New research discovering new species of fussy barnacles

Andrew Hosie's blog | Created 3 years ago

Three new species have just been described as part of an ongoing research project between the WA Museum and Curtin University, which has been examining the diversity of a symbiotic group of barnacles found embedded in the tissues of sponges. There were approximately 20 named species of sponge-inhabiting barnacles in Australia and this latest publication from the project adds an additional six species to the Australian fauna, including the three new species. These species were discovered in northern WA and collected during several expeditions.

Acasta caveata is characterised by very large membranous “windows” between the shell plates that can give the shell a cage-like appearance. These membranous areas are in direct contact with the sponge tissues and their exact function is unknown. One theory is that they serve to reduce the cost of making the shell as they are now protected by the sponge host. Another theory is that they may function to exchange nutrients between the barnacle and host.

The very square and boxy Euacasta acutaflava has a yellow-orange chitinous point at the tip of the opercular plates (these close off the shell when the barnacle isn’t feeding), a little bit like a fingernail. The squat appearance of the shell is thought to be, in part, due to the relatively flat and bladed sponges it inhabits.

Euacasta excoriatrix has much longer limbs (used for feeding, called cirri) than related species. The segments of these whip-like limbs are also well armed with teeth and spines. This led to the species name excoriatrix, which translates to “the flayer” as the teeth are used to scrape away sponge tissue to prevent the barnacle from being overgrown.

The new species were described in the first article of a special issue dedicated to Australian marine invertebrate biodiversity in the journal Diversity .

These new species were found inhabiting a single genus of sponge each, and another publication from the project has demonstrated that sponge barnacles can be very fussy when it comes to where they settle. A survey of all sponges of the order Dictyoceratida for barnacles resulted in 42 barnacle species inhabiting 64 host species, demonstrating that most are actually very picky, with many only found in one host species so far. Analyses on barnacle DNA revealed that when a barnacle species uses multiple sponge species, then these are typically closely related, being members of the same genus, or family, of sponge. On the flip side of this is that closely related species of barnacles are often found in closely related sponges. However, host sharing by different barnacle species is very rare.  A strong evolutionary link between host and symbiont is evident and appears to be driving the biodiversity of these barnacles. The study was published in Molecular Phylogenetics and Evolution and can be accessed here: 10.1016/j.ympev.2021.107179.

This project is ongoing with more species to be named and investigations into the evolutionary origins of sponge-barnacle symbiosis. The research is funded in part by the Australian Biological Resources Study and the Gorgon Net Conservation Benefits Fund.