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Article

Digging in Deep: Size and Site-Specific Variation in Burrow Morphology and Behaviour of the Mud Shrimp, Trypaea australiensis Dana, 1852

by
Renae L. Kirby
* and
Marian Y. L. Wong
School of Sciences, University of Wollongong, Wollongong, NSW 2522, Australia
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(3), 432; https://doi.org/10.3390/jmse13030432
Submission received: 10 February 2025 / Revised: 22 February 2025 / Accepted: 24 February 2025 / Published: 25 February 2025
(This article belongs to the Special Issue Evolution and Ecology of Crustaceans and Their Applications)
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Abstract

:
The importance of habitats, particularly burrows, for intertidal crustaceans is multifaceted. These habitats provide crucial shelter, food sources, and reproductive advantages that are essential for enhancing survival and fitness. However, maintaining these habitats can be costly, influencing whether individuals adapt to or relocate under changing environments. Burrowing mud shrimp present challenges when it comes to studying their behaviours and burrow morphology, owing to their cryptic lifestyle. In this study, we investigated burrow morphology and burrowing behaviour in the mud shrimp, Trypaea australiensis, to better understand the importance of burrows for these organisms. Firstly, we quantified burrow morphology in situ using resin casts and 3D imagery at two locations. Secondly, we examined shrimp burrowing behaviour in custom-made aquarium cuvettes in the laboratory. Resin casts showed that burrows at Shoalhaven Heads exhibited larger burrows with greater variation in the length of burrow measurements compared to burrows at Port Hacking. Laboratory observations of burrowing behaviour demonstrated that shrimp dedicate a large proportion of time to maintaining the structure of their burrows, irrespective of time of day or shrimp sex. Differences were observed between size categories, where smaller individuals were observed sitting significantly more and ventilating significantly less compared to larger individuals. Overall, our study provides a quantitative insight into the importance of burrow habitats and provides the first insights into burrowing behaviour and burrow structure for this endemic ecosystem engineer.

1. Introduction

Intertidal marine habitats are crucial ecosystems that support a rich diversity of organisms and play a vital role in the health and stability of ocean ecosystems [1,2,3]. Intertidal zones act as natural buffers against coastal erosion by absorbing wave energy and nutrient cycling, carbon storage, and water filtration [4]. Protecting and preserving these areas is essential for maintaining the health of coastal ecosystems and the species that depend on them. Such species include a wide range of marine organisms [4,5,6], such as marine benthic crustaceans. In particular, crabs, shrimp, and barnacles rely heavily on the benthic substrate in both juvenile and adult life stages, with the substrate providing protection from predators, safe sites for reproduction, and feeding opportunities [2,3,7]. Therefore, the quality and availability of these habitats are essential for populations of benthic marine organisms and for overall ecosystem health.
Burrowing benthic crustacea, such as burrowing shrimp and crabs, also have an important influence on intertidal marine ecosystems [3,4,8]. For instance, research has shown that shrimp from the family Thalassinidae significantly influence their surroundings, with changes in burrow morphology affecting sediment properties and in turn, crucial ecosystem characteristics such as community composition and nutrient levels [9,10]. Therefore, understanding the burrow morphology of these burrowing crustaceans is crucial for understanding the health and quality of intertidal ecosystems, and previous studies examining burrow morphology have revealed a surprising spectrum of complexity in burrow morphology, feeding ecology and the impact of their bioturbation activities on broader ecosystem dynamics [11,12,13]. These studies have predominantly used resin casts to reveal the intricacies of burrow morphology, providing key insights into spatial and temporal variation in burrow morphology as well as the methods employed by various crustaceans to construct burrows [1,14,15,16,17,18]. Variation in burrow morphology is thought to be influenced by factors such as phylogenetic relationships and geographic latitude. For instance, burrow designs can differ markedly between tropical and temperate species, reflecting adaptations to local environmental conditions [19]. Despite these advancements, significant knowledge gaps remain, particularly with regard to whether burrow morphology differs between crustacean taxa as well as between sites on smaller spatial scales.
Burrowing shrimp create more than just shelters for themselves; their burrows also serve a vital ecological function by providing safe havens for numerous other species [20,21,22]. These burrows form unique microhabitats that support diverse organisms, including commensal bivalves that have evolved to thrive in the low-oxygen conditions typical of the burrow environment [23,24,25]. Bivalves from the Galeommatoidea superfamily [26] depend on these burrows for protection from predators and for stable living conditions [23,24,25]. Given their significant role in maintaining biodiversity, it is crucial to investigate the ecological value of these burrows and the associated symbiotic relationships they support. Studying these interactions helps us better understand ecosystem complexity and the adaptive strategies organisms employ to survive in such specialised environments.
In addition to burrow morphology, understanding the burrowing behaviour of thalassinid shrimp within their burrows is also relatively limited. Current studies have predominantly focused on investigating general aspects of burrowing shrimp ecology and reproductive biology, whereas behaviours have only been inferred via indirect observations [27,28,29,30,31,32]. Indeed, the behaviours of these shrimp are logistically challenging to quantify in situ owing to their cryptic burrowing lifestyle and the delicate nature of their burrows. Therefore, most behavioural studies on thalassinid shrimp have been conducted under laboratory conditions and have focused on the relationship between feeding and burrow morphology [1,13,33]. As such, there is still a notable gap in knowledge surrounding the specifics of burrowing behaviour, including the amount of time spent digging and maintaining burrows compared to other activities, and the energetic costs associated with these behaviours. This knowledge is essential for quantifying the importance of burrows for the organisms themselves, as well as the impact and ecological role of burrowing behaviour on the intertidal ecosystem.
Here, we quantified burrow morphology and examined the burrowing behaviour of the estuarine mud shrimp, Trypaea australiensis. This species occurs along the East coast of Australia, from Queensland to Victoria. It exhibits sexual dimorphism with an enlarged prominent chela that is typically larger in males and breeds seasonally, with ovigerous females seen mainly in warmer months [27,34,35]. T. australiensis is regarded as an “ecosystem engineer”, as its burrowing activities significantly affect water quality and the diversity of surrounding flora and fauna [22,36]. Firstly, we used burrow casting techniques and 3D imagery to determine the typical burrow morphology of T. australiensis at two locations to quantify burrow morphology and infer how shrimp create and use their burrows. Secondly, we conducted laboratory-based behavioural observations in custom-made cuvette aquaria to quantify the behavioural time budgets of individual shrimp in relation to sex, size, and time of day. For burrow morphology, we predicted that there would be differences between sites, given that previous studies show environmental and substrate variations and our selected sites vary in terms of tidal range and sediment profile [19,37,38]. For burrowing behaviour, we predicted that (a) shrimp would invest more time into burrow maintenance behaviours than other behaviours, (b) burrowing behaviour would vary by time of day and shrimp size, and (c) males and females would invest similar amounts of time in burrow maintenance.

