The graptolite Rhabdopleura recondita tube composition, development and morphological invariance (Hemichordata, Pterobranchia)

The phylum Hemichordata is comprised of exclusively marine organisms, and together with the Echinodermata and Chordata forms the Deuterostomia branch on the animal tree of life. In the introductory and second chapters I provide a background on Hemichordata including the solitary Enteropneusta and the colonial Pterobranchia and define them in an evolutionary or phylogenetic context. The enteropneusts are often regarded as the best living proxy of the deuterostome ancestor. Pterobranchs, include the Cephalodiscida and Graptolithina. Graptolites (graptos=written, lithos=rock) are mostly represented by fossil species dating back to the Cambrian Period, more than 500 million years ago. These “writings in the rock” are widely known and studied by paleontologists and are so abundant that they are used as index fossils to identify sedimentary layers. Graptolites are extinct but for five benthic species belonging to the genus Rhabdopleura, members of the Rhabdopleurida, which I extensively review in chapter three. Rhabdopleura recondita from the Mediterranean Sea is the subject of this thesis. It is common along the south coasts of Italy from where I sample it by SCUBA diving. It is unusual in that colonies reside hidden inside of the zoaria of dead bryozoans. Only erect tubes project from the host matrix. Chapters four and five are the most significant contributions of this thesis, with a focus on R. recondita tubes. Chapter four provides observations of tube building by R. recondita kept in captivity. I observed larvae, zooids and colonies abilities to secrete new tubes in the presence and absence of the bryozoan zoarium host material. We discovered that larval settlement and dome secretion can occur without the bryozoan host, but the continued growth of the colony requires the host substrate. Adult zooids can reform new tubes only if they are able to shelter inside of host material. A surprising result from the zooid observations was the secretion of an operculum and a flared tube. Colonies that had erect tubes removed were able to make new tubes, but fewer in number. A parallel study was done on colonies that had tubes removed and then were cultured in channels at four flow velocities. This experiment was designed to induce a phenotypic plastic response to flow. Instead, I found no significant difference in tube length or tube number in response to four flow velocities. This result suggests that the tube development of R. recondita may be canalized, or fixed. It is significant because it suggests that small differences that distinguish primitive, encrusting graptolite species, are good. Chapter five is on the composition of R. recondita tubes. Several hypotheses and numerous analysis have been done on this topic, but none were conclusive. Here I use genomics and bioinformatics, immunochemistry and spectroscopy and reject the hypotheses that the tubes contain keratin or cellulose. Instead I found eight chitin synthase genes in the genome and transcriptome, a complex made of a chitin-like polysaccharide, protein, fatty acid and unexpected elemental components. This study is significant because it closes the door on old hypothesis of graptolite tube composition and reveals that it is a complex structure including chitin. The conclusion chapter is a brief summary of the results and a reflection on fruitful avenues of future research.

Keyphrases: Rhabdopleura, Graptolithina, PhyloCode, Immunochemistry, Phenotypic plasticity, Bioinformatics