Major Cheliped Research Articles

A new ‘Southern Giant Crab’ from a miocene continental slope palaeoenvironment at Taranaki, North Island, New Zealand

ABSTRACT Large-sized extinct crab specimens recovered from Waitoetoe beach, North Island, New Zealand form the basis for a new species of ‘Southern Giant Crab’, Pseudocarcinus karlraubenheimeri n. sp. The specimens originate from the upper Miocene Urenui Formation (approximately 8.8 myr) of the Taranaki Basin, in which a series of volcanoes of the Mohakatino Volcanic Centre erupted offshore, leading to the formation of a specific palaeoenvironment. The well-preserved, articulated specimens were found buried in sediments which include reworked volcanogenic material. The crabs inhabited a deep-marine setting. This is the first evidence that Pseudocarcinus inhabited the region that is now New Zealand. New Zealand Miocene environments apparently offered favourable conditions in terms of food sources, metabolic requirements, and calcium-carbonate supply for Pseudocarcinus karlraubenheimeri n. sp. Pseudocarcinus thrived on both sides of the Tasman Sea until it disappeared in New Zealand waters. Pseudocarcinus crabs are characterised by gigantism, which provided them with significant advantages in competition and defence. Their carnivorous nature is reflected in their exceptionally large major cheliped. The broader use of benthic dwelling gastropods and bivalves as prey seems to have led to subsequent advances in brachyuran claw engineering, and an increase of molluscivorous crabs in the Late Cretaceous and Palaeogene. Zoobank: urn:lsid:zoobank.org:pub:B53DB047-B18C-4CA4-946F-5E9209A581F2

Abnormal symmetric chelipeds in an specimen of snapping shrimp Synalpheus fritzmuelleri Coutiere, 1909

Abstract Morphological symmetry abnormalities in cheliped appendages of alpheid shrimps are extremely rare and poorly recorded in the literature. A symmetric minor cheliped were, for example, observed in queen females belonging to Synalpheus eusocial species. Symmetric major chelipeds were now described in Synalpheus fritzmuelleri individuals living in shallow Brazilian waters. These individuals were found in symbiotic association with the bryozoan Schizoporella sp. (biogenic substrate) adhering to the pilings of Ubatuba Bay docks, São Paulo State. Only one of 20 sampled S. fritzmuelleri individuals presented anomalous symmetric chelipeds. Based on carapace length, size, and morphological features, the analyzed specimens seemed to be juvenile; thus, the hypothesis of anomalous condition can be directly linked to genetic inhibition of the mechanism accounting for major cheliped development in this ontogeny phase. Studies like the present one often provide remarkable information on animal morphology and can be used as reference in evolutionary assessments to be conducted in the future.

Just a matter of size? Evaluating allometry and intersexual heterometry in Pagurus bernhardus using ratios and indices (Decapoda, Anomura).

Heterochely denotes the presence of dissimilarly sized chelipeds on opposite sides of the body, a prevalent occurrence in diverse crustaceans. Conversely, heterometry pertains to the quantifiable disparities in size between these chelipeds. Both chelipeds hold pivotal roles in activities such as foraging, mating, and defense. Consequently, individuals of both genders in heterochelic species exhibit this morphological pattern. Previous studies have identified sexual dimorphism in cheliped size, with males displaying larger major chelipeds compared to females, albeit solely relying on propodus length as a size proxy and focusing solely on the major cheliped. In our study, we meticulously examined 190 specimens of the common European hermit crab Pagurus bernhardus from two collections. We sought to elucidate allometric relationships and assess whether heterometry exhibited sex-based differences when adjusting for body size by using ratios. Our findings revealed that male chelipeds displayed hyperallometric growth relative to females, and all three calculated heterometry indices exhibited significant disparities between the sexes. Consequently, male specimens exhibited larger major and minor chelipeds, even when theoretically matched for body size with females. This phenomenon may be attributed, among other factors, to male-male contests. Should indirect mate selection favor males with larger chelipeds in proportion to their body size, this dynamic could potentiate sexual selection in their favor.

A checklist and key to the Ivorian freshwater crabs, with the description of a new species of Liberonautes Bott, 1955 (Brachyura: Potamoidea: Potamonautidae).