2. Materials and Methods

2.1. Study Sites

This study was conducted at Port Hacking (34°04′37.2″ S 151°07′49.8″ E) and Shoalhaven Heads (34°51′27.4″ S 150°44′52.1″ E), New South Wales, Australia. Port Hacking, a drowned river valley, is characterised by a full tidal range and large flood tide deltas of shelf sand [37,38]. In contrast, the Shoalhaven River, a mature barrier estuary, has narrower inlets, a smaller tidal range, and typically finer sediment due to its location behind wave-deposited beach sands [37,38]. Both locations provided ideal intertidal zones for sampling that encompassed dense populations of the study species, Trypaea australiensis [35].

2.2. Burrow Morphology

Between July 2020 and July 2022, 30 cm2 quadrats were haphazardly placed over several clusters of burrow openings located around the mid-tide mark (including the lower and upper reaches of the high and low tide marks, respectively) at each field site commencing at low tide. Low tide was selected as the appropriate time for sampling, as it reduced the level of water in the burrows, allowing adequate time for the resin to set. Furthermore, low tide allowed greater access to burrows that would otherwise be inundated with water and logistically impossible to sample. An araldite resin kit (Renlam Kit K 3600, Meury Enterprises PTY LTD, Mulgrave, VIC, Australia) was used to make casts of the burrows within each quadrat, with the resin and hardener mixed in a 3:1 ratio, respectively. Once the resin mix had slightly thickened, it was poured into each of the burrow openings in the quadrat until they were full. This resin was then allowed a minimum of 36 h to set before excavation of the casts was conducted by hand and shovels. The excavated casts (N = 10 cast from Port Hacking and N = 9 from Shoalhaven Heads) that were intact and not missing any burrow sections were rinsed with water and transported to the University of Wollongong (UOW) laboratory, where excess sediment adhered to the outer walls of the casts was gently removed using an Ozito 170 W Rotary Tool (Ozito Industries, Bangholme, Australia). To measure the casts, 3D scans of each burrow cast were taken using an Artec Leo handheld scanner (Artec 3D, Senningerberg, Luxembourg) (Figure 1A). The Artec Leo scanner creates 3D models by projecting light or lasers onto an object and capturing the reflections. As the resin was translucent and shiny in some areas of our casts, we applied white matte spray paint as per the Artec Leo user guidelines to ensure all detail was captured without errors. The scanner was moved slowly (enough to ensure overlapping image capture at a frame rate of around 80 frames per second) around the suspended cast to capture all sides, with the live preview on the touchscreen ensuring complete coverage (Figure 1A). Once all angles were covered, the scan was stopped, and the device processed and merged the data into a detailed 3D model.
Each scan was uploaded to Artec Studio 17 (Artec 3D, Senningerberg, Luxembourg) where background noise (outliers) was removed, and alignment was performed. Initial alignment automatically positioned the scans by matching overlapping features. Fine registration was then applied to precisely adjust the scans, ensuring a seamless fit before they were merged into a single 3D model. Measurement tools within the same software were then used to record various measurements of length (mm), perimeter (mm) and geodesic distance (mm) across the entirety of the casts (Figure 1B). Geodesic distances were calculated from two to three given points specified on the scan, showing the length of the shortest path over the surface of the burrow. Measurements were later converted to cm.

