The burrow structure of Minuca osa (Brachyura: Ocypodidae) from the eastern Montijo Gulf, Panamanian Pacific

Abstract

Between November 2022 and March 2023, thirty-seven Minuca osa burrow plaster casts were poured while simultaneously collecting biometric data of the occupants in Ponuga, eastern Montijo Gulf, Panamanian Pacific. Casts revealed a highly variable structure with straight and spiral sections, reaching depths down to 122 cm (mean ± SD = 73.20 ± 28.66 cm). Burrow depth and length did not differ between sexes; however, males exhibited larger burrow diameter (25.9 ± 4.61 mm) compared to females (19.48 ± 1.65 mm; Mann-Whitney, P < 0.001). Male carapace width (23.4 ± 2.15 mm) surpassed that of females (18.42 ± 1.73 mm; Mann‐Whitney, P < 0.001). Female carapace length (r ² = 0.793) and male chela length (r ² = 0.769) were correlated to diameter. This study presents the first description of M. osa burrow structure, providing valuable insights into this understudied species.

Keywords:
Burrow diameter; carapace; fiddler crabs; mangrove; plaster cast; water table

Due to their burrowing activity, fiddler crabs play a crucial role in sediment mixing and bioturbation, with substantial implications for various soil physicochemical processes, thereby influencing a diverse array of species (Kristensen, 2008; Aschenbroich et al., 2016; Agusto et al., 2021). In general, fiddler crab burrows are an essential resource that serves as refuge from predators, a source of water during low tide, and a site for reproduction (Mautz et al., 2011; Pardo et al., 2020).

Minuca osa (Landstorfer and Schubart, 2010) is a burrow‐building fiddler crab species. It was originally described in Golfo Dulce on the Pacific coast of Costa Rica (Landstorfer and Schubart, 2010) and recently reported in the Ponuga River, Panamanian Pacific (Lombardo, 2022). Observations indicate that M. osa constructs burrows in high-mangrove areas with sandy-muddy sediments. These are maintained and defended by the resident crabs (Lombardo, 2023). While burrow structure has been studied in different fiddler crab species (see Qureshi and Saher, 2012; Sen and Homechaudhuri, 2016; Chen et al., 2017; Min et al., 2023), the internal structure of M. osa burrows remains unknown. Thus, the objective of this study was to examine the structure of M. osa burrows using replica casting techniques to characterize their features.

The study site is located in the Ponuga River (07°51'51"N 81°0'52"W) in Veraguas, Panamanian Pacific. During rainy season (April-December), the substrate is covered by the highest monthly tides and sequentially exposed during low tide. During the dry season (January-April), the river level drops and flooding is infrequent (Lombardo, 2022; 2023). From November 2022 to March 2023, 27 focal males and 10 females were selected using binoculars (Bushnell 10×42 mm). Focal individuals were identified by their size and coloration (Lombardo, 2022: fig. 1b, c), as well as their digging activity. Observation of M. osa individuals coincided with diurnal ebbing tides to facilitate burrow ownership assessment while crabs first emerged. The casting mixture consisted of plaster gypsum and water in a 1:0.5 ratio. The mixture was poured slowly, ensuring it filled the burrow completely, forming a solid cast, without voids. During pouring of the plaster mixture, any crabs attempting to escape from the burrow were captured by hand. Crab carapace width (CW), length (CL), chela length (ChL), height (ChH), and total weight (TW) were measured using a Vernier caliper (0.01 mm) and a digital scale (0.01g). Processed crabs were kept for reference in 4% formaldehyde solution, and burrow casts were left to cure for four days. During cast excavation, soil profiles were exposed, allowing the cast to rest on the sediment. Soil profile details such as the total burrow depth (TBD) and water table depth (WTD) were inspected and recorded. In the lab, the burrow length (BL) was registered with a flexible measure tape. As diameter within casts varied along their length, the site for measuring diameter was standardized by dividing each cast into four segments according to their BL (neck = NK, second and third sections = S2 and S3, and end = EN; Fig. 1). Burrow morphology was compared based on sex, size, and rainy (Nov-Dec) versus dry (Jan-Mar) seasons using the Mann-Whitney and Moods median tests, while simple regression was applied to explore the relationships between burrow diameter, WTD, TBD, BL, and crab biometrics.

