The specimen was collected on 17 October 2020 from fish trawl bycatch landing at Digha Mohana, a busy fishing harbor in the East Midnapore district of West Bengal. The site sits at the mouth of the Champa River, which opens into the northern Bay of Bengal, at coordinates 21°37′49.5″N, 87°32′48.3″E. After collection, the crab was cleaned with distilled water, photographed for morphological identification, and preserved in 10 percent formalin. It has since been deposited in the Zoological Reference Collection at the Department of Life Sciences, Hemchandracharya North Gujarat University in Patan, Gujarat, under catalog number LFSC.ZRC-295.

The identification team comprised Krupal J. Patel of The Maharaja Sayajirao University of Baroda, Heris Patel and Jigneshkumar N. Trivedi of Hemchandracharya North Gujarat University, and Santanu Mitra of the Zoological Survey of India in Kolkata. Their careful morphological work confirmed that the crab belongs to Alox chaunos Galil and Ng, 2007, a species originally described from specimens collected at Momo Beach on the island of Panglao in the Philippines. The species was later redescribed based on material from Japan, and its presence in the Bay of Bengal now bridges a considerable gap in its apparent distribution across the Indo-West Pacific.

Pebble crabs belong to the family Leucosiidae, subfamily Ebaliinae, a group of crabs characterized by rounded, often smooth or ornately sculptured shells that give them a pebble-like appearance when resting on sandy or muddy seabeds. The genus Alox was established in 1996 by Tan and Ng as part of a major revision of the related genus Oreophorus. Members of Alox are readily distinguished by a subpentagonal carapace ornamented with mushroom-shaped tubercles, a distinct groove running parallel to the anterolateral margin of the shell, and antennular fossae that are completely sealed by the basal segment of the antennule when the animal retracts it. These features make the genus one of the more visually distinctive groups among leucosiid crabs.

The Indian specimen, with a carapace length of 8.35 millimeters and a carapace width of 11.60 millimeters, displays all the hallmark features of the species. Its shell is subpentagonal and moderately convex on the upper surface, about 1.4 times broader than long, and densely covered with mushroom-like tubercles interspersed with irregular pits. The frontal area is produced forward and bilobed, with an elongate narrow apical cavity, while a prominent Y-shaped median keel widens toward the rear and forms a straight longitudinal ridge extending to the midpoint of the cardiac region. This keel is flanked by a pair of nostril-like postfrontal cavities, and a deep, broad groove runs parallel to the anterolateral margin, its anterior half connecting to the postfrontal cavity.

Beyond the shell, the reproductive anatomy of the male specimen provided decisive confirmation. The first male gonopod, or G1, is slender and slightly twisted in its distal half, tapering toward an obtuse tip that bears a terminal opening and is directed dorsally. The second gonopod, G2, is slender, bent dorsally, and roughly one-third the length of the G1, terminating in a petaloid process. Notably, the researchers found that the shape of the G2 illustrated in a previous study by Ohtsuchi and Kawamura appears to be aberrant, and the new specimen allowed them to provide what they consider a more accurate illustration of this structure. Such corrections matter in taxonomy, where the fine morphology of male reproductive structures often serves as the ultimate arbiter of species identity.

Distinguishing Alox chaunos from its closest congener, Alox rugosum, which is already known from India, required a detailed comparison of multiple characters. The carapace of A. chaunos is 1.4 times broader than long, compared with 1.5 times in A. rugosum. The hepatic region of A. chaunos is straight to moderately convex owing to a median protuberance, whereas in A. rugosum it is convex. The intestinal region is only moderately produced in A. chaunos but strongly produced in A. rugosum, and the posterior margin of the shell is slightly concave in the former versus slightly convex in the latter. The groove between the branchial region and the posterolateral margin is absent in A. chaunos but prominently granule-floored and lined in A. rugosum. Even the claws differ: the chela fingers are 1.2 times as long as the palm in A. chaunos, against 1.5 times in A. rugosum. The telson, the terminal segment of the abdomen, is distinctly narrow and elongate in A. chaunos, nearly three times longer than its basal width, while in A. rugosum it is triangular and only about 1.15 times longer than its basal width. Finally, the G1 narrows distally in A. chaunos but is slightly dilated in A. rugosum.

The Indian specimen does show a handful of minor differences from the original descriptions, including prominently inflated branchial regions that are not separated from the posterolateral margin, a slightly concave posterior margin, and a third pleonal somite that is not eroded while the fourth through sixth somites are eroded laterally. The authors attribute these variations to geographical variation, a common phenomenon in widely distributed marine invertebrates whose populations can diverge subtly in form across ocean basins. They also raise a cautionary note: older records of Alox rugosum from India may require re-examination, since the superficial similarity between the two species means some historical identifications could have conflated them.

The broader context of the discovery underscores how much remains to be documented along India’s coasts. The Indian coastline stretches 11,099 kilometers and spans dramatically different oceanographic and environmental settings, from the tropical shores of the southwest to the temperate-influenced waters of the northeast. Successive regional surveys and taxonomic revisions have steadily expanded the known Indian leucosiid fauna, which currently stands at approximately 101 species representing 38 genera. Of the 14 species now recognized in the genus Alox, only four had previously been reported from India: Alox latusoides, A. ornatum, A. patella, and A. rugosum. The addition of A. chaunos brings that tally to five.

The distribution pattern of the genus Alox across the Indo-West Pacific is itself of ecological interest. Some species, such as A. liklik, A. naispela, and A. patella, inhabit tropical environments, while others, including A. zalion, A. uru, and A. rugosum, are reported from temperate waters. Certain species, notably A. ornatum and A. patella, range from the tropical coasts of Australia to the temperate shores of Japan, indicating a capacity for broad ecological tolerance and dispersal. The presence of A. chaunos in both the Philippines and Japan, and now in the northern Bay of Bengal, fits this pattern of a genus whose members can traverse considerable latitudinal distances, whether through planktonic larval dispersal on ocean currents or other means.

For the researchers, the find is also a reminder of the scientific value hidden in ordinary fishing bycatch. Trawl nets that sweep the seafloor routinely bring up benthic invertebrates that would otherwise escape the notice of taxonomists, and harbors such as Digha Mohana, where landings are concentrated and accessible, offer a practical window into the diversity of adjacent seabed habitats. A single male crab, carefully photographed, described, and deposited in a reference collection, has now added a genus-level range extension to the faunal inventory of the Indian subcontinent. The authors thank Professor Peter K. L. Ng of the Lee Kong Chian Natural History Museum at the National University of Singapore for his valuable comments on the manuscript, and Santanu Mitra acknowledges the support of the Director of the Zoological Survey of India. As surveys of India’s coastal waters continue, specimens like this one suggest that the true diversity of the region’s pebble crabs, and of its benthic fauna more generally, is likely still underestimated.

Subject of Research: First record of the leucosiid pebble crab Alox chaunos in Indian waters based on a trawl bycatch specimen from West Bengal

Article Title: First record of the pebble crab Alox chaunos from India

Article References: Patel, K. J., Patel, H., Mitra, S., & Trivedi, J. N. (2026). First record of the pebble crab Alox chaunos from India. Discover Oceans, 3(1), Article 28. https://doi.org/10.1007/s44289-026-00143-3

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00143-3

Keywords: Alox chaunos, Leucosiidae, pebble crab, Brachyura, first record, West Bengal, Digha Mohana, Bay of Bengal, trawl bycatch, taxonomy, Indo-West Pacific, marine biodiversity