The heart urchinBrisaster fragilisis an important bioturbator found in the Estuary and Gulf of St. Lawrence. Several adaptations allow it to move within fine sediments (e.g., test shape, spine morphology, and distribution), which are compared here to those of its Pacific sibling speciesB. latifrons. While ventral spatulate spines and dorsal and anterolateral curvilinear spines are similar between the two species, anterior spines differ significantly: sigmoid-shaped forB. fragilisand curvilinear forB. latifrons. This morphological difference, in addition to a narrower plastron forB. fragilis, suggests a different digging strategy.In situvideo observations ofB. fragilisshow a “dig and move” strategy: anterior spines “dig” forward at the sediment while the plastron spines “move” the urchin into the newly created space.B. latifronson the other hand uses an oblique rocking motion. This suggests that generalizations derived from one or few species displaying similar body shapes may not be possible. Factors such as sediment depth (i.e., the amount of sediment above the urchin) are likely to affect movement and force the animal to employ a different digging strategy, even within a single species. The role of spines for locomotion is further discussed, with additional reference to tubercle morphology.
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