A new freshwater crab species of Liberonautes Bott, 1955 from Mt. Tonkori in the Upper Guinea rainforest in Côte d'Ivoire, West Africa is described. Liberonautes constantini n. sp. is morphologically distinct from other species of this genus found in Côte d'Ivoire and the neighboring countries. The new species is recognised by a unique combination of morphological characters of the carapace, thoracic sternum, major cheliped, and adult male first gonopod. Illustrations of L. constantini n. sp. are provided and differences from congeners found elsewhere in West Africa are discussed. The addition of L. constantini n. sp. raises the number of species of freshwater crabs known to occur in Côte d'Ivoire to seven. An updated checklist and key are provided to the Ivorian species of freshwater crabs, and their conservation status is discussed.

Polka-dotted treasures: Revising a clade of ascidian- and bivalve-associated shrimps (Caridea: Palaemonidae)

Abstract Most marine shrimp species of the family Palaemonidae are characterized by symbiotic associations with hosts that belong to a wide range of invertebrate phyla. One clade of related endosymbiotic species has evolved to live inside the branchial chambers of ascidians and the mantle cavities of bivalve molluscs. The phylogeny of this clade (comprising 15 bivalve-associated species in the genera Anchistus, Neoanchistus, and Paranchistus, and three species of ascidian-associated species in the genus Dasella) is the topic of the present study, which is based on both morphological and molecular data. A concatenated phylogeny reconstruction was built by using the markers coi and 16S. With the help of a total evidence approach (with a scored morphological datamatrix), species could be added for which no molecular data were available. An ancestral character state analysis was performed to detect host switches. In contrast to another endosymbiotic clade, the ancestral host state was found to be slightly in favour of bivalves. The phylogenetic relevance of morphological features is discussed with a focus on a tympanal organ located in the major chelipeds of some bivalve-associated shrimp species. This little-known anatomical structure is illustrated by sem and µCT scans. Its possible function is discussed. In the phylogeny reconstructions, both Anchistus and Paranchistus were found to be polyphyletic. They were reclassified to obtain a more natural classification: Anchistus custoides and Anchistus custos were transferred to the resurrected genus Ensiger. Paranchistus liui, P. nobilii, P. pycnodontae, and P. spondylis were transferred to Polkamenes gen. nov. Anchistus pectinis and P. ornatus were transferred to Tympanicheles gen. nov. Per genus, a key to the species is provided.

A new species of the alpheid shrimp genus Athanas Leach, 1814 (Decapoda: Caridea) from the Sea of Japan.

A new species of the alpheid shrimp genus Athanas Leach, 1814 is described based on a holotype from Wakasa Bay, Sea of Japan side of Honshu, Japan, at a depth of 90 m. Athanas exilis n. sp. appears close to A. sydnyensis Anker & Ahyong, 2007, but is distinguished from the latter by some morphological features, including the different shape of the telson, the shorter antennular stylocerite, more strongly produced distal lamella of the antennal scaphocerite, and less developed armature of the merus of the major cheliped. Preliminary genetic analysis using the mitochondrial 16S rRNA gene was performed to compare interspecific genetic divergence within the genus.

Salmoneus shojaei, a new species of mangrove-dwelling alpheid shrimp (Decapoda: Caridea) from Iran.

During a survey of the mangrove infauna on the Iranian side of the Persian Gulf and the Gulf of Oman several specimens of a new alpheid shrimp, Salmoneus shojaei sp. nov., were collected around pneumatophores of mangrove trees, mostly in association with the larger burrowing snapping shrimps of the genus Alpheus Fabricius, 1798. The new species appears to be a member of the S. gracilipes species group and is morphologically closest to S. colinorum De Grave, 2004 and S. alpheophilus Anker Marin, 2006. However, a unique combination of morphological characters, such as the carapace without rostral carina, the unarmed ischium of the major cheliped, the armed ischia of the minor cheliped and second pereiopod, the very slender dactyli of the fourth and third pereiopods, and the posterior margin of the telson with a deep U-shaped notch, distinguishes the new species from all other members of the S. gracilipes group. In addition, S. shojaei sp. nov. presents a diagnostic, albeit very faint, banding of the pleon, which separates it from most other species of the S. gracilipes group with known colour patterns. A DNA barcode (a partial fragment of the mitochondrial gene, CO1), as well as partial fragments of the mitochondrial 16S rRNA and the nuclear H3 genes, are provided to genetically characterise the new taxon.

Two axiidean ghost shrimps (Crustacea: Decapoda) from India, Guyanacaris keralam sp. nov. (Axiidae) and Paragourretia galathea (K. Sakai, 2017) (Ctenochelidae).

