The Clam Lifecycle

Small clam being measured

Collecting data about clam shell length.

There are many reasons that it will be useful for students to have an understanding of the softshell clam (Mya arenaria) life cycle as they work as part of CSI-Maine. It will help them understand when clams are most vulnerable to predators, how long it takes for a clam to reach legal size, why there can be lots of seed clams even in areas where there are no adult clams, and much more. Understanding the clam lifecycle will also be important for using CSI-Maine in support of learning goals related to ecological concepts such as predator/prey relationships and population dynamics.  So … here is a “very short course” on the topic.

The picture on the right shows a student measuring a clam that was collected in the fall, after a summer of growth. If you click on the picture to see a larger version, you can see that this little clam has a shell length (SL) of about 7.5 mm. That turns out to be a pretty good amount of growth when you consider that the clam probably measured only about 0.2 mm (the thickness of a human hair is about 0.1 mm) when it settled into the mud earlier that spring. The diagram below provides an overview not only of that first year of growth, but also of the full softshell clam lifecycle from starting as an egg through adulthood and reproduction.

Clam Lifecycle

Lifecycle of the soft shell clam (reproduced from a 1983 Maine Department of Resources report by C.T. Newell)

Let’s walk through this diagram, starting on the lower left. Male and female clams release sperm and eggs into the water when the water temperature is around 10° C (50° F). After about 12 hours the fertilized egg has developed into its trocophore larva form that is less than 0.1 mm long — about the width of a human hair. A trocophore is a kind of free-swimming plankton. (Here is a link to a Wikipedia article with some pictures of trocophores. You can also find YouTube videos that show them swimming.)

After a couple of days, the clam turns into a different kind of larva, called a veliger larva, which is when it begins to develop its shell. The veliger larva is still free-swimming, though most of the distance that it covers is because it is carried along by ocean currents.  The veliger larva feed on phytoplankton.

The veliger stage can last several weeks, depending on how warm the water is. The warmer the water, the faster that the veliger grows. As it grows, its shell thickens, making the tiny clam heavier. The veliger also begins to develop the muscular foot that enables it to move around once it settles out of the water column. Once the clam settles down into the mud or rocks along a shoreline, carried in by current and waves, it can attach itself to the bottom by secreting a sticky thread called a byssus. At this point, the clam is still very small, with a SL of only about 0.2 mm, or about twice the thickness of a human hair. During this very early transition from the veliger stage to being a juvenile clam, the young clam can still use its foot to move around a bit in order to find a more favorable location, which is usually a mixture of sand and silt that will allow the clam to burrow into the mud.

Settling into the Mud: Life as an Adult Clam

Clams protect themselves from birds, crabs, and other predators that live above the surface of the mud by digging down into the mud. If you look back at the diagram, above, you will see that the clams that are in the mud extend a long siphon up to the top of the mud. Clams are “filter feeders,” which means they take in seawater and then filter out all of the very tiny organisms that are floating around in the seawater. These tiny organisms are the food that keep clams alive and enable them to grow. So, the siphon that clams extend up through the mud to the water is how they get food while living in the mud. Adult clams can dig down as far as 30 cm, or about 1 foot, into the mud.

In Maine, clams that are less than a year old–the ones that settled in the spring and grew over the summer–typically have a SL of less than 15 mm. (In areas where the water is warmer, like Chesapeake Bay, they can reach twice that size by the end of their first growing season.) Smaller clams do not dig down very far — usually only a centimeter or two. This has a couple of consequences.  The first is that it easy for crabs, birds, and other predators to dig them up. Another is that they are still vulnerable to being picked up and moved around by wave action.

Clams do not grow much over the winter. The ones that survive their first winter start growing again the next year as the water warms up, with most growth happening between July and September. We know that clams grow at different rates in different coves and at different locations along the “tidal gradient,” which you can think of as a line stretching from where the edge of the water is at high tide down to where it is at low tide. In fact, one of the important questions that students will explore in this study is how clam growth changes from place to place. But, acknowledging all of that, it is safe to say that in many places we would expect to see clams grow 10mm or more during their second year, reaching a size of 25 mm or more.

A clam needs to have a SL of 2 inches, or about 51 mm, in order to be of legal size for harvesting. So, that means that it will need to spend at least one and perhaps two more summer seasons living in the mud and growing before it can be harvested.

Starting the Cycle Again

When female softshell clams reach a SL of about 40 mm they can begin to produce eggs. When one considers the information about clam growth in the preceding paragraphs, this means that most female clams need to survive for at least 3 years before they can begin to replenish clam stocks.

References

Most of the information provided here comes from Abraham and Dillon’s 1986 report to the Fish and Wildlife Service. Here is a link to the report; the full reference is below. Some teachers might want to share this report with students who want to dig more deeply into the life history of softshell clams. The report uses technical terminology that is common in zoology; students would need to be encouraged to look up such terms using an online resource such as Wikipedia. Teachers and students should keep in mind that the Abraham and Dillon report is focused mostly on clams in mid-Atlantic states such as Maryland, where the water is warmer and clams grow much more rapidly.

The Maine Clam Handbook, linked here and also referenced below, is another useful resource, though it is now somewhat dated with regard to the impact of predators. It is perhaps most useful in providing an overview of the arrangements that towns can make to manage their clam flats.

Research papers by Dr. Brian Beal, of the Downeast Institute, and his collaborators were the source for growth rate and other information that is specific to Maine and its colder water temperatures. A few of these papers are referenced below. They would be challenging reading for most students.

Abraham, B. J., & Dillon, P. L. (1986). Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (Mid-Atlantic): Softshell Clams (No. Biological Report 82(11.68)). National Wetlands Research Cener, Fish and Wildlife Service. Retrieved from https://www.nwrc.usgs.gov/wdb/pub/species_profiles/82_11-068.pdf

Beal, B. F., & Gayle Kraus, M. (2002). Interactive effects of initial size, stocking density, and type of predator deterrent netting on survival and growth of cultured juveniles of the soft-shell clam, Mya arenaria L., in eastern Maine. Aquaculture, 208(1), 81–111.

Beal, B. F., Nault, D., Annis, H., Thayer, P., Leighton, H., & Ellis, B. (2016). Comparative, Large-Scale Field Trials Along the Maine Coast to Assess Management Options to Enhance Populations of the Commercially Important Softshell Clam, Mya arenaria L. Journal of Shellfish Research, 35(4), 711–727. http://doi.org/10.2983/035.035.0401

Ellis, K. L. (Ed.). (1998). The Maine Clam Handbook: A Community Guide for Improving Shellfish Management. Maine/New Hampshire Sea Grant College Program. Retrieved from http://www.seagrant.umaine.edu/files/pdf-global/98clamhandbook.pdf

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