residual of -0.012, with a notable range depicted by a high of 0.959 and a low of -1.153, with the lightest carnivore weighing an average of 0.177 g (Certasoma ammonium) and the heaviest (Monoplex pathenopeum) at 97.8 g (Fig. 1b). Furthermore, active species displayed an average residual of -0.269, while inactive species had an average residual of 0.487 (Fig. 3).
Fig 1. A = The allometric relationship (m-w) of log mass (g) and log resting metabolic rate (MO2) of invertebrates recorded in February 2024. Recorded in 18॰C water on February 2024 at Leigh Marine Laboratory, NZ. B = The mass (non-log) of examined invertebrates.
Fig 2. The average residual of the allometric relationship (m-w) between log mass (g) and log resting metabolic rate (MO2) in herbivorous, omnivorous and carnivorous invertebrates. Recorded in 18॰C water on February 2024 at Leigh Marine Laboratory, NZ.
Fig 3. The average residual of the allometric relationship (m-w) between log mass (g) and log resting metabolic rate (MO2) in active and inactive invertebrates. Recorded in 18॰C water on February 2024 at Leigh Marine Laboratory, NZ.4. Discussion
4.1. Relationship of m-w vs diet and activity levels
The m-w relationship in herbivores, carnivores, and omnivores exhibited distinct patterns. Herbivores displayed m-w well below the average trend line, potentially due to the intricate carbohydrate structure of plants posing digestion challenges (Choat & Clements, 1998). Notably, within this study, herbivores had the highest mass. Previous research consistently links body mass with metabolic rates, where smaller species exhibit faster metabolism and larger species exhibit slower (White & Kearney, 2014). Given that the larger species in this study were predominantly herbivores, it clarifies why their average m-w was much lower than predicted, prompting the need for understanding. Negative residuals of m-w may be attributed to shell weight, as each of the four herbivorous species exhibited shells. Because shell weight accounts for body mass but not respiration, there is a potential for misleading data. In contrast, omnivores had the lowest mass and demonstrated an average m-w higher than the trend line, displaying positive residuals. Seven of eight species were classified as active, justifying the positive residuals, as smaller body mass and higher activity levels likely influenced metabolic rates.While carnivores showed m-w slightly below the trend line, the relationship is closer to the trend line than herbivorous counterparts. This data can be attributed to the readily digestible nature of captured prey (Stevens, 1988) compared to complex plant structures. Additionally, while carnivorous diets often lead to greater energy expenditure and metabolic rates due to high-energy prey items (Hirt et al., 2017), this study presented a different scenario. Only one of the four carnivorous species, O. oratoria, was classified as active, explaining the highest outlying data point above the m-w trendline. Furthermore, carnivores displayed a minor negative residual with a large error bar. The residual can be attributed to carnivore m-w aligning closely to the trend line, while the error range displays the vast difference in overall body mass from the carnivorous species examined.
While variability from the trendline is approximately 45.61%, this can be accounted for through diet and activity levels. Regardless, the p-value was <0.05, indicating the model was statistically significant.4.2. Potential data discrepancies
Upon further examination, certain factors may have influenced the observed metabolic rates. Handling invertebrates multiple times and placing them in confined environments could have induced stress, potentially resulting in higher metabolic rates (Mercier et al., 2006), supporting the high residuals seen within active species. The composition and additional weight of shell-bearing animals may have contributed to lower metabolic rates relative to their mass (Rosenberg & Hughes, 1991).
Students were taught how to run the experiment on the day; therefore, a lack of experience potentially contributed to errors. Factors such as shared handling of invertebrates among multiple students throughout the day could have led to inaccurate recorded time and metabolic levels or mixing of test species amongst groups. Additionally, during MO2 testing, not all invertebrates were tested equally, as some species’ tests were repeated 3-4 times, while others only once. This lack of repetition may have created small inaccuracies within our data. Furthermore, we could not account for cofounding traits within the invertebrates, such as health, age and sex, which may have impacted the m-w relationship (Schuster et al., 2019). These potential sources of error should be considered when interpreting the results.4.3. Conclusions
Our findings support the hypothesis of an allometric relationship between resting metabolic rates and body mass in herbivorous, omnivorous, and carnivorous invertebrates. The distinct patterns observed suggest the significant influence diet and activity levels have on metabolic rates. Despite potential sources of error, the statistical significance of our model suggests a significant relationship.
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