ABSTRACT
The competent larvae of articulate brachiopods have fully developed larval muscles, a pedicle-forming distal region constricted from the pedicle lobe, and a change in behaviour from photophilous to photophobic. Gliding or creeping movements and frequent contacts of their anterior lobe or ventral surface on the bottom of the laboratory culture dish appear to be exploratory attempts to test suitability of the substratum for attachment or settlement. Settling larvae first glued a small random area at the tip of their pedicle to the substratum with a sticky secretion for tentative attachment. Since these postlarvae may easily be flushed off the substratum with a jet of water from a pipette, or be mechanically separated from substratum without injury to pedicle lobe, presumably during this tentative attachment or settlement, postlarvae could detach themselves or allow themselves to be detached from the substratum by currents or wave action in post-settlement selection for more suitable sites. Subsequently, after a layer of cuticle is secreted between the glued pedicle and substratum to achieve irrevocable permanent attachment, postlarvae cannot be flushed off the substratum nor mechanically removed without injury to the pedicle. Pre- and post-settlement selections of attachment sites or responses of competent larvae to appropriate stimuli from the substratum and the environment, before and during tentative settlement may account for preferences of larvae for certain substrata, and also for natural occurrences of articulate brachiopods in concavities or grooves, and in crypts.
