Identity and Host-Specificity of Sponge-Dwelling Barnacles (BSc Thesis)

Free-living barnacles are ubiquitous components of many intertidal and shallow subtidal zones worldwide, and have been extensively studied. However, the symbiotic relationships of balanomorph barnacles with their host organisms remain less well understood. These symbiotic species are found on motile hosts (e.g., whales, sirenians, turtles) or embedded within sessile invertebrates such as sponges, cnidarians, and bryozoans. Sponge-inhabiting barnacles are particularly diverse and often known to be species-specific. These barnacles embed their bodies within host tissue and extend their cirri through openings on the sponge surface for suspension feeding, preferentially positioning themselves on the inhalant side where water currents are strongest. Studies suggest that as barnacles become morphologically more specialised, the range of potential hosts narrows, leading to symbiotic associations.

Sponges of the genus Petrosia are known to produce a diverse array of secondary metabolites, likely mediated in part by their associated microbiome, that serve as effective chemical defences against predators and fouling organisms. This chemical defensive system may also explain why relatively few critters are found within their tissue. The branching worm R. kingghidorahi and these consistently co-occurring barnacles are notable exceptions to this pattern. While the Ramisyllis-sponge association has been partially characterised, the identity, host-specificity, and ecological role of the barnacles within this symbiotic system are still unknown.


Project description
This project aims to identify the barnacle residing in the Petrosia sponge that also hosts Ramisyllis, and to investigate host-specificity using an integrative taxonomic approach combining molecular and morphological data. You will work with barnacle specimens obtained from sponge samples, originating from Sado Island, Japan. Using multilocus sequencing, you will generate molecular data for phylogenetic inference. Morphological analysis will be conducted using existing 3D CT-scan data and optical and electro microscopy. By integrating molecular phylogenetics with morphological characters and comparing your findings to published literature on sponge-associated barnacles, you will address the following questions:

Main questions:

(I) What barnacle species are we dealing with?
(II) Is this barnacle obligately associated with the sponge host from Sado Island, or has it been reported in other taxa as well?
(III) Does this represent a potentially undescribed species endemic to the Japanese seas?
(IV) What morphological specialisations (if any) are associated with this symbiotic lifestyle?