Ghost shrimps collected from the Indian Exclusive Economic Zone by the Fishery Oceanographic Research Vessel Sagar Sampada during two cruises in the eastern (105 metres depth) and southeastern Arabian Sea (101 and 326 metres depth) were studied. One male, identified as Guyanacaris keralam sp. nov. of the Axiidae, differs from its congeners in the unique armature of the rostrum, gastric region of carapace, pleonal pleural margins, dorsal margins of cheliped fingers, ventral margin of second pereopod merus, and lateral margins of telson; the presence of corneous spines on the propodi and dactyli of third and fourth pereopods, and the relative sizes of the ocular peduncles, antennular peduncle, scaphocerite and cheliped fingers. Guyanacaris K. Sakai, 2011 is re-diagnosed based on the presence of short supraocular spine and post-cervical carina on carapace, dentate margins of first two pleonal pleura, third to fifth pleura with angular to straight posterior margins, antennal scaphocerite with mesial spine at base, and presence of male first pleopod 1. Bruceaxius thailandensis K. Sakai, 2015 is tentatively retained in Bruceaxius K. Sakai, 2011, owing to absence of mesial spine at the base of scaphocerite, tuberculate upper margins of carpus, propodus and dactylus of major cheliped, and tri-segmented appendix masculina on male second pleopod. In addition, two males, diagnosed herein as Paragourretia galathea (K. Sakai, 2017) of the Ctenochelidae, differ from the holotype female only in the relative length of the terminal antennular article and the presence of a distomedian spinule on the telson, of all the features.

Heterochely and cheliped dimorphism in the porcellanid crab Petrolisthes japonicus (De Haan, 1849) (Decapoda: Porcellanidae)

Cheliped dimorphism is common in decapod crustaceans, and many decapods have a pair of different-sized chelipeds, i.e., one side being larger (major) than the other (minor), a phenomenon known as heterochely. Anomuran crabs from the family Porcellanidae exhibit a true crab-like body shape, with relatively large chelipeds for their body size, and they show a sexual size dimorphism of chelipeds (i.e., males with larger chelipeds). However, very little is known about heterochely in porcellanid crabs. Therefore, we aim to elucidate heterochely and cheliped dimorphism in the porcellanid crab Petrolisthes japonicus (De Haan, 1849). We examined the laterality of the chelipeds and performed allometric growth analyses of the cheliped dimensions. Heterochely was observed in most of the specimens, and major chelipeds occurred randomly on either the right or left sides of the body, regardless of the crab size. Major and minor cheliped size of males surpassed female cheliped size, and males increased the growth of both major and minor chelipeds to the same degree during the breeding season, thereby inducing the distinct intersexual size dimorphism of the cheliped. This is probably attributable to sexual selection (i.e., male-male competition). Intersexual dimorphism as an energy allocation strategy was also evident with females allocating their energy evenly to both major and minor cheliped growth without changing the chela shape; regardless of the body size, whereas males were investing more energy in major cheliped growth, resulting in wider chelae with increasing body size. Consequently, the major cheliped may have a greater role in male-male competition in P. japonicus.

Plasticity of growth rates and sizes at sexual maturity in different populations of the fiddler crab Minuca vocator (Herbst, 1804) within the same latitudinal range.

The growth rates and size at maturity of decapod crustaceans usually vary throughout latitudes. Here, we evaluated whether these life history traits can be stable within the same latitude range, where some environmental factors are constant, especially temperature. To this end, we estimated and compared the growth rates and size at sexual maturity of three populations of the fiddler crab Minuca vocator, located at latitude 23° S, in mangroves of the subtropical Brazilian coast. Variations in the growth rate of the abdominal width of females and of the propodus of the major cheliped of males were assessed by the allometric technique. The abdomen growth rates of females and the cheliped growth rates of males varied among the three populations, resulting in different sizes at maturity. These results do not support our prediction that growth rates and size at sexual maturity would remain stable in populations of the same latitude range. The differences in these life history traits could be a consequence of contrasting local conditions among mangroves. Therefore, we suggest that these crabs show plasticity and responsiveness to local-scale environmental variations, to optimize the energy allocation for maintenance, reproduction, and somatic growth.

