For a species of large abalone shellfish to develop from a species of smaller ones, they must spend less energy on finding food and avoiding predators, and more on competition in mating
So it is surprising that the fossil record shows that a species of large abalones developed from a smaller one only after otters, which prey on abalones, began to dominate the waters in which the abalones lived
Otters also prey on the abalones' competitors for food and so indirectly make it easier for abalones to get food.