2.3. Quantifying Burrowing Behaviour

At Shoalhaven Heads only, shrimp (N = 27) were collected from their burrows using nipper pumps and measured and sexed as described in [35]. Individuals were then collected and transported back to the Ecological Research Centre (ERC) at UOW. To transport the shrimp, each was placed in a separate plastic container inside a larger insulated container. Each small container included some sediment from the study site, providing a burrowing option to minimise stress during transit. The insulated container was filled with seawater, equipped with a battery-powered aerator for oxygenation, and a thermometer was added to monitor the temperature.
A marine aquaria rack at the ERC consisting of a recirculating system with eight 50-litre glass aquaria was used for the laboratory trials to quantify burrowing behaviour. Each rack consisted of 8 interconnected aquaria (60 cm × 30 cm × 30 cm) in which three custom-sized glass “cuvette” tanks and up to four sediment-filled plastic containers (dimensions) were placed (Figure 2A). These plastic containers were used as holding containers for shrimp until burrowing trials commenced (Figure 2B, Table A1). Each cuvette tank was 20 cm × 20 cm in length and height, but they differed in depth from 2 cm to 6 cm (Figure 2C). This ensured that they could be matched to the appropriate shrimp of different sizes, enabling the shrimp to be clearly observed (Table A1). Sediment collected from the field site was added to each cuvette until up to ~15 cm high (Figure 2C). The cuvette tanks were then left submerged in the larger aquaria for 2–3 weeks to allow the sediment to settle and stabilise [33]. After the sediment had stabilised, a single shrimp was added to two of the sediment-filled cuvettes and the shrimp were given ~2–3 weeks to create a burrow that was optimal for viewing behaviours (i.e., where the main section of the burrow was visible and/or enough sections were visible). If there an insufficient amount of burrow was visible, the shrimp was removed and placed into another settled cuvette to burrow again for a 2–3-week period. The third cuvette was kept empty for sediment settling, so there was always an extra cuvette ready for the addition of a new shrimp.
Once each resident shrimp had made a visible burrow within its designated cuvette tank, two observations were made: one during the day between 0800 and 1600, and one during the night after dusk. For each resident shrimp, burrowing behaviours were recorded with a Kodak PlaySport camera (during the day) and an infra-red Wolfcub 4 × 40 digital monocular (for night trials) for 30 min. Behavioural scoring was conducted in person using a previously established ethogram (for a similar species) [33] and later verified by scoring the recorded video. After the 30 min period, the percentage of time spent performing each behaviour(s) was calculated. The time spent(s) hidden from view was also recorded and later calculated as the percentage of time spent visible. Any trials falling below 50% time spent visible were repeated (with the same shrimp) to obtain more accurate results.

2.4. Statistical Analysis

All analyses were conducted using R v3.6.3 and RStudio v1.4.1106 [39,40], using the standard statistical packages for Generalised Linear Models (GLMs), as well as the package Vegan for multivariate analyses [39,41].
To explore potential differences in burrow morphology between the two sites, four GLMs were conducted on the same set of cast measurements to examine differences in morphological variables between the sample sites of adult burrows. The morphological cast measurements included as response variables were (1) maximum width (cm), (2) geodesic depth (cm), (3) geodesic u-section length (cm) and (4) tunnel width (cm), with the site as the predictor variable in all cases (Figure 1B). To further visualise and summarise the data, the means, standard errors (±SEs), and ranges for each morphological variable were calculated. Violin plots were then used to graphically display the trends and variability between the sites.
To assess the difference in time spent exhibiting burrowing behaviours, we began by condensing our initial 12 recorded behaviours [33] into 5 general behavioural categories. These included burrow maintenance [tamping (Ta), stirring (St), pumping (P), carrying (C), dropping (D) and bulldozering (B)]; locomotion [turning (T) and walking (W)]; grooming (G); sitting (S); and ventilation (V).
Then, to determine whether the combination of behaviours exhibited by shrimp was associated with the time of day, sex, or size category, we used a PERMANOVA to examine differences in the behavioural assemblages depending on the 3 associated predictors. The Bray–Curtis index of dissimilarity was employed for these analyses. Both untransformed and square root-transformed datasets were utilised for multivariate analyses. Transformed data were used to reduce bias from dominant behaviours that could potentially overshadow other patterns in the data. It is important to note that given that the findings for the effect of time of day were not significant (p < 0.05), the PERMANOVA analyses regarding sex and size were conducted only based on daytime observations, with means and standard errors (±SE) calculated and reported for these observations. Daytime observations were used, as PERMANOVA analyses would not allow for a paired samples analysis if the data were pooled. For any significant findings, further SIMPER tests were conducted to determine which behaviours contributed most to the differences in predictor variables. Additionally, an nMDS plot was used to show the relationships between shrimp behaviours, where the ellipses represent a 95% confidence interval for each size category.

