Morphological descriptions and morphometric discriminant function analysis reveal an additional four groups of Scylla spp.

Hanafiah Fazhan,Yushinta Fujaya,Hongyu Ma,Mohammad Farhan Darin Azri,Md Sheriff Shahreza,Emilia Quinitio,Khor Waiho,Juliana C Baylon,Nita Rukminasari,Mhd Ikhwanuddin

doi:10.7717/peerj.8066

Abstract

There are four species of mud crabs within the genus Scylla, and most of them live sympatrically in the equatorial region. Apart from a report in Japan about the finding of a natural Scylla hybrid more than a decade ago after the division of genus Scylla into four species by Keenan, Davie & Mann (1998), no subsequent sighting was found. Thus, this study investigates the possible natural occurrence of potential hybridization among Scylla species in the wild. A total of 76,211 individuals from mud crab landing sites around the Malacca Straits, South China Sea and Sulu Sea were screened. In addition to the four-purebred species, four groups (SH 1, n = 2, 627; SH 2, n = 136; SH 3, n = 1; SH 4, n = 2) with intermediate characteristics were found, mostly at Sulu Sea. Discriminant Function Analysis revealed that all Scylla species, including SH 1 - 4, are distinguishable via their morphometric ratios. The most powerful discriminant ratios for each character and the top five discriminant ratios of males and females were suggested. The carapace width of SH 1 males and females were significantly smaller than pure species. Based on the discriminant ratios and the description of morphological characters, we hypothesize that the additional four groups of Scylla with intermediate characteristics could be presumed hybrids. Future work at the molecular level is urgently needed to validate this postulate.

Highlights

Mud crabs belonging to the genus Scylla are distributed along the Indo-West-Pacific region and are divided into four distinct species, namely S. serrata, S. olivacea, S. tranquebarica and S. paramamosain (Keenan, Davie & Mann, 1998)
The four pure-species, S. olivacea, S. paramamosain, S. tranquebarica and S. serrata found in this study conformed with the morphological description provided by Keenan, Davie & Mann (1998)
Present study confirmed that the morphological characters of the four pure Scylla species (S. olivacea, S. paramamosain, S. tranquebarica and S. serrata) described by Keenan, Davie & Mann (1998) was similar to the morphological characteristics of all four pure-species found in this study

Summary

Introduction

Mud crabs belonging to the genus Scylla are distributed along the Indo-West-Pacific region and are divided into four distinct species, namely S. serrata, S. olivacea, S. tranquebarica and S. paramamosain (Keenan, Davie & Mann, 1998). The detailed morphological and morphometric differences among the four species in the genus Scylla—S. serrata, S. tranquebarica, S. paramamosain, S. olivacea—were described by Keenan, Davie & Mann (1998) and were used as the basis for species differentiation in most studies on mud crabs (Klinbunga, Boonyapakdee & Pratoomchat, 2000; Imai et al, 2004; Ikhwanuddin et al, 2011; Fazhan, Waiho & Ikhwanuddin, 2017b; Waiho et al, 2017b; Waiho et al, 2017c; Waiho et al, 2018), including the current study. Due to the acclimatization to their natural habitat (upper river mouths with lower salinity and constant exposure to intertidal currents; (Fazhan et al, 2017a), S. olivacea are the sturdiest

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