Length of a sexually selected ornament-armament in fiddler crabs (Decapoda: Brachyura: Ocypodidae): One way, over deep time and space

Abstract Using three data sources, we measured claw lengths of the extremely large major claws of 70 species of fiddler crabs, spread over four major biogeographic regions and all 11 genera, as a function of carapace length. Despite a diversity of biogeography, body size, type of sexual signaling and mating behavior, and details of claw morphology, all species groupings follow the same linear relationship. No subgroupings could be determined, by region, among diverse genera, by ancestral-derived status, or by data source. Major claw size is known to be implicated in female selection of mates, which might be expected to diversify claw lengths among species and genera, as might differences in combat might be expected to diversify claw length, as it does in other claw traits. The constancy suggests a possible functional constraint on major claw length across the fiddler crabs, which is to be determined. Efficient folding of the claw as the male rapidly retreats into a burrow, or a protection of the carapace from frontal attack by predators or combatants in an initial aggressive encounter might select for this pervasive relationship. Tests are suggested. The sexually selected major claw is more variable than the naturally selected minor claw, but this difference in variability could be attributed partially to regeneration of the major cheliped and even variation in shape. A surprising similar trend of closing force index for the major claw exists for a large number of species spread throughout the genera and major regions occupied by fiddler crabs, but a considerable variation exists in a variety of functional traits. Thus, the constancy of length relationship stands out distinctly from a considerable background variation in form. As a sexually selected trait, the claw length constancy begs for an explanation.

Distinguishing ten sympatric species of fiddler crab (Decapoda: Ocypodidae) using a suite of phenotypic characteristics.

Ten species of fiddler crab are reported inhabiting the intertidal zone of a shore on Kaledupa Island, Indonesia. This is one of the highest recorded numbers of fiddler crab species living in sympatry, equating to over two-thirds of those known from the Wallacea biogeographic region and more than half of all those recorded from Indonesia. The descriptions to identify and distinguish these ten species are provided using a suite of characters e.g., carapace, major cheliped, male gonopods, gastric mills, life colouration in males and females, and notes on their ecology and distribution. Specimens were observed and collected in the Wakatobi National Park, near the village of Ambeua on Kaledupa island, Sulawesi Tenggara, Indonesia. Gastric mills are described for the first time for Gelasimus jocelynae, Paraleptuca crassipes, Tubuca coarctata, T. demani and T. dussumieri. A tabulation of anatomical features and colouration for all species in this study is provided as a support for field studies. It identifies features that support the recently proposed taxonomic revision of fiddler crabs by Shih et al. (2016).

Two new species of Macrobrachium Spence Bate (Decapoda: Palaemonidae) from Ayeyarwady River, Myanmar with a note on Macrobrachium lamarrei (H. Milne Edwards 1837)

Two new species of the genus Macrobrachium Spence Bate have been described from near Mandalay of Ayeyarwady (Irrawaddy) River, Myanmar (22°2′37″N; 96°2′37″E). Macrobrachium univmandalay sp. nov. is closely related to Macrobrachium spinosum Cai & Ng, 2002 and Macrobrachium yui Holthuis, 1950 and is characterized by the presence of 10–12 dorsal teeth of which 3–4 post-orbital, first tooth at about 1/4th of anterior carapace; second chelate legs are dissimilar in size and shape in which carpus of major leg distinctly shorter than palm, cutting edges of fingers with 2 large and 2–3 smaller denticles at proximal part and without denticles on the distal half. Minor leg in which merus longer than carpus and palm, carpus shorter than palm as well as fingers and fingers without denticles in the cutting edges. M. spinosum and M. yui differ from this species in all these characters above.Macrobrachium ayeyarwadiense sp. nov. shows close affinity with Macrobrachium horstii de Man, 1892 and M. spinosum and can be easily separated on the basis of difference in characters of rostrum, position and arrangement of teeth on upper and lower margins, position of first dorsal tooth, distance between post-orbital edge and first dorsal tooth and its relation to rostrum length, length of antennular peduncle in relation to post-orbital carapace, shape and structure of 2nd cheliped, nature, size and arrangement of spines on outer and inner margins of ischium, merus, carpus, palm and fingers and nature of denticles on cutting edges of major and minor legs. The species can at once be identified on the absence of tubercles at distal half beyond larger denticles on fingers, convex nature of merus, carpus and disposition of spines on them, swollen palm and curved nature of movable finger and crossed nature of fingers distally of major 2nd peraeopod. Nature of long stiff setae on inner side of ischium, merus and carpus of major cheliped is characteristic to the species. Minor 2nd cheliped is also species specific in which the fingers are slender and curved so that a wide gap exists when closed. A note on Macrobrachium lamarrei (H. Milne Edwards 1937) is also given in the text.

