If nonpit urchins grow faster than pit urchins, it could explain the larger size of nonpit urchins compared to pit urchins in the same tidepool. Chemical tagging methods are commonly used to measure the growth of sea urchins and other marine invertebrates. I used a tetracycline-injection technique to measure the growth of sea urchins, and found that nonpit urchins do indeed grow faster than pit urchins. In addition, growth rates varied among sites.

 

To illustrate this concept, consider a sea urchin that is 30 years old. At Cape Blanco, such an urchin from a pit had a test diameter of approximately 6 cm, while an urchin outside a pit had a diameter of approximately 7 cm. At Middle Cove of Cape Arago, however, a 30-year old pit urchin had an approximate test diameter of 6.9 cm compared to 8.4 cm in nonpit urchins. However, while growth differences between microhabitats were statistically significant, they are insufficient to explain the greater difference in the sizes of pit and nonpit urchins.