3. Results

3.1. Burrow Cast Morphology

Overall, 19 complete casts (15 adults and 4 juveniles) were excavated and deemed optimal for measuring, including 10 casts from Port Hacking and 9 casts from Shoalhaven Heads. The burrow morphology of Trypaea australiensis at both sites comprised the typical U-shaped burrow (Figure 3), though there were differences concerning burrow sizes. For both Shoalhaven Heads and Port Hacking, larger burrows (presumably adult burrows) comprised a typical U-shaped top section (two openings) while smaller burrows (presumably juvenile burrows) had only one opening (Figure 3). From the bottom of the U-shaped section of the burrow, the remaining main tunnel descended/spiralled with occasional peripheral tunnels adjacent to the many turning chambers along this section. Notably, one cast excavated from Port Hacking showed connections between three burrows (one juvenile burrow and two adult burrows) (Figure 3A).
The mean maximum width of adult burrows at Port Hacking (mean length: 16.22 cm, ±SE: 1.20 cm, range: 10.07 cm–21.39 cm) was less than that of Shoalhaven Heads (mean length: 24.27 cm, ±SE: 4.20 cm, range: 10.46 cm–47.10 cm) (Figure 4A). However, when examining predictors of burrow morphology, site had no significant effect on maximum burrow width (X2 = 241.66, df = 1, p = 0.07) (Figure 4A).
The site had a significant effect on tunnel width (X2 = 0.49, df = 1, p = 0.019). The mean tunnel width for Port Hacking (mean length: 1.11 cm, ±SE: 0.057 cm, range: 0.78 cm–1.31 cm) was significantly smaller than that of Shoalhaven Heads (mean: 1.47 cm, ±SE: 0.14 cm, range: 0.87 cm–2.04 cm) (Figure 4B).
The mean geodesic depth (mean: 54.77 cm, ±SE: 3.68 cm, range 38.29 cm–71.22 cm) (Figure 4C) and geodesic u-section length (mean: 27.13 cm, ±SE: 3.83 cm, range 12.46 cm–40.37 cm) (Figure 4D) were lower at Port Hacking that at Shoalhaven Heads (geodesic depth, mean: 67.19 cm, ±SE: 8.67 cm, range: 43.10 cm–111.42 cm) (Figure 4C) (geodesic u-section length, mean: 29.62 cm, ±SE: 3.85 cm, range: 20.28 cm–52.07 cm) (Figure 4D). However, there was no difference in geodesic depth (X2 = 575.25, df = 1, p = 0.20) (Figure 4C) or geodesic u-section length (X2 = 23.21, df = 1, p = 0.67) between the two sites (Figure 4D).

3.2. Burrowing Behaviour

A total of 27 shrimp were observed for behavioural trials, with each individual being recorded during both the day and night (N = 54 observations). Multivariate analyses showed that time of day was not related to the assemblage of behaviours exhibited by shrimp in their burrows for both untransformed (Pseudo-F = 1.10, df = 1, p = 0.34) and square root-transformed data (Pseudo-F = 0.69, df = 1, p = 0.54). As the effect of time of day was not significant, only the daytime behaviour of the shrimp was considered. When considering day observations, it was found that shrimp spent most of their time on burrow maintenance (mean: 28.95% ±SE: 2.36), closely followed by ventilating (mean: 24.02%, ±SE: 3.13) and sitting (mean: 23.76%, ±SE: 2.87) within the burrow (Figure 5).
When considering the effects of sex and size (daytime observations only, N = 27), PERMANOVA analyses showed that the behavioural assemblage exhibited by shrimp within burrows did not vary between males and females for both untransformed (Pseudo-F = 0.62, df = 1, p = 0.59) and square root-transformed data (Pseudo-F = 0.54, df = 1, p = 0.63), or untransformed data for size (Pseudo-F = 2.43, df = 1, p = 0.089). However, these behaviours did differ between small and large shrimp for transformed data only (Pseudo-F = 3.10, df = 1, p = 0.046) (Figure 6 and Figure 7). Moreover, SIMPER showed that ventilating behaviour contributed the most to the difference between the two size categories, followed closely by sitting (Table 1), with small shrimp spending more time sitting and less time ventilating compared to large shrimp (Figure 7).

