Australia's Diverse Sponge Barnacles

Andrew Hosie's blog | Created 2 years ago

When people think of barnacles they normally picture a small, conical shell clinging to rocks at low tide or to ships’ hulls and normally thought of as being a nuisance. What most people don’t realise is that barnacles are crustaceans (so closely related to crabs and shrimps) that have specialised and adapted to almost every marine environment, from the depths of over 5000m to being exposed at low tide for hours a day.

Most barnacles will attach to pretty much anything they can, but many have become exceptionally fussy. Barnacles show remarkable adaptations to living in association with other organisms including living under sea anemones, embedded in corals, attaching near the mouthparts and gills of crabs, parasitising deep sea sharks and living on the backs of whales and turtles.

One of the most species-rich groups of barnacles have adapted to living embedded in sponges. The sponge completely covers the barnacle’s shell except for a small hole that allows the barnacles to feed.

Representative specimens of sponge-inhabiting barnacles from Australia.

Representative specimens of sponge-inhabiting barnacles from Australia. Top left and centre show a sponge inested with barnacles (lumps with holes) and a cut-away showing the barnacle nestled in the sponge.
Image copyright WA Museum

Barnacles are filter feeders and use their long feathery-limbs to trap plankton and other food particles from the water. Sponge barnacles presumably gain some form of protection from the elements or predators by being embedded in the sponge, but it is unclear if the sponge gains from or is harmed by the barnacles. These sponge-inhabiting barnacles belong to the subfamily Acastinae and there are presently 19 species reported in Australian waters (>80 worldwide).

WA Museum staff, in collaboration with Curtin University, are currently researching this group trying to determine just how many species there are in Western Australia, what hosts they inhabit and how they interact with their hosts.

A micro-CT scan of a sponge barnacle inside its host sponge. most of the sponge

A micro-CT scan of a sponge barnacle inside its host sponge. most of the sponge tissue has been removed from the image revealing the barnacle within.
Image copyright WA Museum

This research project is using advanced techniques such as DNA sequencing and micro-CT scans to unravel the diversity of the group. The below video shows a 3D scan of a barnacle still inside its host, at the beginning you can see the sponge’s tightly packed skeleton, these spicules can then be removed, revealing the barnacle beneath.