What is a Brachiopod? (And Why You Should Care)

The Champ, a brachiopod

Last week in my Fossil Friday post, I featured a brachiopod specimen I called “The Champ”.  If you are not a palaeontologist, you have likely never heard of a brachiopod, and may assume it is some obscure group of little interest.  To be fair, if you are a modern biologist, it is true that brachiopods are of limited significance (yes, brachiopods still exist today, but in small numbers and obscure habitats).  However, for the palaeontology crowd, brachiopods have been the subject of much study, as they are arguably one of the most common groups of marine invertebrates throughout the Palaeozoic.  In fact, brachiopods are unique in that we know more about the fossil critters than those still alive today.

Geologic Time Scale of the Phanerozoic (when multicellular life exists). Brachiopods have been around since the late Cambrian. Image credit: https://www.geol.umd.edu/~tholtz/G204/lectures/204rockclox.html

So what is a brachiopod?  In simple terms, it is a two shelled marine invertebrate, much like a clam or mussel.  But having two shells is about all clams and brachiopods have in common.  One of the first ways we teach students to differentiate brachiopods and clams is to look at the symmetry of the two shells.  The plane of symmetry in clams runs between the shells (there is a left and right shell), whereas the plane of symmetry in brachiopods runs down the midline of each shell (there is bottom and top shell which are bilaterally symmetrical).

Brachiopod vs. clam symmetry. Image credit: https://www.digitalatlasofancientlife.org/learn/brachiopoda/brachiopoda-vs-bivalvia/

Brachiopods belong to their own phylum, whereas clams are a class within the phylum Mollusca (which also includes octopods, snails, and several other smaller groups).  Brachiopods are also part of a larger group of organisms called lophophorates, all of which have a special organ called the lophophore.  The lophophore is a coiled organ with many cillia (tentacles) which actively beat and pump water, providing respiration, and drawing food towards the mouth (located at the base of the lophophore).  Brachiopods are therefore considered “active” filter (suspension) feeders, in that they actively pump the lophophore to pull tiny bits of food out of suspension.  There are two other groups within the lophophorates, bryozoans and phoronid worms, which I will discuss another time.

Internal anatomy of a brachiopod. Image credit: https://paleoaerie.files.wordpress.com/2014/03/brachiopod-2-en.gif

Live brachiopod with coiled lophophore.  Image credit: https://www.inaturalist.org/users/2645718

Another important bit of anatomy that is unique to brachiopods is a fibrous (sometimes muscular) organ called the pedicle, which protrudes from a hole near the back of one shell, and helps to attach the brachiopod to the substrate/surface like an anchor or sucker.  Pedicles can be very strong, and the attachment is usually permanent.  For the brachiopod pictured above, if you tried to pick it up, you would likely rip the shells away from the pedicle and the rest of its internal organs (so if you are ever lucky enough to spot one, please be gentle!).

Inarticulate brachiopod with large muscular pedicle. Image credit: https://classconnection.s3.amazonaws.com/258/flashcards/3427258/png/3336-14048A13B7938A0D0F9.png

Articulate brachiopod with smaller, permanent pedicle. Modified from: https://www.niwa.co.nz/sites/niwa.co.nz/files/styles/large/public/Fig1_comp.jpg?itok=YdB-loHB

There are two main groups of brachiopods: the Inarticulata (above left image), and the Articulata (above right image).  Inarticulates have a large, muscular pedicle, which they use to burrow into soft sediments, leaving only the very top of the shell exposed, whereas articulates have smaller, more permanent pedicles, which help to anchor and keep the brachiopod elevated above the substrate.  Atriculate brachiopods generally do not deal with mud and sediment very well (although check out Richardson 1981 for a further discussion of pedicles and mud).  The chemical structure of the shells is also different, with inarticulate shells not as likely to preserve in the fossil record.  Inarticulate brachiopods are known as “living fossils”, in that they have barely changed since they first appeared in the late Cambrian.  Articulates, on the other hand, are very diverse and abundant throughout the fossil record (although only 3 groups have managed to survive to today).  Some articulates can be shaped like potato chips or have lots of spines, be as big as baseballs or smaller than a fingernail, and even have only one shell.  Most palaeontologists, including myself, study articulate brachiopods.

Some Devonian articulate brachiopods (Fig. 3 Barclay et al. 2013)

So why should you care about brachiopods?

There are endless reasons that brachiopod workers could go on about why brachiopods matter, but I will give you a few, based on my experiences.

