The featured crab was encountered in a terrestrial forest inhabited by scorpions, whip scorpions, centipedes and other terrestrial arthropods. It was initially mistaken for a freshwater crab of the genus Geosesarma. The individual had probably wandered from the mangroves on the other side of the track, which runs alongside the creek (Sungei Puaka). Perisesarma are found largely in mangroves, and the species known from Singapore also includes Perisesarma eumolpe and P. indiarum (Wang & Ng, 2011). Although Perisesarma fasciatum is known from Singapore (Wee & Ng, 1994: 82 as Parasesarma fasciatum), this is believed to be the first record of the species on Pulau Ubin (Joelle C. Y. Lai, personal communication). The specimen has been deposited in the Lee Kong Chian Natural History Museum at the National University of Singapore.

Periophthalmodon schlosseri is the largest mudskipper in Singapore, growing up to 30 cm total length, and characterised by a distinctive black stripe on its body (Fig. 5; Larson & Lim, 2005). It is strictly carnivorous, consuming fish, crabs and insects (Su & Lim, 2016); and occurs in Singapore’s mangrove habitats. Intraspecific behavioural displays are most prominent when the tide rises as individuals compete for available space along the water’s edge, such as against a prop root, a fallen branch, or even a man-made structure. The giant mudskipper is highly territorial and fin-erecting displays between conspecifics readily escalate to aggressive chases. A conflict typically ends when the loser swims away in search of another perch (personal observations). In the featured observation, the two Periophthalmodon schlosseri swimming together in close proximity seems atypical and could possibly represent courtship behaviour. Another interesting observation is the absence of the black lateral stripe that is diagnostic of the giant mudskipper (Fig. 5). The two fish in the present observation could be displaying courtship colouration or territorial aggression. As this species is not externally sexually dimorphic, it is challenging to identify heterosexual pairs and courtship behaviour in the field.