Fiddler crabs are commonly distributed in sandy mudflats, mangroves, and wetlands in tropical and subtropical regions (Crane, 1975; Rosenberg, 2014). Out of the 107 extant species listed on the website “Fiddler Crabs” (http://www.fiddlercrab.info/ index.html) (Rosenberg, 2014), 29 are found on the Pacific coast of Panama (Crane, 1975; Shih et al. 2015; Shih et al. 2016; Lombardo, 2022). Some species may exhibit cryptic characteristics (e.g., small size and blending colors), making their presence easily overlooked (Crane, 1941; Hendrickx, 1979). Even brightly colored species, such as Minuca osaLandstorfer & Schubart, 2010, may remain unnoticed; for example, it was reported in Panama only twelve years after its initial discovery in Costa Rica (Lombardo, 2022). Given this context, we hypothesized the presence of other unreported fiddler crabs in Panama. One potential case is that of the Lesser Mexican Fiddler Crab, Minuca zacae Crane, 1941 which was initially reported in Corinto and San Juan del Sur in Nicaragua and Golfito in Costa Rica (Crane, 1941; Crane, 1975). Since Crane (1941), reports for M. zacae in the eastern Pacific region had settled its range from Mexico to Costa Rica (Crane, 1975; Hendrickx, 1979, Hendrickx & Salgado-Barragán, 1992; Hendrickx, 1995; Rosenberg, 2002; Landstorfer & Schubart, 2010; Rosenberg, 2014; 2020). However, reports from Bahía de Málaga in Colombia by Lazarus & Cantera, 2007 accounted for the presence of M. zacae at four locations, suggesting a distribution gap in Panama (Fig. 1A). Filling the gaps in fiddler crab distribution is important as they impact ecosystem functioning through their burrowing and bioturbation activities (Smith et al. 2009; Mokhtari et al. 2016; Booth et al. 2019; El-Hacen et al. 2019). Therefore, the objective of this study was to confirm the identity of fiddler crabs found in the Montijo and Parita Gulfs exhibiting M. zacae characteristics, in order to update its distribution range.

Our survey focused on four sites, three of which were located in the Gulf of Montijo: Ponuga (07º 51ʼ 44.769ʼʼ N, 081º 01ʼ 01.559ʼʼ W) was visited in June 2022 and October 2023; Las Huacas (07° 53ʼ 03.584ʼʼ N, 081° 08ʼ 39.767ʼʼ W) was sampled in January 2024; and Arenas de Quebro (07° 20ʼ01.022ʼʼ N, 080° 53ʼ 03.281ʼʼ W) in February 2024. The fourth site was located in the Gulf of Parita, Los Aromos (07º 55ʼ 57.649ʼʼ N, 80º 19ʼ 51.173ʼʼ W) and was surveyed in April 2024 (Fig. 1B). Specimens

Fig. 1

were observed directly in the field, hand-caught (N = 10) and placed in 75 ml plastic containers. In the laboratory, crabs were rinsed with tap water and euthanized in a freezer for five minutes to facilitate analysis and photography. The carapace widthfront width ratio and chela height and length were measured (precision 0.01 mm) for scaling. Carapace width was compared with biometrics reported in the literature. Morphological features of interest were photographed using a stereoscope (SZ2-ILST) equipped with a camera (EP50) and processed with EPview software (Olympus, Japan). Species identification was conducted using specialized literature (Crane, 1941; Bott, 1954; Crane, 1975; Landstorfer & Schubart, 2010; Rosenberg, 2014; Shih et al. 2015, 2016; Rosenberg, 2020).

Among the ten Minuca zacae specimens collected, eight were male and two were female, none ovigerous. The average carapace width was 10.17 ± 1.02 mm. These specimens were found on mudflats near mangrove vegetation, alongside Minuca herradurensisBott, 1954, and Minuca galapagensisRathbun, 1902. The size range of our sampled M. zacae suggests they are, at first glance, similar in size to those in previous reports but smaller than their sympatric species (Table 1). Males and females (Fig. 2A) exhibited traits fitting the description of Minuca zacae, including: broad frontal region with two small protuberances on the upper side (Fig. 2B-C), and an average carapace width-front width ratio of 4.6:1 (Fig. 2B); pits on the carapace surface with an indistinct pattern; no pubescence on the carapace besides pile traces in the grooves of the H-like depression of the cardiac-mesogastric region (Fig. 2C); dorsally, the anterolateral angles are rectilinear and do not project beyond the front (Fig. 2A-C); ocular orbits significantly, but not excessively, oblique (Fig. 2B); anterolateral margins distinct, short, slightly convex, and posteriorly angled; dorsolateral margins slightly converging and align with the lower edge of the cardiac-mesogastric region (Fig. 2C); two posterolateral striae, with the upper longer than the lower, the latter barely noticeable in small specimens (Fig. 2D); in frontal view, the upper portion of the pterigostomial region (adjacent floor of orbit) with a concave ridge with minute crenations, this region and the suborbital region not densely setose, crenations increase moderately in size externally (Fig. 2E); abdominal segments 3-6 partially fused; meri of ambulatories slender, with dorsal edges almost straight (Fig. 2A, D); walking legs naked, except for small patches on the dorsal margin

