The horny spider crab genus Menathius H. Milne Edwards, 1834 (Epialtidae Macleay, 1838) contains only two species: Menaethius monoceros (Latreille,1825) and Menaethius oriantalis (Sakai, 1969) [1,2,3]. They are common in the Indo-West Pacific extending from the Red Sea, Eastern Africa, and the Indian Ocean to Japan in the Pacific Ocean [1]. Recently, M. monoceros had its distribution extended and was recorded from the Mediterranean Sea by [4]. These species are known to inhabit weedy intertidal rocks in shallow waters [1, 2, 5, 6]. This genus can be distinguished by the following morphological characters: the presence of two small lobes or tubercles on the branchial margins, a small post-orbital lobe, propodi of the first walking leg is smooth ventrally, and with sexes similar in form. It is easy to differentiate M. monoceros from M. oriantalis by the presence of a slender rostrum, basally narrow; the dorsal branchial region has tubercles or rounded elevation; and walking legs are not carinate in M. monoceros. On the other hand, carinate walking legs are present in M. orientalis [1]. However, this genus has a geographical variation, as many authors have described several species which synonymized under M. monoceros [2] and listed by [7] treated in the list of Griffin and Tranter as synonyms [1].
Majidae is considered as a diverse group within brachyuran crabs, containing over 800 species, and was recently transferred to the superfamily Majoidea Samouelle, 1819 [3, 8,9,10]. Despite the revision of the majoid crabs by many authors [1,2,3, 11,12,13,14] throughout the Indo-West Pacific region and the list of [15] from the Arabian Gulf, some genera and species still have taxonomic confusion, and many workers have difficulty in identification using the keys within subfamilies to recognize the genera and species.
The eastern Egyptian coast of the Red Sea includes the eastern and western coasts of the Suez Gulf and the western Aqaba Gulf coast, with a total length of about 1300 km [16]. The Suez Gulf is considered as the boundary between Asia and Africa having an important role in the faunal migration between the Red Sea and the Mediterranean Sea [17] via the Suez Canal. The coasts of the Red Sea are represented by several habitats: soft, rocky, and coral reef habitats. These different habitats comprise suitable substrates as refuges and preferable habitats for different marine invertebrates.
The crabs of the Red Sea were listed and reviewed by [18,19,20,21,22,23,24,25]. Other later workers reviewed the majoids [11] and listed 32 species, as well as accounts of Red Sea majoids by [1, 26] who listed 46 spider crab species within Majoidea including 12 species in the family Epialtidae: Acanthonyx elongatus Miers, 1877; A. dentatus H. Milne Edwards, 1834; Huenia heraldica (De Haan, 1837) [= Maja (Huenia) proteus De Haan, 1839]; Menaethiops contiguicornis (Klunzinger, 1906); M. dubius Balss, 1929; M. ninii Guinot, 1962; M. nodulosus (Nobili, 1905); Menaethius monoceros (Latreille, 1825); Perinia tumida Dana, 1851; Simocarcinus pyramidatus (Heller, 1861); S. simplex (Dana, 1852); and Xenocarcinus tuberculatus White, 1847.
The recent revisions of brachyuran crabs depending only on morphological descriptions have many issues [27, 28]. And therefore, the use of DNA barcoding can help accelerate the identification of confused and even cryptic species [27, 29]. Molecular phylogenetics is a valuable tool to study the morphological evolution of decapod crustaceans, which may reflect their behaviors and distributions [27, 30,31,32,33].
Furthermore, using mitochondrial cytochrome oxidase subunit I gene in molecular studies in crustacean’s taxonomy is useful for species delimitation [29, 34]. Besides COI, the 16S rRNA gene is commonly used in constructing animal phylogeny because of the combination of the variable and conserved region with the same gene [30]. Also, the sequence lengths obtained with these two markers are in the range of sequences available in the GenBank database. With this in mind, the present study applied the DNA barcoding information of the spider crab M. monoceros exclusively from the Red Sea, Egypt, and successfully used phylogenetic analyses to confirm two possible morphotypes.