I was assisting the Estuary Care Foundation to check their oyster baskets at Outer Harbor during October, when I was asked if I had seen a particular crab before. My response may have been something like, “Holy cow, what is that?”. I quickly grabbed my phone and took a couple of photos of the strange crab.

My next instinct was to crab my specimen sampling kit and some fresh seawater to be able to collect the crab for the SA Museum.

I took the crab home and quickly sent my photos to the Museum. Their immediate response was that the crab was Trigonoplax longirostris. They said that they already had some specimens, including some from Outer Harbor. They suggested that I release the crab at a nearby beach. I promptly took the crab to my local beach and let it go in the water.

I later found that Trigonoplax longirostris is also known as the Triangle Crab. I soon posted my three photos of the crab on iNaturalist. I soon discovered that there had been only two previous sightings of the crab in SA recorded on iNaturalist. The last time was 7 years ago, and the first one was 7 years before that. Both sightings were made at Edithburgh.

It appears that Edithburgh is a good spot to find Trigonoplax longirostris. My posting of the crab photos on Facebook led to revelations that SA divers “see them regularly on Edithburgh night dives” (Karolyn Landat).  Karolyn added that she “thought they must be pretty common so didn’t bother logging them or letting anyone know!”

Karolyn kindly sent me this photo of the crab: –

Paul Macdonald commented, “We photographed / filmed one at Edithburgh in September. They are so cool. But we also thought they must be common if we found it.” Dean Spraakman also commented that he had also filmed them at Edithburgh.

Elizabeth Solich kindly sent me this photo: –

Further research revealed that we had our own South Australian Marine Species Lists (of) Crabs & Lobsters – Decapoda online. That list includes Trigonoplax longirostris as follows: –

Trigonoplax longirostris, Triangle crab – 1908, McCulloch

iNaturalist lists the taxonomy of Trigonoplax longirostris as follows: –

Arthropods – Phylum Arthropoda

Crustaceans – Subphylum Crustacea

Typical Crustaceans – Superclass Multicrustacea

Malacostracans – Class Malacostraca

Decapods, Isopods, and Allies – Subclass Eumalacostraca

Decapods and Krill – Superorder Eucarida

Decapods – Order Decapoda

Crabs, Lobsters, and Allies – Suborder Pleocyemata

True Crabs – Infraorder Brachyura

Eubrachyuran Crabs – Zoosection Eubrachyura

Heterotrematan Crabs – Zoosubsection Heterotremata

Superfamily Hymenosomatoidea

Family Hymenosomatidae

Subfamily Hymenosomatinae

Genus Trigonoplax

Trigonoplax longirostris


(My thanks go to Karolyn Landat, Paul Macdonald and Elizabeth Solich for their assistance and photos. My thanks also go to the staff at the SA Museum for their assistance with the identification of the crab species.)

By Steve Reynolds

Steve Reynolds is the current President of MLSSA and is a long-standing member of the Society. Steve is a keen diver, underwater explorer, photographer and is chief author of the Society's extensive back catalogue of newsletters and journals.

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