Above image: Left, Brachiopod Paraspirifer brownockeri on exhibit in the Houston Museum of Natural Science, Houston, Texas. Image by "Daderot" (Wikimedia Commons; Creative Commons CC0 1.0 Universal Public Domain Dedication). Right, Clams Myretrix lyrata from a market in Haikou City, Hainan Province, China. Image by Anna Frodesiak (Wikimedia Commons; Creative Commons CCO 1.0 Universal Public Domain Dedication).
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To the untrained eye, one might assume that brachiopods and bivalves belong to the same group. Indeed, many early naturalists considered them to both be mollusks. They are, however, classified as completely different animal groups. Brachiopods belong to Phylum Brachiopoda, whereas bivalves belong to Phylum Mollusca, along with snails and cephalopods (e.g., octupuses and squids). (Learn more about bivalves here.)
Brachiopods: unequal valves (shell halves), lophophore, pedicle
Bivalves: mollusk (calcareous shell, mantle, gills), identical paired valves
Differences in Shell Symmetry
The key to distinguishing brachiopods from bivalves is determining their lines of symmetry. Bivalves have a plane of symmetry that cuts between their two valves. They have symmetry similar to our hands. If you were to separate their two halves (or your two hands), each side perfectly mirrors the other, even though the shell shape itself might not be symmetrical. Bivalves are often described as having left and right valves.
Brachiopods have a plane of symmetry that cuts across the two valves. This you can think of if someone to cut your body in half down the middle, each side would have an eye, arm, and leg that matches the other side. If you were to cut a closed brachiopod shell in half down the center, both halves would mirror each other but the valves themselves may be different shapes or sizes.
Symmetrical differences. Left: brachiopod; Right: bivalve. Image by Jaleigh Q. Pier is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Being so similar in appearance, it’s been assumed that brachiopods and bivalves play similar ecological roles in marine environments (their ecological niche), but this is not true. Bivalves are by far much more diverse in what they eat, where they live, and what they do. Brachiopods, although diverse in shape, share a similar ecology.
Brachiopods have thrived in warm, shallow seas through Earth’s history, although today competition has pushed them into cold, low-light regions of the modern ocean. Brachiopods are prolific survivors in places where their low metabolism, thick shells, and low body mass allows them to persist. Today they live in every ocean and major benthic (ocean-bottom) habitat. You likely haven't seen one because the vast majority inhabit areas not frequented by human activity. Bivalves, however, live along every shoreline and you may have picked up their shells at the beach.
Another important difference separating these two groups is the ability to move around. Most species are almost entirely sessile (stationary) and have adapted to life above the sediment or hard substrate. Lingulid brachiopods are exceptions. They create burrows in the sediment and use their pedicles anchor them down, while the shell (with the lophophore) sits close to the sediment surface to filter feed.
Burrowing differences; Left: Inarticulate lingulid brachiopod, Right: Burrowing bivalve diverstity. Images adapted from Lingula anatina Lamarck, 1801 (Public Domain), The wonders of the shore by Charles Kingsley, 1859 (Public Domain).
Bivalves, however, are burrowing specialists and have diversified their ability to move and dig burrows of varying depths. To achieve this, they use a straw-like structure called a siphon to bring food and oxygenated water into their shells while buried. Other bivalves, in particular scallops, have evolved the ability to swim by rapidly opening and closing their valves.
Swimming bivalve scallops. Video by Subsurface Media.
Brachiopods and bivalves have likely been competitors since they first arose in the Cambrian. That said, brachiopods were much more diverse than bivalves throughout the Paleozoic, right up until the end-Permian mass extinction. This was the worst mass extinction in Earth’s history and completely decimated marine life. Brachiopods particularly suffered relative to bivalves and never fully recovered to their previously dominant numbers.
Bivalves have blossomed in diversity during the recent geological past; their gills and higher metabolism likely aided in their success. Bivalves also live across a larger range of habitats, including both marine and freshwater ecosystems, while brachiopods are restricted only to marine environments. Brachiopods have no gills and instead use their lophophore to both eat and breathe, which seems to support a much lower metabolism.
Brachiopod and bivalve diversity curves through time. Notice the switch in dominance at the Permian Mass Extinction (252 Ma). Image by Jaleigh Q. Pier is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
References and further reading
Boardman, R.S., Cheetham, A.H., and Rowell, A.J. 1987. Fossil Invertebrates. Blackwell Scientific Publications. 713 pp.
Carlson, S.J. 2016. The Evolution of Brachiopoda. Annual Reviews of Earth and Planetary Sciences, 44:409-438.
Gould, S., and Calloway, C. 1980. Clams and brachiopods—ships that pass in the night. Paleobiology, 6(4), 383-396.
Peck, L.S. 2001. Ecology of Articulated Brachiopods, in Carlson, S.J. and Sandy, M.R. ed., Brachiopods Ancient and Modern. The Paleontological Society Papers, Volume 7:171-184.
A. Selden editor. 2007. Treatise on Invertebrate Paleontology, Part H, Brachiopoda Revised, Volume 6. The University of Kansas and Geological Society of America. 3226 pp.
Tree of Life Web Project. 2002. Brachiopoda. Lamp shells. Version 01 January 2002 (temporary). http://tolweb.org/Brachiopoda/2494/2002.01.01 in The Tree of Life Web Project, http://tolweb.org/
Unless otherwise indicated, the written and visual content on this page is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. This page was written by Jaleigh Q. Pier. See captions of individual images for attributions. See original source material for licenses associated with video and/or 3D model content.