4. Discussion

This study investigated the burrow morphology and burrowing behaviour of Trypaea australiensis. We quantified burrow morphology in situ using resin casts and 3D imagery at two locations, finding that burrows at Shoalhaven Heads were larger and more variable than those at Port Hacking. Laboratory observations revealed that shrimp spend significant time maintaining burrows, with the time spent ventilating and sitting behaviours differing between small and large individuals. This study provides the first quantitative insights into the importance of burrow habitats for T. australiensis by demonstrating significant variation in burrow morphology and characterising key burrowing behaviours.
Overall, burrows at Port Hacking and Shoalhaven Heads were similar in size and shape. Specifically, the casts from each site were characterised by a U-shaped upper section with a spiral-like tunnel structure connected to the base of the U-shape. This overall burrow structure found at both Port Hacking and Shoalhaven Heads is similar to that of other reported thalassinid species [15,17] and may therefore indicate a universality in burrow morphologies within the family. However, variation in burrow morphology can be seen at higher-order levels, such as between decapod crustaceans, rather than intraspecific differences. Much of the design differences relate to the feeding needs of a given species [42,43]. Feeding ultimately dictates the space needed for shrimp to feed either by deposit feeding (constant burrowing/working of sediment), filter/suspension feeding (needing flow-through chambers to collect particulate in the water column), or as drift catchers (actively collecting food that drifts past the burrow opening) [12,42]. Therefore, it is likely that T. australiensis shows similar morphologies across sites, given the similar feeding needs of this species.
When considering site comparisons of burrow morphology, it was evident that variation in morphology was greater at the Shoalhaven Heads site than at the Port Hacking site. It has been suggested that nutrient densities can directly impact the burrow morphology of these burrowing shrimp [17]. As seen in Callianassa filholi, a decrease in the depth and width of burrows was correlated with an increase in organic content within the sediment [29]. Our results suggest that nutrient density at Shoalhaven Heads was more variable than at Port Hacking, as indicated by the greater variability in burrow morphology. As deposit feeders, T. australiensis obtains their nutrients from the sediment while excavating [12,15], burrowing continuously to access food [12,13,28]. Therefore, burrow structures could, to some degree, reflect levels of burrowing activity and the availability of nutrients. Future studies may therefore aim to quantify nutrients within the sediments at both sites to examine this possibility.
Of all the burrow morphology measurements taken, only tunnel width varied between sites. Tunnel width is likely positively related to shrimp size [15] and reflects site-specific constraints [35], with variation in the tunnel width potentially linked to environmental factors (such as sediment type and availability), that may influence the ability of shrimp to construct burrows of different sizes [15,35]. Additionally, the lack of significant differences in the other morphological variables may be due to the highly conserved nature of burrow morphology in general, or limited variation in environmental conditions between sites (owing to close proximity), which would further suggest that environmental factors, such as sediment composition, play a significant role in burrow structure. Therefore, future work should consider the role of shrimp size in combination with environmental factors on burrow morphology, as well as increasing the sample sizes and spatial range of burrow casts.
Analysis of shrimp burrowing behaviours revealed that the time of day did not significantly influence the amount of time spent on any of the behavioural categories measured, contrary to initial predictions. Time of day was initially expected to influence behaviours, as a previous study indicated the potential for active movement outside the burrow to find a mate [35]. More specifically, it was expected that shrimp would likely be more active during the night, as darkness would reduce the risk of predation from visual predators. Hence, the lack of effect of time of day on behaviour was surprising and indicates that shrimp are unaffected by diurnal changes in light. Additionally, our burrow casts revealed connections between burrows, which may help explain the absence of differences between day and night behaviour, as shrimp seldom leave their burrows. Instead, it is possible that tides (rather than times) may have a stronger effect on behaviours, as shrimp may be more attuned to water level rather than light changes [44,45,46]. Indeed, intertidal organisms experience significant abiotic fluctuations due to tides, as tidal cycles increase oxygenation at high tides and deplete it at low tides [45,46]. As seen in other studies, shrimp (Neotrypaea uncinate) have been shown to increase ventilation behaviour during low tide to oxygenate their burrows to resist hypoxic stress [45,46,47,48]. As such, further studies should consider the effects of temperature and tidal fluctuations on burrow behaviours.