Figure 1.
Minuca osa burrow structure from Ponuga, Veraguas, Panama. A. Female burrow length (BL; dotted line, 97 cm). B. Male total burrow depth (TBD, solid vertical bar, 104 cm). Sections: neck = NK, second S2 and third S3 sections, and end = EN. C,D. Detail of male burrow variation with chamber (scale = 55 mm) and funnel shape (scale = 60 mm). Male and female crab scale bars are 20 mm and 15 mm, respectively.

Seven male and two female burrow casts were unrecoverable, and one male and one female could not be captured for biometry. Twenty-six complete burrow casts revealed a structure with straight and spiral sections, reaching depths down to 122 cm. Burrow depth and water table depth were positively correlated in both sexes (♂, r ² = 0.964, F _(1,12) = 350.92, P < 0.001; ♀, r ² = 0.954, F _(1,4) = 104.53, P = 0.001). The males exhibited greater size and weight compared to females (Tab. 1). There was no statistical difference among burrow section diameters (Moods median test, d.f. = 3, χ² = 4.80, P = 0.187); however, male burrow cast sections NK, S2, and S3 exhibited wider diameters compared to the females. In the dry season, the water table was farther away from the surface (dry = 93 cm, rainy = 72 cm, U = 108, P = 0.004), and the burrows were deeper compared to the rainy season (dry = 92 cm, rainy = 59 cm, U = 122.5, P = 0.028); no differences between medians were detected in the rest of the burrow features versus season. In contrast, there was no difference in WTD, TBD, BL, or EN between male and female burrows (Tab. 2). Five male biometric variables demonstrated a positive correlation with cast diameter (S2 and NK). Among the females, only CW and CL exhibited significant association with burrow diameter S2 (Fig. 2). Biometric variables showed no discernible correlations with WTD, TBD, or BL (Tab. 3).

Table 1.
Descriptive statistics and comparison of biometric variables between sexes in Minuca osa from Ponuga, Veraguas, Panamanian Pacific. Carapace width (CW), length (CL), chela length (ChL), height (ChH), and total weight (TW). All measurements are reported in millimeters, except for total weight (TW) which is reported in grams.

Biometric variable	♂	♀	Mann‐Whitney test
	Median ± SD Min.-Max.	Median ± SD Min.-Max.
CW	22.99 ± 1.99; 18.64-27.61	18.84 ± 1.31; 17.02-20.95	U = 342, P = 0.001
CL	15.45 ± 1.07; 13.09-17.51	13.21 ± 0.74; 12.63-14.84	U = 260, P = 0.003
TW	5.76 ± 1.29; 2.91-8.14	2.73 ± 0.45; 2.10-3.33	U = 270, P < 0.001
ChL	35.44 ± 6.47; 19.51-43.56
ChH	12.85 ± 1.32; 8.71-15.08

Table 2.
Comparison of burrow features between male and female Minuca osa from Ponuga, Veraguas, Panamanian Pacific. Water table depth (WTD), total burrow depth (TBD), burrow length (BL), burrow diameters: neck (NE), second section diameter (S2), third section diameter (S3), and end diameter (EN). Depth and length are given in centimeters, while diameter is in millimeters.

Burrow features	♂	♀	Mann‐Whitney test
	Median ± SD Min.-Max.	Median ± SD Min.-Max.
WTD	80.3 ± 23.95; 18-120	81.75 ± 29.02; 30-115	U = 286, P = 0.859
TBD	74.6 ± 28.06; 18-122	67.3 ± 32.44; 27.80-114	U = 304.5, P = 0.476
BL	79.47 ± 29.03; 24.1-119.4	69.71 ± 34.05; 24.5-113.2	U = 303, P = 0.525
NE	27.18 ± 5.72; 15.95-40.19	20.08 ± 2.36; 17.3-24.61	U = 352, P = 0.002
S2	25.9 ± 4.61; 19.38-35.23	19.48 ± 1.65; 16.93-22.06	U = 360.5, P < 0.001
S3	26.23 ± 6.45; 17.83-39.27	20.04 ± 4.26; 15.04-27.56	U = 338, P = 0.016
EN	24.13 ± 5.32; 13.72-34.23	21.83 ± 2.55; 18.69-25.58	U = 313, P = 0.252

Table 3.
Relationships between burrow features and the biometry of male and female Minuca osa from Ponuga, Veraguas, Panamanian Pacific. Water table depth (WTD), total burrow depth (TBD), burrow length (BL), burrow diameters: neck (NK), second section diameter (S2), third section diameter (S3), and end diameter (EN). Carapace width (CW), length (CL), chela length (ChL), height (ChH), and total weight (TW).

