Understanding growth rings in geoduck clams and their historical environmental significance

North Pacific climate recorded in growth rings of geoduck clams: A new tool for paleoenvironmental reconstruction

Robert C. Francis, Nathan J. Mantua, Edward L. Miles, David L. Peterson

Summarized by Baron Hoffmeister

What data were used? Growth chronology (i.e growth patterns that accumulate over years in the shell of the organism, similar to tree rings) of geoduck clams (see figure 1) collected in Washington, USA were used to reconstruct sea-surface temperatures (SST) in the Strait of Juan de Fuca.  

This is an image of a geoduck. These are known to have life spans lasting over 165 years. From How Stuff Works.

Methods: This study used growth ring data in geoduck clams to determine how sea surface temperatures affected the shell growth (something called “accretion”) within these organisms over their life span. 

Results: Geoduck clams are a part of the class Bivalvia (i.e., a marine or freshwater mollusk that has its soft body compressed by a shell; this includes other organisms like snails and squids). These organisms produce their own shells, and the shells continue to grow as these organisms age (unlike organisms like mammals, who stop growing at a certain age). The shell accretion of these organisms can be observed under a microscope from samples of the shells. These are called growth lines and the spacing in between lines indicates how much new shell material the organism produced during a certain period of time (see figure 2). The growth lines of the geoduck clams found within Strait of Juan de Fuca correlated strongly with sea-surface temperatures. Researchers found that when the water was warmer, more growth was observed. This is common for a number of marine bivalves, and these proxy methods help construct a better understanding of sea surface temperatures from the past. 

The top panel is an SEM micrograph of the ring structure in a 163-year-old geoduck clam. An SEM is a scanning electron microscope that uses a focused beam of electrons that interact with the sample and produce signals that can be used to collect data about the surface composition and surface structures. The bottom panel shows the growth index (solid black line) with local air temperatures (dotted line) from 1896 to 1933. From 1900 to 1910, shell accretion correlated with warmer air temperatures.


Why is this study important? This study helps reconstruct environmental conditions and researchers can use this data in conjunction with other climate proxies to better understand how current climate patterns and ocean temperatures can affect marine ecosystems in the North Pacific basin.

The big picture: This study is important, not only for creating a more cohesive climate proxy database, but also indicating that shell accretion in specific marine organisms can provide important climatic data. Bivalves have a large geographic range and the data collected from these organisms through shell accretion studies can allow us to have a better understanding of historic climate conditions worldwide. 


Francis, R. C., Mantua, N. J., Miles, E. L., & Peterson, D. L. (2004). North Pacific climate recorded in growth rings of geoduck clams: A new tool for paleoenvironmental reconstruction. Geophysical Research Letters, 31(6).

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