Most importantly, however, is that brachiopods can be used for studies in conservation palaeobiology (for most of the reasons I list below).  Conservation palaeobiology is a new, hot field in science which seeks to use palaeontological data to help ecologists and biologists figure out how to preserve modern ecosystems that are facing pressures from human-induced change (i.e. climate change).

1. Brachiopods are probably the single most common fossils for the majority of the history of life.  That means brachiopod workers often have data sets that are well into the thousands, which can have great statistical power for answering lots of questions, especially when it comes to things like changes to community diversity through time, and how communities deal with things like mass extinction events (kind of like the one we are currently experiencing).  You just can’t do that with most vertebrates (no offense to all my vertebrate colleagues.  We all know T. rex is rad, right?).
2. Because they are so common, brachiopods are a great for studying how biotic interactions can develop or change through time.  For example, brachiopods are a major source of food for shell crushing fish and crustaceans, as well as some shell-drilling snails. We can use evidence of predation to observe any changes to the success or frequency of attacks through time, which can not only shape the evolution of the brachiopods, but also their predators.  Plus, no food = no predators. I also look at the relationships between brachiopods and the organisms that encrust them to try and determine the nature of the relationship (parasitism, mutualism, etc.) over time.  Understanding how brachiopods can develop a relationship with their encrusters provides many insights into how other organisms relate.

Living brachiopods being encrusted by other organisms. Image credit: http://www.otago.ac.nz/geology/otago072187.jpg

 Throughout the fossil record, brachiopod numbers and diversity get whacked several times.  A lot of brachiopod workers try to figure out what it is about each group (body shape? geographic spread? diversity?, etc.) that either causes them to go extinct, or allows them to survive.  For example, there is a catastrophic mass extinction at the end of the Permian (much worse than the extinction that killed the dinosaurs), and most brachiopod groups are decimated.  However, a few manage to survive, but never become as diverse or as common as they were during the Palaeozoic.  At the same time, molluscs such as clams become much more abundant.  Many researchers have dedicated their entire careers to try and explain why brachiopods are never able to recover, and why molluscs seem to “take over”.

Brachiopods have relatively simple, but highly varying shapes, which allows us to study the mechanisms of how shape relates to lifestyle (i.e. the study of functional morphology).  To use a very simple example, sponges are usually shaped like tubes because it allows water to pass through them and over their cells just like a chimney sucks out smoke.  We can do lots of experiments that help us determine the basic physics of how brachiopods interact with moving water, which can tell us how they feed and live (e.g. Alexander 1984; LaBarbera 1977, 1978).  Shape relates a lot to how any organism survives and thrives in its environment, and can be applied to the evolutionary history of most groups of organisms, from plants to dinosaurs.

Did I mention we probably know more about the fossil brachiopods than the modern brachiopods?  I’ve been studying brachiopods for about six years, and have yet to see a live one.  In fact, most modern brachiopod researchers rely on palaeontological literature as a basis for their studies.  That doesn’t happen very often.  Fun fact for those of you that read this far: brachiopods may be poisonous.  There is this urban legend people always bring up about some professor that fed his/her students some brachiopods, and they ended up in the hospital (Taylor and Lewis, 2005).

A Jurassic brachiopod with lophophore supports intact from the Natural History Museum collections.  Image credit: http://www.nhm.ac.uk/our-science/collections/palaeontology-collections/brachiopod-collection.html

TL;DR – Brachiopods are neat, and very useful for solving modern biological problems. And potentially poisonous… so don’t eat them…

References:

Alexander, R., R. 1984. Comparative hydrodynamic stability of brachiopod shells on current-scoured arenaceous substrates. Lethaia 17:17–32.

Barclay, K. M., C. L. Schneider, and L. R. Leighton. 2013. Palaeoecology of Devonian sclerobionts and their brachiopod hosts from the Western Canadian Sedimentary Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 383–384:79–91.

LaBarbera, M. 1977. Brachiopod orientation to water movement. 1. Theory, laboratory behavior, and field orientations. Paleobiology 3:270–287.

LaBarbera, M. 1978. Brachiopod orientation to water movement: functional morphology. Lethaia 11:67–79.

Richardson, J.R. 1981. Brachiopods in mud: resolution of a dilemma. Science 211:1161–1163.

Taylor, P. D., and Lewis, D. N. 2005. Fossil Invertebrates. Harvard University Press.