Queensland Coast: September 2021

Did oysters disappear from Queensland mangroves or were they never there to start with. There have been some big environmental changes but they occur slowly and go unnoticed. This is the shifting baseline phenomena, where powerful changes occur too slowly for people to see from personal experience but which totally change places from generation to generation.

A document from the NSW government included a photo of an aboriginal man harvesting oysters from the mangroves. Even similar swamps in southern Queensland are oyster free. Did the oysters disappear from Queensland swamps before my time? If so why?

Aboriginal man harvesting oysters (photo: NSW museum)

Oysters thrive in the tropics and are present on the rocky or concrete breakwaters, often as thick crusts. In smaller mangrove creeks where stilt roots descend from high branches and dangle into deep water, clumps of oysters form at the club ends where the roots stop growing. Obviously, it is not a water quality issue that prevents oysters from growing in the mangroves.

Oysters on thriving on rocks inside Cairns Marina

I began to notice broken shells on the mud below mangrove trees and even patches of bark crushed off the place where an oyster was attached. This implicates something that can nip or bite. As the oysters on dangling roots are fine, we can rule out fish. This leaves crabs. Famously, Queensland has mud crabs (Scylla serrata) and these very powerful crabs can crush shellfish. The lumps in the gape of their nippers are even called molars and these lumps serve to crush hard shells. Another crab is the thunder crab, which is so named as you need thunder to get them off if they latch on to you.

Remains of an oyster

Crunch marks on mangrove stilt roots

Thunder crabs (Myomenippe fornasinii) are probably specialised oyster predators. These heavily built crabs are very slow moving but powerful. They are relatively hard to find but live in mangrove estuaries and passages. They are probably the ones that crush oysters from mangrove roots.

Thunder crab in action!

Thunder and mud crabs are probably not entirely responsible for depleting oysters in the mangroves, as oysters are abundant in places that these crabs cannot reach or in more southerly areas where cold water temperatures limit crab activity. Historical photos suggest that at least some mangrove areas were oyster encrusted and is possible that crabs were eaten or controlled by indigenous people to allow this to happen. At river mouths and lake entrances in NSW, there are carpets of oysters such as can be seen in the top photo in this post.