Males and females spent similar amounts of time on each of the five burrowing behaviours, which is in line with our initial predictions and likely related to their social structure. Previous studies have shown that these shrimp predominantly live solitary lives with only temporary bouts of social interaction [35,49]. Given their solitary nature, both males and females are likely to invest similar amounts of time maintaining their burrows, ensuring oxygen levels are adequate and maximising growth and body condition through grooming and food acquisition. In contrast, shrimp that live predominantly in pairs or groups have been shown to exhibit a division of labour, with time budgets varying substantially between the sexes or different group members. For example, eusocial marine snapping shrimp (Synapheus spp.) live within the canals of sponges [50]. Individuals reside in large groups where there is a division of labour in terms of defence and offspring care [50,51,52,53]. Typically, one queen and her mate are the reproductive individuals for the group [52,53], while the remaining group members spend most of their time on parental care and colony defences [50,51,52,53].
Unlike sex, the time spent exhibiting different behaviours did differ with shrimp size. Specifically, we found that smaller shrimp spent less time ventilating their burrows and more time sitting on average, compared to larger individuals. Although we did not make specific predictions regarding size effects, the fact that smaller shrimp spent less time ventilating their burrows is not entirely surprising for several reasons. Firstly, smaller shrimp, specifically juveniles, have been observed to connect their burrows to an adjacent adult burrow [16,35]. Therefore, smaller juveniles may be benefiting from utilising the ventilation conducted by the neighbouring adult, reducing the need for them to ventilate their burrows. Secondly, the juvenile burrows found in the current study only had a single burrow opening, which has been confirmed by previous studies [17]. This single opening would make it more difficult to expel and ventilate the burrow, as one opening hinders the water flow throughout the burrow [13,47]. Therefore, the costs of ventilating frequently may be too high for smaller individuals. Thirdly, smaller individuals have less need to ventilate if they have lower metabolic rates and reduced oxygen demand [54]. Therefore, future studies should monitor the ventilation behaviour and metabolic rates of shrimp of varying sizes under different levels of environmental stressors (i.e., hypoxia), to fully understand this size effect.
The fact that burrow maintenance did not differ with time of day, males and females, or large or small individuals, combined with our pooled data showing that shrimp spent the largest portion of their time on burrow maintenance, suggests that maintaining burrows is exceptionally important for T. australiensis. As mentioned previously, T. australiensis individuals need their burrows for protection from predation and to access their primary food sources (located in the sediment), hence the need for burrowing exceeds the associated costs of ongoing burrow maintenance (namely time and effort on excavation) [17,28]. Burrow maintenance may need to be ongoing owing to the fluctuating intertidal environment, where burrows may constantly be inundated and gain access to food. Furthermore, from the apparent burrow connections we observed, we hypothesise that T. australiensis will avoid leaving their burrows, even to find mates. The potentially high risk of predation from many predators, and the energy required to rebuild a burrow [48], likely far outweigh the costs of maintaining their current burrow.
T. australiensis also spent a considerable amount of time ventilating and sitting in their burrows. As burrow maintenance involves the moving, lifting, sorting and compacting of sediment, it is understandable that T. australiensis could be spending just as much time conserving energy by sitting and ventilating to increase available oxygen within the burrow. It is well known that these burrow habitats are anoxic in nature [22,55], and burrowing shrimp species have adapted to live successfully under these conditions by ensuring ventilation [55]. In addition, previous studies have shown that burrowing shrimp typically have low rates of oxygen consumption [56] and supplement oxygen levels in the water via their feeding strategies (namely deposit and resuspension feeding) and ventilating and pumping behaviours [15,55,57]. Therefore, it is reasonable to expect that the ventilation of a given burrow is just as important for their survival as processing sediment for the maintenance of burrow walls.
Ultimately, this study has demonstrated that T. australiensis expends substantial time in creating and maintaining complex burrows, as well as ventilating the burrows, and hence quantifies the importance of burrows for shrimp. The time and energy dedicated to burrow construction and ventilation emphasise how vital these structures are for maintaining stable oxygen levels and reducing exposure to predators and environmental stressors like hypoxia. This underscores the need for effective management strategies to protect these essential habitats. Overfishing, particularly through practices that damage or destroy burrows, can severely disrupt these delicate ecosystems, threatening shrimp populations and the broader intertidal community. Given the importance of burrow integrity for shrimp survival, managing and preventing overfishing alongside habitat protection should be a priority to ensure the long-term stability of these species and their ecosystems. Future research could investigate the effects of shrimp size on ventilation and burrow behaviour, as well as how different environmental stressors, such as temperature fluctuations, hypoxia and sediment profiles, affect shrimp across various size classes. This would provide a deeper understanding of the adaptive responses of shrimp to environmental changes, supporting the case for sustainable fishing practices and more comprehensive management efforts.