Burrow features	Biometrics
	♂CW	♂CL	♂ChL	♂ChH	♂TW	♀CW	♀CL	♀TW
WTD	r2 = 0.2678 F (1,7) = 2.56 P = 0.154	r2 = 0.0502 F (1,13) = 0.69 P = 0.422	r2 = 0.210 F (1,12) = 3.19 P = 0.099	r2 = 0.072 F (1,12) = 0.94 P = 0.352	r2 = 0.2112 F (1,12) = 3.21 P = 0.098	r2 = 0.0378 F (1,4) = 0.16 P = 0.712	r2 = 0.033 F (1,4) = 0.01 P = 0.913	r2 = 0.1984 F (1,4) = 0.99 P = 0.376
TBD	r2 = 0.2806 F (1,7) = 2.73 P = 0.142	r2 = 0.1755 F (1,13) = 2.77 P = 0.120	r2 = 0.1838 F (1,12) = 2.70 P = 0.126	r2 = 0.1119 F (1,12) = 1.51 P = 0.242	r2 = 0.3943 F (1,12) = 7.81 P = 0.016	r2 = 0.2373 F (1,4) = 1.24 P = 0.327	r2 = 0.1474 F (1,4) = 0.69 P = 0.452	r2 = 0.00 F (1,4) = 0.00 P = 1.00
BL	r2 = 0.3056 F (1,7) = 3.08 P = 0.123	r2 = 0.2207 F (1,13) = 3.68 P = 0.077	r2 = 0.2283 F (1,12) = 3.55 P = 0.084	r2 = 0.2206 F (1,12) = 3.40 P = 0.090	r2 = 0.4219 F (1,12) = 8.76 P = 0.012	r2 = 0.2880 F (1,4) = 1.62 P = 0.272	r2 = 0.2086 F (1,4) = 1.05 P = 0.363	r2 = 0.049 F (1,4) = 0.02 P = 0.895
NK	r2 = 0.0136 F (1,7) = 0.10 P = 0.765	r2 = 0.3652 F (1,13) = 7.48 P = 0.017	r2 = 0.4858 F (1,12) = 11.34 P = 0.006*	r2 = 0.4307 F (1,12) = 9.08 P = 0.011	r2 = 0.3634 F (1,12) = 6.85 P = 0.002	r2 = 0.0469 F (1,4) = 0.20 P = 0.680	r2 = 0.014 F (1,4) = 0.01 P = 0.945	r2 = 0.1780 F (1,4) = 0.87 P = 0.405
S2	r2 = 0.4342 F (1,10) = 7.67 P = 0.020	r2 = 0.6050 F (1,13) = 19.91 P = 0.001*	r2 = 0.7689 F (1,12) = 39.92 P < 0.001*	r2 = 0.5773 F (1,12) = 16.39 P = 0.002*	r2 = 0.7358 F (1,12) = 33.43 P < 0.001*	r2 = 0.6770 F (1,4) = 8.38 P = 0.044*	r2 = 0.7929 F (1,4) = 15.32 P = 0.017*	r2 = 0.4565 F (1,4) = 3.36 P = 0.141
S3	r2 = 0.1606 F (1,7) = 1.34 P = 0.285	r2 = 0.2711 F (1,13) = 4.83 P = 0.047	r2 = 0.3023 F (1,12) = 5.20 P = 0.042	r2 = 0.2059 F (1,12) = 3.11 P = 0.103	r2 = 0.3218 F (1,12) = 5.69 P = 0.034	r2 = 0.1199 F (1,4) = 0.54 P = 0.501	r2 = 0.2241 F (1,4) = 1.16 P = 0.343	r2 = 0.0190 F (1,4) = 0.08 P = 0.794
EN	r2 = 0.260 F (1,7) = 0.02 P = 0.896	r2 = 0.1427 F (1,13) = 2.16 P = 0.165	r2 = 0.0149 F (1,12) = 0.18 P = 0.678	r2 = 0.1192 F (1,12) = 1.62 P = 0.227	r2 = 0.076 F (1,12) = 0.99 P = 0.339	r2 = 0.020 F (1,4) = 0.00 P = 0.979	r2 = 0.0816 F (1,4) = 0.36 P = 0.583	r2 = 0.001 F (1,4) = 0.00 P = 0.985