Two new species of emTypton/em Costa, 1844 from tropical American waters, withtaxonomic notes on emT. tortugae/em McClendon, 1911 and a new record of br /emT. granulosus/em Ayón-Parente, Hendrickx amp; Galvan-Villa, 2015 (Decapoda: Caridea: Palaemonidae).

Two new species of the palaemonid shrimp genus Typton Costa, 1844 are described based on material from Panama and Mexico. Both species are closely related to T. tortugae McClendon, 1911, a species originally described from the Dry Tortugas, off southern Florida, USA, and later scarcely recorded from other western Atlantic localities, from Bermuda to Mexico and Brazil. Some clarification and additional illustrations are provided for the type material of T. tortugae. Typton jonkayei sp. nov., is described based on material from fouling-encrusting communities dominated by sponges, growing on submerged roots of the red mangrove, Rhizhophora mangle L., in Bocas del Toro, Caribbean coast of Panama. This new species differs from T. tortugae in several morphological details, for instance, on the minor and major chelipeds (second pereiopods), telson, uropod, frontal margin and ambulatory pereiopods. Typton cousteaui sp. nov. is described based on a single ovigerous female dredged in the southern Gulf of California off Baja California Sur, Mexico, previously reported as T. tortugae. This new taxon seems to represent a true cryptic species with no significant morphological divergence from the allopatrically isolated T. tortugae, except for slight morphometric differences. In addition, T. granulosus Ayón-Parente, Hendrickx Galvan-Villa, 2015 is recorded from the Pacific coast of Panama, based on material collected in the Coiba Archipelago. Some taxonomic, distributional and ecological remarks are provided for T. granulosus and the closely related T. serratus Holthuis, 1951.

Life History Traits of the Snapping Shrimp Alpheus carlae (Decapoda: Alpheidae) from the South-eastern Coast of Brazil.

The life history traits of the snapping shrimp Alpheus carlae were investigated. We evaluated the relative growth, morphological sexual maturity, sexual dimorphism, handedness, fecundity, and egg volume. Sampling was performed monthly in an estuarine region of Cananéia, in the south-eastern coast of Brazil. The following structures were measured to perform the relative growth analysis: length of carapace and second pleonal pleuron, length, width and height of major cheliped propodus, and length of appendix interna and appendix masculina. Juveniles and adults males and females have different growth patterns, indicating distinct strategies of energy allocation. The estimated carapace length at the onset of morphological sexual maturity of females and males was 5.6 mm and 6.2 mm, respectively. The sexual dimorphism of specific body structures was evident, mainly in the appendix interna of females and major cheliped of males. There was no evidence of handedness in females or males. The mean fecundity of females was 364 ± 204 eggs (mean ± SD) eggs and was positively correlated with the carapace length. The egg volume differed between developmental stages I and II, and I and III, with an overall volume increase of 73.23% and 95.45%, respectively. The results contribute to the knowledge about A. carlae and its life history in natural environment.

Climbing behavior of guarding males in the hermit crab Pagurus minutus: the effect of rivals, female size, and being weaponless

Male crustaceans engage in precopulatory mate-guarding to monopolize a female. However, a conspecific rival often takes the female from the guarding males during male–male contests, so guarding males should invest in behavior to decrease the rate of encounters with rivals, especially when they are potentially weak and guard a valuable female. We examined this idea using a net-climbing (avoidance) response by precopulatory guarding males of the hermit crab Pagurus minutus. Guarding males more often climb revetment walls in the field than solitary males and females, and the guarders were larger in body size than the solitary males on the walls. Guarding males increased climbing effort in the presence of rivals compared to controls (no stimulus), but not when faced with a non-threat organism. Compared to intact guarding males, those without a major cheliped (“weaponless”) were less likely to succeed in defending their partner in direct fights, but increased climbing investment overall, rather than especially when a rival was present. Although female size positively correlates with clutch size in this species, male climbing height decreased with increasing size of the guarded female. We discuss the effectiveness and importance of climbing in this species as it relates to conditions at our study site and the general vulnerability of guarding males depending on weapon status.

