A tiny freshwater crab Amarinus lacustris with three eyes collected from a New Zealand river represents a unique malformation that has never been described before in a crustacean or any other arthropod.

First described by zoologist Charles Chilton in 1882, Amarinus lacustris is an omnivorous, freshwater spider crab found in rivers of south-eastern Australia, New Zealand and nearby islands.

The species grows up to 1 cm wide and has an H-shaped groove on its back.

The malformed specimen was found in Hoteo River, a river that feeds into Kaipara Harbour, north of Manukau and Waitemata Harbours, near Auckland, New Zealand. It has three compound eyes and a third antenna-like structure on the back of its carapace.

“In New Zealand, Amarinus lacustris is the only freshwater crab and is common in a number of streams and rivers of the Auckland and Waikato Regions, and all specimens collected thus far have been normal,” Dr Stephen Moore from the University of Auckland with co-authors wrote in the paper published in the journal Arthropod Structure & Development.

They described the three-eyed mutant: “the lateral two eyes are situated in the outer angles which are formed between the lateral sides of the two rostra and the carapace margin, i.e. in the expected position.”

“The third eye lies at the same horizontal level in the middle between the two lateral eyes, underneath the anterior opening of the notch between the two rostra.”

“The median eye is slightly larger than the lateral ones and it has an oval shape with its large axis horizontally oriented.”

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Bibliographic information: Gerhard Scholtz et al. A crab with three eyes, two rostra, and a dorsal antenna-like structure. Arthropod Structure & Development, published online November 7, 2013; doi: 10.1016/j.asd.2013.10.007