Mud crabs are categorised into four species within the genus Scylla which are Scylla Serrata, Scylla Paramamosain, Scylla Olivacea and Scylla Tranquebarica. Though all mud crabs are anatomically similar, every species can be identified through several characteristics and structures. These species are distributed along the Indo-West-Pacific region. Below are the countries that are defined as Indo-West-Pacific region:
Australia
Bangladesh
Bhutan
Brunei
Cambodia
Fiji
India
Indonesia
Japan
Laos
Malaysia
Maldives
Myanmar
Nepal
New Zealand
Papua New Guinea
Philippines
Singapore
Sri Lanka
Taiwan
Thailand
Timor Leste
United States
Vietnam
In RAS Aquaculture, two species of mud crabs; Scylla Olivacea and Scylla Tranquebarica can be found in the crab farm.
Scylla Olivacea is the most common species of Scylla in markets in Sundaic Southeast Asia and Thailand. Its habitat is intertidal to subtidal, estuarine, found on mangroves (Herbst, 1796). For Scylla Tranquebarica, they inhabit mangrove areas down to sublittoral parts (Fabricius, 1798).
The differences between Scylla Olivacea and Scylla Tranquebarica can be detected from their colour, size, and morphological structures.
Scylla Olivacea has orange or brown to black claws. Meanwhile, Scylla Tranquebarica mostly has purple claws.
Secondly, the size. For Scylla Tranquebarica, they can amazingly grow up to 1 kg. However for Scylla Olivacea, they do not seem to grow bigger than 800 g. According to studies (Waiho, Fazhan & Ikhwanuddin, 2016b; Fazhan et al., 2017a), Scylla Tranquebarica is the second largest species of Scylla after Scylla Serrata while Scylla Olivacea is considered the smallest among the four species. Yet, Scylla Olivacea is the sturdiest when it comes to acclimatisation to their natural habitat.
Next, the morphological structures of both species are different too. The shape of Scylla Olivacea’s frontal lobe spines is more rounded while Scylla Tranquebarica is blunt shaped.
Secondly, the propodus spines on the crab’s claws. Scylla Olivacea’s propodus spines are reduced, Scylla Tranquebarica got it obvious on both spines.
Thirdly, Scylla Olivacea does not have polygonal patterning on all claws and legs. Scylla Tranquebarica on the other hand (or on the other claw), have polygonal patterning on the third, fourth, and fifth legs.
In conclusion, Scylla Olivacea and Scylla Tranqeubarica can be differentiated through their claws' colour, size, and three morphological structures - frontal lobe spines, propodus spines, polygonal patterning.
References:
Fazhan, H., Waiho, K., Quinitio, E., Baylon, J. C., Fujaya, Y., Rukminasari, N., ... & Ikhwanuddin, M. (2020). Morphological descriptions and morphometric discriminant function analysis reveal an additional four groups of Scylla spp. PeerJ, 8, e8066.