Author Contributions

R.L.K.: study concept and design, data collection, statistical analysis, writing, reviewing and editing, M.Y.L.W.: study concept and design, supervision, reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was generously funded by the Department of Primary Industries—Recreational Fisheries Trust (Project LS048). R.K. was supported by an Australian Government Research Training Program Scholarship.

Institutional Review Board Statement

No approval of research ethics was required, as experimental work was conducted with an invertebrate species not regulated by the University of Wollongong ethics committee. However, the authors declare that all necessary approvals for the sampling of specimens have been obtained and conducted under a Fisheries New South Wales permit (permit no. F95/269-8.2), ensuring animal welfare and correct sampling were completed.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original data presented in the study are openly available in Knowledge Network for Biocomplexity (KNB) at https://doi.org/10.5063/F1QZ28GZ.

Acknowledgments

The field trips were conducted with respect for the traditional owners past and present of the Eora and Tharawal people. We would like to thank the following people for helping with fieldwork at various times during this project: Kurtis Croucher, Brett Kirby, Katelyn Kirby. Thank you to the UOW Ecological Research Centre for use of the marine aquaria system for our laboratory trials, and to Joshua Mattock from the UOW Biomechanics Research Laboratory for their assistance with the 3D scanning of our resin casts. Last, but not least, thank you to Courtney Hildebrandt for helping with data visualisation. We would also like to thank the anonymous reviewers and editor for their guidance and constructive feedback.

Conflicts of Interest

All authors have no competing interests to declare that are relevant to the content of this article.

Abbreviations

The following abbreviations are used in this manuscript:
GLMGeneralised Linear Model.
PERMANOVAPermutational Multivariate Analysis of Variance.
PHPort Hacking.
SHShoalhaven Heads.
SIMPERSimilarity Percentages Analysis.

Appendix A

Table A1. Cuvette tank dimensions (cm) and corresponding resident shrimp size ranges (cm).
Table A1. Cuvette tank dimensions (cm) and corresponding resident shrimp size ranges (cm).
Cuvette Tank DimensionsResident Shrimp Size
20 cm length × 20 cm height × 2 cm depthLess than 3 cm
20 cm length × 20 cm height × 4 cm depthBetween 3 cm and 4 cm
20 cm length × 20 cm height × 5 cm depthBetween 4 cm and 5 cm
20 cm length × 20 cm height × 6 cm depthGreater than 5 cm

