In a recent study, the biomolecules making up brachiopod shells were examined to understand shell evolution.
Brachiopods are shelled sea creatures that are best known for their rich fossil record. They appeared during the Cambrian era about 500 million years ago and still have some living members today (though most of the 30,000 species are known only from fossil records). Brachiopods have two shells but are distinct from more familiar shelled organisms such as clams or oysters, which are bivalves.
The distinction is made in shell symmetry: brachiopods have a line of symmetry perpendicular to their shell hinge, which means looking from top down each half of their body is symmetrical, while bivalves have parallel symmetry along the hinge because their bottom and top shells are mirror images.
It seems like a subtle difference, but to taxonomists (researchers who study evolutionary relatedness) it is an important clue that these two groups, although extremely similar in appearance and lifestyle, are less related than one may suspect. However, determining relatedness based on only physical appearances can be misleading (think convergent evolution), so researchers often study biomolecules such as DNA and proteins to more fully understand evolutionary origins.
In a recent study researchers at the Ludwig-Maximilians-Universitaet in Munich conducted a molecular characterization of the proteins in brachiopod shells. Their work is published in the latest edition of Genome Biology and Evolution.
Shells in marine environments are a composite of calcite or calcium phosphate, proteins, and polysaccharides. The latter two materials are often specialized to certain lineages and give shells distinct material properties.
The study showed that some molecules used to produce brachiopod shells are unique when compared to other common shell-bearing sea creatures. On the other hand, many proteins show deep evolutionary roots because of their highly conserved structures between brachiopods and other organisms.
To understand the evolution of these conserved proteins, the team used techniques referred to as proteomics and transcriptomics to probe the physical and genetic features of the proteins. This approach of exhaustively characterizing molecular content of marine shells has been applied to other organisms such as corals, sea urchins, and mollusks (bivalves), but not to brachiopods. This enabled the researchers to compare their results to these other shelled organisms to determine similarities in the proteins involved in shell formation.
Their results let them to a few interesting conclusions. First, they observed that the seven most abundant proteins found in Magellania venosa (the study’s model brachiopod organism) were completely unique to this group. However, despite this unique set, there was significant similarity in amino acid sequence (the monomers that make up proteins) in most other proteins shared between brachiopods and their fellow shelled metazoan.
This shows that even though there are probably long timescales separating brachiopods and other shelled lineages, such as bivalves, there is deep evolutionary conservation of protein structure and function in shell formation. This could point to important molecules acting as a sort of glue for healthy shells that are strongly and positively favored by natural selection. The researchers also contributed several important methodologies to the field for isolating proteins from shells and detailed their different preparation strategies.
Overall, this research sheds light on the evolution of biological molecules involved in shell formation across different marine lineages. It shows that many proteins are highly conserved and are likely to be crucial in shell formation and integrity. Though brachiopods and bivalves ultimately evolved to exhibit different morphological characteristics, molecular protein similarities show their ancestral relatedness.
However, the study also provides evidence that brachiopods have evolved their own distinct proteins over time following an ancient split from their shelled relatives. This is evidence that there is more than one way to build a shell and that niches existed in the early oceans for brachiopods to differentiate. But how did these niches come about? Are niches of anthropogenic origin being produced in our world today that could lead to future evolution?
These are tantalizing and important questions in marine biology today. Research into the biochemistry of shells in the ocean is increasing in the face of chemical changes in ocean waters, such as decreased pH. It is not clear, however, what effects these changes will have on organisms that rely on shells for survival. Recent research has shown that some fish species may be able to recover or even thrive in acidifying oceans, but shelled organisms that heavily rely on ocean chemistry may not be so lucky.