Fiddler crabs (Ocypodidae: Uca Leach, 1814) are distributed along most tropical and semitropical coastlines of the world (Rosenberg, 2020).

Research on fiddler crabs in Panama has been published on various topics, such as, functional morphology in Petruca panamensis, Leptuca terpsichores and L. beebei (Lim & Goh, 2021), vision in L. stenodactylus (see How et al. 2015), sexual selection (Dennenmoser & Christy, 2013), construction of structures (Pardo et al. 2020) in L. terpsichores and L. beebei, and larval ecology in L. terpsichores (Christy, 2003), and L. deichmanni (Kerr, 2015). Even though fiddler crabs are widely distributed in Panama, sampling in all the aforementioned studies was conducted exclusively at the Pacific entrance of the Panama Canal, whereas studies from other provinces are lacking.

There are currently 35 recognized fiddler crab species in Panama. On the Atlantic coast, there are seven species, distributed into three genera: Minuca with four species, Uca with two, and a single species of Leptuca (L. thayeri). In the Pacific, there are 28 species grouped into four genera, of which sixteen are in the genus Leptuca, six within Uca, five within Minuca, and a single species of Petruca (P. panamensis) (Rosenberg, 2014).

In the Ponuga River, Eastern Montijo Gulf, Veraguas Province, fiddler crabs were observed waving their claw in synchrony. The major claw, bright red-orange with a white tip, was the most striking feature of these fiddler crabs. A species fitting this morphological characteristic was described by Landstorfer & Schubart (2010) from the Golfo Dulce, Pacific coast of Costa Rica (Fig. 1A ). The authors named the new species, Minuca osa, and suggested that it might be endemic. Based on geographical proximity of Golfo Dulce and our study area, it seemed plausible that M. osa could also be found in Panama (Fig. 1B, C ). Therefore, the objective of this study was to identify the specimens found in the Eastern Montijo Gulf and report on the possible geographical extension of M. osa distribution range.

Sampling was conducted on April 27th, 2021, and August 13th and 14th, 2022 at two sites on the Eastern Montijo Gulf (Fig. 1D); the Ponuga River (07° 51ʼ 51.3756ʼʼ N,-081° 00ʼ 52.6248ʼʼ W) and Mata Oscura (07° 27ʼ 29.0124ʼʼ N, -080° 55ʼ 12.8532ʼʼ W). Fiddler crabs (n = 22) were caught by hand, rinsed, and later stored in a freezer unit. Morphological characters of interest were photographed; specimens’ carapace width and length, as well as chela height and length, were measured (mean ± SD; 0.01 mm), and taxonomical keys (Landstorfer & Schubart, 2010; Cra- ne, 2015; Shih et al. 2016; Rosenberg, 2020) were used for the identification of the collected material. Morphological characters between Golfo Dulce (Landstorfer & Schubart, 2010) and the Eastern Montijo Gulf were compared with the t-test.

Fig. 1 First record of the Fiddler Crab, Minuca osa from the Eastern Montijo Gulf, Panama

Features of the genus Minuca were found in 22 individuals averaging 21.44 ± 2.96 mm and 14.60 ± 1.62 mm in carapace width and length, respectively; 14 were from Ponuga and eight from Mata Oscura. Specimens were larger than those from Golfo Dulce, except in major chela height, where no difference was detected (Table 1). In general, compared to other Minuca species, sampled individuals from Ponuga and Mata Oscura can be considered large within the genus (Table 2 ).

Table 1

EMG-CW	22	21.44 ± 2.96	12.66	25.98	4.21; 18	0.001
GD-CW	12	16.22 ± 3.70	9.28	21.10
EMG-CL	22	14.60 ± 1.62	11.10	17.89	4.01; 15	0.001
GD-CL	12	11.32 ± 2.57	6.12	14.70
EMG-QH	22	11.04 ± 2.34	4.09	14.66	1.55; 13	0.146
GD-QH	7	9.76 ± 1.76	7.83	11.90
EMG-QL	22	30.27 ± 7.80	7.13	42.23	2.31; 18	0.033
GD-QL	8	24.41 ± 5.42	18.32	31.05
EMG-Front	22	7.47 ± 1.04	4.42	9.09	3.85; 13	0.002
GD-Front	10	5.59 ± 1.37	3.11	7.17

Table 2

Morphological characters of the genus found in sampled specimens include: H-like depression in the cardiac-mesogastric region, carapace width to front ratio of approximately 35% (wide front; Fig. 2A), with long anterolateral margins curving inwards (Fig. 2A, D ), small suborbital crenulations increasing towards outer angle (Fig. 2B ), narrow gape minor chelipeds with small serrations (Fig. 2C ), and two postero-lateral striae in the carapace (Fig. 2D ).

