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Menaethius monoceros

Spider crab, Decorator crab

Chi Chiu Lo (2014)



Fact Sheet


Brief Introduction


Physical Description



Relationship With Other Organisms

Life History & Behaviour

Anatomy & Physiology


Biogeographic Distribution

References & Links


Infraorder Brachyura
(fig. 5)

Menaethius monoceros is classified under the infraorder Brachyura (true crab) that is separated into two major groups, Eubrachyura (advancedcrabs) and Podotremata (primitive crab). Podotremes were seen as primitive because of their larval morphology and more ancestor-liked adult (Ahyong et al. 2007).

Figure 5. Classification of the Branchyura based on single minimal length topology derived MP analysis. (Ahyong et al. 2007)

Subfamily Epialtinae
There were a few studies divided Epialtinae into groups by using different methods. Guerao and Abello (1996) divided Epialtinae into three groups based on the dorsalspine development on the carapace of the zoeae. Species in subgroup 1 has a dorsalspine that is longer than the eye diameter, such as Pugettia spp. (fig. 6, right). On the other hand, subgroup 2 species has a dorsal spine that is shorter than the eye diameter, such as Acanthonyx spp. (fig. 6, left). Subgroup3 species does not have a dorsal spine which only includes one species, Menaethius monoceros.

Figure 6. Left: Lateral view of Acanthonyx lunulatus (second stage) (scale 0.5mm) (Guerao andAbello 1996). Right: Lateral view ofPugettia intermedia (first stage) (scale=0.1mm) (Ko 1998)

On the other hand, Terada (citedby Ko 1998) divided Epialtinae into 2 groups, (1)
Menaethius and Huenia, and (2)Pugettia based on the chromatophore location, the setation of the maxilule and maxilla, and the properties of the telson. While Pugettia has a seta on the proximal segment of the maxillule, Menaethius and Huenia does not.

Genus Menaethius
Barnaed (1950) stated that Menaethius spp. has simple dorsal-ventrally depressed rostrum which flanked by salient supraorbital spines, non-subchelate walking legs, cylindrical sixth joint, short eyes butextend beyond the carapace margin.

Other than that, both Ko (1998)and Guerao and Abello (1996) pointed out that
Menaethius spp. zoeae can be easily distinguished by recognising the absent of dorsal spine on the carapace.

Species complex
Menaethius monoceros is the only species of its own genus. However, when Plaisance et al. (2009) analysed the gene of reef-associated crustacean, they found that M. monoceros probably is a species complex. Mitochondrial cytochrome oxidase subunit I (COI) gene is commonly used gene sequence for delimitation of the marine invertebrates. The COI gene sequence of the M. monoceros sample collected from middle Pacific area, only matched COI gene sequence of the M. monoceros in the GeneBank by less than 85%. This implied that M. monoceros probably is a species complex.