Thermal stress: The role of body size and the giant major claw in survival and heat transfer of a fiddler crab (Leptuca pugilator)

Mobile intertidal crabs are often exposed to the sun while displaying outside of moist and cooler breeding burrows, which imposes increased heat stress and water. Individuals of a field population in mid-summer of the fiddler crab Leptuca pugilator had body temperatures that declined with increasing body size. All crabs were cooler than the sediment surface. Laboratory experiments simulated a male leaving its burrow and standing on hot sand heated by the sun. Smaller crabs succumbed to thermal stress sooner than larger crabs. Incapacitation keyed to field sediment temperatures occurred usually in about 30 min. Water loss ranged 1–8% at incapacity Small crabs heated up as fast as large crabs. Males with a major claw gained body heat more than clawless males and succumbed sooner. The major cheliped loses water proportionally more slowly than the body, which is likely explained by the more impervious surface of the claw, suggesting the claw might be a heat sink. I hypothesized that heat may be accumulated in the relatively impermeable major cheliped and transferred to the body, increasing thermal stress in the body where crucial organs must function. Several experiments were done to attempt to demonstrate body part differences in heating or heat transfer between cheliped and body. Thermal images demonstrated a temperature excess in the manus, and a lower temperature of the dactyl, consistent with the impermeability of the manus and its larger surface area intercepting heat, and perhaps radiation from the dactyl. Immersion of major claws in warm and cold water demonstrated heat transfer between cheliped and body. If claws are painted black, they heat up faster than unpainted claws, but there was no evidence of the transfer of increased heat to the body when claws were darkened. Thus the potential for heat transfer between claw and body is probably quite limited, which is further suggested by the very narrow connection at the basal ischium segment of the cheliped.The results suggest that smaller male fiddler crabs may have reduced success in the upper intertidal partially because of their higher vulnerability to thermal stress. Smaller males may choose to avoid the hotter upper intertidal until they become larger. Experimental results show no evidence for the role of the major claw in alleviating thermal stress to the fiddler crab carapace.

A new species of Callichirus (Axiidea, Callianassidae) from the Pleistocene bioclastic sediment of southeast Brazil

A new species of ghost shrimp, Callichirus santensissp.nov., is described from the Upper Pleistocene Cananéia Formation, southeast Brazil. The new species is described based upon preserved ischium, carpus, propodus, and dactylus of the male major cheliped. Diagnostic features of the new species include: (i) ischium with distinctive lateral carina; (ii) carpus unarmed; and (iii) cutting edge of dactylus consisting of large subrectangular tooth proximally, large acute tooth centrally, and small blunt teeth distally. The new species represents the first record for the genus from the Pleistocene of Brazil.

A new species of Alpheus (Crustacea, Decapoda, Alpheidae) from theArabian Sea, Kerala, India.

A new species of Alpheus, A. samudra nov. sp., is described from the bycatch of trawlers operating between 275-375 m depth on the Quilon Bank (Kerala, India). The new species belongs to the brevirostris group, but can be easily distinguished from all but one species, by the extremely laterally compressed major cheliped. Alpheus samudra nov. sp. is very similar to A. leptocheles Banner Banner, 1975 in the form of the major cheliped, but the proportions of the fingers to the palm and the shape of the fingers easily separate both species.

Synalpheus amintae sp. nov., a new species of sponge-dwelling snapping shrimp(Crustacea: Decapoda: Alpheidae) from Parque Nacional Natural Isla Gorgona, Pacific Coast of Colombia.

A new species of sponge-dwelling snapping shrimp, Synalpheus amintae sp. nov. from the Parque Nacional Natural Isla Gorgona, Pacific coast of Colombia, is described. This new species pertains to the denominated Synalpheus "Gambarelloides" species group sensu Coutière (1909)-previously known as "Laevimanus" group. It was collected in the southern part of the Island, during episodes of extreme low tides, known locally as "puja". Synalpheus amintae sp. nov. is closely related to the only two other known species of the S. "Gambarelloides" species group from the eastern Pacific: Synalpheus occidentalis Coutière, 1909, from Gulf of San José, Lower California, and Synalpheus mulegensis Ríos, 1992, from Bahía Concepción, Gulf of California, and can be differentiated from them by the poorly developed distal portion of pollex of the major cheliped, the armature of the distal segment of third maxilliped, and the number of acute teeth of exopodal uropod. A discriminative analysis dealing with other three species of Synalpheus from the western Atlantic, with distinctive pollex of major chela reduced, is presented. A dichotomous key was elaborated to identify the species of Synalpheus "Gambarelloides" species group from the eastern Pacific. The number of valid species described of Synalpheus from the eastern Pacific is increased to 23.