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Jmse 13 00432 g001
Figure 1. (A) Diagram of apparatus used to suspend the resin casts. To the left, the handheld Leo scanner in operation, taking a 360° scan of the resin cast; (B) diagram of burrow morphology measurements taken of a typical adult T. australiensis burrow: geodesic u-section length (A—green), tunnel width (B—red), geodesic depth (C—purple), and maximum cast width (D—black).
Figure 1. (A) Diagram of apparatus used to suspend the resin casts. To the left, the handheld Leo scanner in operation, taking a 360° scan of the resin cast; (B) diagram of burrow morphology measurements taken of a typical adult T. australiensis burrow: geodesic u-section length (A—green), tunnel width (B—red), geodesic depth (C—purple), and maximum cast width (D—black).
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Figure 2. (A) Diagram of the recirculating marine aquarium tank setup (at the UOW Ecological Research Centre) used to house the shrimp. (B) One of the eight 50 L aquariums used for behavioural trials, featuring three cuvettes with sediment for resident shrimp and several smaller containers housing shrimp for subsequent trials. (C) Diagram of the custom cuvettes designed to observe T. australiensis behaviour, with varying widths to match shrimp sizes while keeping the height and length constant.
Figure 2. (A) Diagram of the recirculating marine aquarium tank setup (at the UOW Ecological Research Centre) used to house the shrimp. (B) One of the eight 50 L aquariums used for behavioural trials, featuring three cuvettes with sediment for resident shrimp and several smaller containers housing shrimp for subsequent trials. (C) Diagram of the custom cuvettes designed to observe T. australiensis behaviour, with varying widths to match shrimp sizes while keeping the height and length constant.
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Figure 3. Typical burrow morphology of T. australiensis. (A) Field photo of excavated burrow cast of two adult burrows and one juvenile burrow, (B) 3D scan of an adult burrow cast, and (C) 3D scan of a juvenile burrow cast.
Figure 3. Typical burrow morphology of T. australiensis. (A) Field photo of excavated burrow cast of two adult burrows and one juvenile burrow, (B) 3D scan of an adult burrow cast, and (C) 3D scan of a juvenile burrow cast.
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Figure 4. Violin plots showing the distribution of response variables by site. Port Hacking is denoted by “PH” and Shoalhaven Heads “SH”. (A) Max width (cm), (B) tunnel width (cm), (C) geodesic depth (cm), and (D) geodesic u-section length (cm) across sites for adult shrimp burrows, illustrating the variation in each variable by site. The centre dot indicates the mean of each distribution.
Figure 4. Violin plots showing the distribution of response variables by site. Port Hacking is denoted by “PH” and Shoalhaven Heads “SH”. (A) Max width (cm), (B) tunnel width (cm), (C) geodesic depth (cm), and (D) geodesic u-section length (cm) across sites for adult shrimp burrows, illustrating the variation in each variable by site. The centre dot indicates the mean of each distribution.
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Figure 5. Shrimp behaviours exhibited during the day and night within the burrow under laboratory conditions. Burrow maintenance includes tamping (Ta), stirring (St), pumping (P), carrying (C), dropping (D) and bulldozering (B). Locomotion involves turning (T) and walking (W) behaviours (N = 27).
Figure 5. Shrimp behaviours exhibited during the day and night within the burrow under laboratory conditions. Burrow maintenance includes tamping (Ta), stirring (St), pumping (P), carrying (C), dropping (D) and bulldozering (B). Locomotion involves turning (T) and walking (W) behaviours (N = 27).
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Figure 6. nMDS plot showing the relationship between samples, with points coloured by size category. Large (L) categorised points are blue, and points categorised as small (S) points are pink. The ellipses represent the 95% confidence intervals for each size category.
Figure 6. nMDS plot showing the relationship between samples, with points coloured by size category. Large (L) categorised points are blue, and points categorised as small (S) points are pink. The ellipses represent the 95% confidence intervals for each size category.
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Figure 7. Graph of average time spent exhibiting solitary behaviours, considering the differences between small and large categorised shrimp (untransformed data).
Figure 7. Graph of average time spent exhibiting solitary behaviours, considering the differences between small and large categorised shrimp (untransformed data).
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Table 1. SIMPER analysis of transformed data considering size categories (large (L) and small (S)) across five behavioural categories. The table provides the average dissimilarity (Average), standard deviation (SD), the dissimilarity ratio, and the average values for each behaviour in the “large” (Ava) and “small” (Avb) size categories. It also includes the cumulative contribution (Cumsum) to the overall dissimilarity and the p-values (p) for each behavioural category.
Table 1. SIMPER analysis of transformed data considering size categories (large (L) and small (S)) across five behavioural categories. The table provides the average dissimilarity (Average), standard deviation (SD), the dissimilarity ratio, and the average values for each behaviour in the “large” (Ava) and “small” (Avb) size categories. It also includes the cumulative contribution (Cumsum) to the overall dissimilarity and the p-values (p) for each behavioural category.
AverageSDRatioAva AvbCumsump
Ventilating0.0730.0511.442.854.870.290.05
Sitting0.0640.0491.295.824.280.550.17
Burrow Maintenance0.0550.0441.265.165.430.770.21
Grooming0.0400.0281.413.042.680.930.67
Locomotion0.0180.0150.233.543.491.000.30
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Kirby, R.L.; Wong, M.Y.L. Digging in Deep: Size and Site-Specific Variation in Burrow Morphology and Behaviour of the Mud Shrimp, Trypaea australiensis Dana, 1852. J. Mar. Sci. Eng. 2025, 13, 432. https://doi.org/10.3390/jmse13030432

AMA Style

Kirby RL, Wong MYL. Digging in Deep: Size and Site-Specific Variation in Burrow Morphology and Behaviour of the Mud Shrimp, Trypaea australiensis Dana, 1852. Journal of Marine Science and Engineering. 2025; 13(3):432. https://doi.org/10.3390/jmse13030432

Chicago/Turabian Style

Kirby, Renae L., and Marian Y. L. Wong. 2025. "Digging in Deep: Size and Site-Specific Variation in Burrow Morphology and Behaviour of the Mud Shrimp, Trypaea australiensis Dana, 1852" Journal of Marine Science and Engineering 13, no. 3: 432. https://doi.org/10.3390/jmse13030432

APA Style

Kirby, R. L., & Wong, M. Y. L. (2025). Digging in Deep: Size and Site-Specific Variation in Burrow Morphology and Behaviour of the Mud Shrimp, Trypaea australiensis Dana, 1852. Journal of Marine Science and Engineering, 13(3), 432. https://doi.org/10.3390/jmse13030432

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