Evo-Ed: Integrative Cases in Evolution Education

Cases for Evolution Education


Soft shell clams (Mya arenaria) are bivalves native to the east coast of North America. Some populations of the clams have individuals living in them who are resistant to a toxin called saxitoxin, while others do not. To understand how saxitoxin gets into the soft shell clams and where it goes from there we need to examine the clams as part of a marine food web.

Photograph of a sea otter eating a soft shell clam.

The Source of Saxitoxin

illustration of a dinoflagellate

Soft shell clams are filter feeders. They feed by filtering water through their gills and consuming particulate matter that is entrapped. During algal blooms, soft shell clams consume large quantities of microscopic dinoflagellate algal protists. These protists (Alexandrium spp.) naturally produce a paralytic shellfish toxin, saxitoxin.

Algal blooms, also known as red tide, occur during warm summer months when nutrient conditions and water temperature stimulate dinoflagellate reproduction. High concentrations of dinoflagellates produced during algal blooms leads to high exposure of soft shell clams to saxitoxin.

Selection Pressure and Biomagnification of Saxitoxin

On the Atlantic coast of North America it appears as though algal blooms act as an agent of natural selection, reducing the fitness of (or killing) soft shell clams that are not resistant to saxitoxins. Those that are resistant to the toxin begin to accumulate it as they filter feed.

A marine food web containing dinoflagellates, clams, and a variety of predators.

Humans and other animals at higher trophic levels such as gulls, green crabs, moon snails, otters, and sturgeon can be poisoned by saxitoxin when they eat resistant clams during or after algal bloom events. When clams are not resistant to the toxin, they often die, giving humans and other would-be predators an indication that they are not safe to consume. However, when clams are resistant they can accumulate the toxin without giving any indication that they are unsafe to eat. These concentrations of saxitoxin in resistant clams are small when compared to the body mass of would-be predators. However, predators can ingest large quantities of the toxin, resulting in biomagnification of the toxin level. For example, if a fish feeds on many toxic clams and a human feeds on many toxic fish, the total amount of saxitoxin consumed by a given human can end up being equivalent to the total toxin present in the dozens or hundreds of clams originally consumed by the fish.

On the west coast of North America a closely related bivalve, the butter clam (Saxidomus gigantean), is also resistant to paralytic shellfish toxins and some scientists suggest that the clams sequester it for use as a defense against sea otter predation. An increasing frequency of algal blooms (possibly due to global warming: Paul, 2008) has resulted in increasingly toxic butter clam populations. It is possible that this increase in toxicity has hindered a natural re-expansion of sea otters into the southern part of its historical range where algal blooms are now more common (Kvitek and Bretz, 2004).