Table 1

Species	Carapace width (Mean ± SD)	Mean difference	Location	References
M. zacae	10.17 ± 1.02	1.06	Gulfs of Montijo and Parita, Panama	This study
	9.11 ± 2.55		Altata bay, Mexico to Golfito, Costa Rica	Crane (1975); Hendrickx & SalgadoBarragán (1992); Rosenberg (2002)
M. osa	16.22 ± 3.70	-6.05	Golfo Dulce, Costa Rica	Landstorfer & Schubart (2010)
	21.44 ± 2.96	-11.27	Gulf of Montijo, Panama	Lombardo (2022)
M. galapagensis	20.11 ± 2.00	-9.94	Rodman, Panama	Crane (1975).
M. herradurensis	16.49 ± 5.86	-6.32	Diablo Creek, Port of Rodman, and Taboguilla Island, Panama	Rosenberg (2002)

of the carpus and propodus; merus of major chela with two rows of small serrations on the ventral margin, serrations changing distally to small tubercles, particularly in the anterior row (Fig. 2F); on average (n = 8), the width of the manus of major chela contained 2.1 times within its length; pollex and dactyl slightly longer than palm, lower part of the outer manus with a row of small, bead-like supramarginal tubercles near the base, tubercles larger and spaced out towards the distal half, ending at the pollex base; a clear shallow, triangular depression at the base of the pollex just above the beaded margin (Fig. 2G), with less than 0.5 of the outer upper manus covered by large tubercles, extending to the dorsal margin of the dactylus diminishing distally (Fig. 2H); palm lacking an oblique tuberculate ridge, central surface smooth, beaded inner edge of the dorsal margin curving

Fig. 2

downward around the carpal cavity, without connection between the carpal cavity and upper palm faint, depression displaying weak tuberculation, two ridges associated with the dactyl base, proximal not diverging from distal (Fig. 2I), dactyl curving downward beyond the pollex as typical for the genus; inner row of tubercles in pollex prehensile edge obsolescent to absent, tubercles in the outer row weak, median row prominent, near the tip a large tubercle close to the outer edge, followed by two smaller tubercles, dactyl prehensile edge featuring rounded, enlarged tubercles in proximal 0.5, spaced equally apart (Fig. 2G, I); small chela gape narrower than dactyl width, pollex and dactyl with weak serrations on the distal third with scare setae (Fig. 2J), fingertips of the minor chela slightly curving inward, spoon-like shaped; gonopod flanges broad of similar width and length, slightly concave, inner process translucent, narrow, tapering distally, pore opening posteriorly, with flanges seemingly fused in front of channel, thumb setose, fully formed, inserted much lower than flanges and extending below their base (Fig. 2K-L); female gonopore crescent-shape, with thickened lip, not unevenly raised, without tubercle (Fig. 2M-N).

Compared to its sympatric species, M. zacae is smaller, a feature also reported by Crane (1975) and Rosenberg (2023). This overlap implies adults can be mistaken for juveniles of M. herradurensis and M. galapagensis(Crane, 1975; Hendrickx, 1979; Rosenberg, 2023). Minuca zacae differs from these two species as follows: it possesses more oblique orbits, thick brow; the oblique tuberculate ridge on the palm is absent; fingers are longer than the palm; although leptochelous, the major chela is stouter than in other species of Minuca. Merus of ambulatory legs is slender, which is also noticeable in females (Crane, 1975). The gonopod in male M. zacae lacks a distinct anterior flange, as in M. galapagensis and M. herradurensis. In M. herradurensis, the anterior flange always extends beyond the posterior. Moreover, the inner process of the gonopods in M. zacae is relatively subtle, unlike in other species of Minuca(Crane, 1975). Minuca osa is another species whose juveniles may be mistaken for M. zacae due to similar carapace color and spot pattern (Landstorfer & Schubart, 2010). However, a notable difference is the presence of a prominent oblique tuberculate ridge on the palm of M. osa(Landstorfer & Schubart, 2010; Lombardo, 2022), absent in M. zacae.

Based on their morphology, specimens collected in Montijo and Parita belong to M. zacae. This report bridges the M. zacae distribution gap between Costa Rica (Rosenberg, 2020) and Colombia (Lazarus & Cantera, 2007), covering a range from Bahía Altata, northern Mexico (Hendrickx & Salgado-Barragán, 1992) to Bahía de Málaga, southern Colombia (Lazarus & Cantera, 2007)