Figure 2.
Relationship between biometrical variables and burrow cast diameters of Minuca osa from Ponuga, Veraguas, Panama. A, B. Female and C-G. Male regressions. Burrow neck diameter (NK) and second section diameter (S2). Carapace width (CW), length (CL), chela length (ChL), height (ChH), and total weight (TW). All measurements are given in millimeters, except for total weight (TW), which is reported in grams.

The size of crabs is typically proportional to the depth and size of their burrows, with larger crabs constructing larger and more spacious burrows (Ens et al., 1993; Lim and Diong, 2004; Qureshi and Saher, 2012). This correlation might not hold entirely in M. osa because WTD, TBD, and BL seem independent of crab size and sex. Contrastingly, wider burrow diameter has been attributed to the larger carapace length to width ratios, implying such crabs require wider burrows for comfortable movement (Lim, 2006; Qureshi and Saher, 2012). Mangrove and floodplain tree root biomass decreases with soil depth. This is especially relevant since burrow segments NE and S2 are located in the upper soil layers where roots are abundant. Crabs likely adapt to such root obstacles by excavating tunnels with acute-angle turns (Dembowski, 1926; Lim and Diong, 2004); thus, a large asymmetrical claw would require wider tunnels to pass through. This seems to be the case in M. osa, particularly in male crabs, where the relationship between ChL and burrow diameter (S2) was strongest.

The moisture content of the substrate influences the depth of burrows (Dembowski, 1926) and said moisture is dependent on rain, tides and water table dynamics (Laio et al., 2009). The correlation between BL and WTD, as well as the similarities in WTD, TBD, and BL between male and female M. osa, emphasize the importance of accessing water to prevent desiccation (Thurman, 1998; Yoder et al., 2005). This is in line with M. osa adaptive behavior in response to variation in WTD, resulting in burrow depth variability between rainy and dry seasons. Similar behavior has been reported in other fiddler crabs, for example, in tidal flats with shallow WTD, crabs dig burrows 10-40 cm deep. This behavior may prevent unnecessary energy expenditure on further excavation (Kristensen, 2008; Chen et al., 2017). In contrast, when confronted with increasing WTD and greater distances from the water source, fiddler crabs tend to construct deeper burrows, 90-180 cm (Thurman, 1984; Klaassen and Ens, 1993; Chen et al., 2017; Tina et al., 2017). This behavior, possibly aimed at mitigating desiccation, provides a plausible explanation for the deep burrows constructed by M. osa, particularly in this site, where the WTD reached depths of 120 cm.

This is the first study describing the burrow structure of M. osa, where relationships were found between burrow diameter and crab phenotypic traits under sexual selection. Interestingly, burrow depth and length appear independent of crab size and sex, providing valuable insights for future research on the response to environmental fluctuation in this understudied species.

ACKNOWLEDGEMENTS

Our gratitude goes to Arilis Peralta and Héctor Santos for their assistance during field work. We also thank the anonymous reviewers for their comments to improve the manuscript.

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• Funding and grant disclosure
  There were no external funding sources for this study.
• Study association
  The research presented here was not part of the acquisition of an academic degree.
• Study permits
  The methods were compliant with Panamanian law.
• Data availability
  All data generated and analyzed during this study are presented in this article.
• Zoobank:
  http://zoobank.org:pub:5659A205-FA76-457F-8330-8F7C6C2E5FAF

Edited by

• Associate Editor:
  Tânia Marcia Costa

• Editor-in-chief:
  Christopher Tudge

Data availability

All data generated and analyzed during this study are presented in this article.

Publication Dates

• Publication in this collection
  24 Feb 2025
• Date of issue
  2025

History

• Received
  04 June 2023
• Accepted
  09 Apr 2024