Sesarmops
A genus of Indo-West-Pacific mangrove and forest stream crabs.
Sesarmops is a genus of crabs belonging to the family Sesarmidae. These crabs are found across the Indo-West-Pacific oceanic region, living in freshwater forest streams near the coast and in mangrove habitats.
The genus was established in 1970 when Serène and Soh reorganized the existing genus *Sesarma*, initially including five species: *S. atrorubens*, *S. impressus*, *S. intermedius*, *S. mindanaoensis*, and *S. sinensis*. They defined *Sesarmops* by several key features: the carapace is as long as or longer than its width measured at the external orbital teeth; the frontal margin has a deep median concavity; the postfrontal lobes are prominent; the external orbital tooth is clearly separated from the rest of the lateral margin by a distinct cleft; and the gastric and cardiac regions are well defined.
Later additions to the genus include *Sesarma weberi*, provisionally added by Ng et al. in 2008, and *Sesarma angustifrons*, transferred by Paulay and Starmer in 2011. The genus underwent a major revision in 2020, with two papers published in January and December of that year. During this revision, species were split and redefined, and two species were moved to a new genus.
The type species, *Sesarmops impressum* (formerly *Sesarma impressa*), and related species were reviewed by Ng et al. in 2020. They found that *S. impressa* sensu lato from the West Pacific differs in colouration and gonopod structure from Indian Ocean populations. Consequently, Ng, Li, and Shih restricted *S. impressus* to the West Indian Ocean, defining *S. indicus* in the East Indian Ocean and *S. imperator* in the West Pacific. The type locality for *S. indicus* is the Andaman Islands, with records from northern Sumatra, the Mentawai Islands Regency, and the Enggano Islands provisionally referred to the same species.
The original types for *S. atrorubens* and *S. similis*, collected by Hess and labeled from "Sydney," are lost. Scholars believe Hess received the specimens via Sydney but that they were collected elsewhere. Ng et al. designated neotypes from Fiji and Samoa, respectively.
Mitochondrial DNA comparisons revealed that *S. impressus* and *S. imperator* form a clade sister to *Bresedium eurypleon* and *B. brevipes*, indicating that *Sesarmops* is paraphyletic. *S.
Quick Facts
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- Sesarmops
Facts from the source article.
Lore & Background
Sesarmops was defined in 1970 when Serène and Soh re-organised the existing genus Sesarma, including five species: S. atrorubens, S. impressus, S. intermedius, S. mindanaoensis, and S. sinensis. They defined the genus by several carapace features: the carapace is as long or longer than the width measured at the external orbital teeth; the frontal margin has a deep median concavity; the postfrontal lobes are prominent; the external orbital tooth is clearly separated from the rest of the lateral margin by a distinct cleft; and the gastric and cardiac regions are well defined.
Later additions included Sesarma weberi provisionally added by Ng et al. in 2008, and Sesarma angustifrons transferred by Paulay and Starmer in 2011. The genus was revised in January and December 2020, with species split and redefined, and two species transferred to a new genus. The type species Sesarmops impressum and related species were reviewed by Ng et al. in 2020, restricting S. impressus to West Indian Ocean populations, defining S. indicus in the East Indian Ocean and S. imperator in the West Pacific. Hess's original types for S. atrorubens and S. similis, from 'Sydney', are lost; scholars believe the specimens were collected elsewhere, and neotypes were designated from Fiji and Samoa, respectively.
Reader's Guide
The revision of Sesarmops in 2020 resolved long-standing taxonomic confusion within the genus, particularly around the widespread species S. impressus sensu lato. By splitting this complex into three distinct species—S. impressus (West Indian Ocean), S. indicus (East Indian Ocean), and S. imperator (West Pacific)—the revision clarified biogeographic patterns and morphological variation. Comparisons of mitochondrial DNA revealed that Sesarmops is paraphyletic relative to Bresedium, with S. impressus and S. imperator forming a clade sister to Bresedium eurypleon and B. brevipes, while S. mora is sister to the impressum–Bresedium clade. The intermedius–patshuni–dehaani group was re-classified into a new genus Orisarma by Schubart & Ng in 2020, containing five species. As of 2023, the World Register of Marine Species recognises nine species of Sesarmops, with two former species moved to Orisarma. The genus remains significant as a model for understanding Indo-West-Pacific mangrove and freshwater crab diversity, and its taxonomic history illustrates the challenges of defining genera based on morphology alone.
Did You Know?
- The type species Sesarmops impressum was formerly known as Sesarma impressa.
- Comparisons of mitochondrial DNA found that Sesarmops is paraphyletic relative to Bresedium.
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