A total of 16 males and six females exhibited traits that fit the description of Minuca osa. Specifically, segments of the pleon were not fused, carapace surface with small pits; no pubescence or setae on carapace (Fig. 2A, D ). There was pile with few long setae in the ambulatories dorsal margin of the carpus and propodus, in agreement with that reported for M. osa (Fig. 2E ). The dorsal margins of ambulatory meri in males had a soft dorsal tuberculate ridge (Fig. 2F ); dactyls of ambulatory legs have three distinct rows of short setae (Fig. 2G ). In males, the external surface and dorsal edge of the manus was tuberculate, with a minuscule hollow space, proximal to the predactylar groove. Tubers in the major chela diminished on lower manus and were minuscule on pollex and dactyl (Fig. 2H ). The pollex was curved upwards at the tip; both, the pollex and dactyl were flattened. The dactyl was longer than the pollex and was curved at approx. 90° downward at tip (Fig. 2H, K, M ). The pollex showed three rows of tubercles, and one large carina beyond the midpoint (Fig. 2I ). There was one large subdistal tooth at the gape midrow of the pollex, forming three distinct tubercles (white) on the tip (Fig. 2J, K, M ). The dactyl had three to four larger tubercles (Fig. 2K, M). The inner surface of the manus had a ridge of dense tubercles lining the margin of the carpal cavity, warping in direction to the palm with tubercles growing in size and ending in a patch in the central section of the manus, almost connecting to the oblique tuberculate edge (Fig. 2L ). The ventral manus tuberculate ridge was oblique with a high apex, sloping into pollex ventral surface. This ridge had distinctive tubercles decreasing in size towards the pollex with increased irregularity. The pollex and manus articulation had a deep sulcus which ran between the pollex ventral margin and two predactylar tuberculate ridges; the nearest was divergent and curled into the inner dorsal tuberculate ridge of the pollex (Fig. 2L, M ). The carpal surface (dorsal) of major chelipeds bearded four to five sharp tubers, and the merus showed numerous sharp tubercles on the dorsal edge (Fig. 2N ). The gonopod had two flanges and a thumb; the posterior flange was longer than the anterior. The genital pore and flanges were covered by extension of the shaft. Setae were on the opposite side of the flanges, on thumb and tip of gonopod (Fig. 2O ).

Fig. 2

Minuca burgersi, M. vocator, and M. ecuadoriensis are morphologically similar to M. osa (Landstorfer & Schubart, 2010; Rosenberg, 2020); however, M. burgersi and M. vocator are only distributed in the Western Atlantic, while M. ecuadoriensis is only found in the Eastern Pacific (Masunari et al. 2020; Rosenberg, 2020). Furthermore, M. burgersi also differs from our specimens in that the oblique tuberculate ridge on the major palm has small tubercles (not a single row); tubercle ridge does not diminish distally on outer pollex (Crane, 2015; Shih et al. 2016). In Minuca vocator and M. ecuadoriensis, the carapace as well as the ambulatories on all segments have a profuse pile (except dactyls) in an irregular pattern (see Masunari et al. 2020), while our specimens had no such pile. In M. ecuadoriensis, the oblique ridge in the major palm has a very low apex; tubercles are small and irregular, sometimes vestigial or absent. In contrast, the apex in M. osa is high and tubercles are large (Landstorfer & Schubart, 2010; Crane, 2015); all of our specimens showed the latter. Considering its behavior and coloration, M. osa is a conspicuous fiddler crab species; thus, it is unclear why it was not been identified other than in Golfo Dulce (Landstorfer & Schubart, 2010) prior to this report. Based on the morphological differences between M. osa, M. burgersi, M. vocator, and M. ecuadoriensis, it can be concluded that M. osa is also present in the Eastern Montijo Gulf, Panama