GAPE, FEEDING CURRENTS AND VALVE SNAPPING IN THECIDELLINA MEYERI FROM CURAÇAO, NETHERLANDS ANTILLES: BIOMECHANICAL ANALOGUE FOR TRACE-MAKING PALEOZOIC STROPHOMENATES?

The ability of Ordovician strophomenates Sowerbyella and Rafinesquina to move sediment and create moat-like depressions has led to questions about mechanisms. Anatomical studies suggest a gape of more than 45°, likely critical to trace-making abilities. Strophomenates are extinct, but thecidellinids are reasonably good analogues; they also gape widely and have a similar lophophore structure. They differ in their small size, 3 - 5 mm, lack of concavo-convex form, and by pedicle valve cementation. Nevertheless, their physiology could illuminate biomechanical constraints on strophomenate-sediment interactions.

For this study, we analyzed 1 hour of video showing 30+ specimens collected with the fragment of dead plate coral, Agaricia, to which they were cemented from the fore reef slope at ~15 meters depth near Slangenbaai, Curaçao. Video was recorded after brachiopods were transferred to a tank where they rapidly recovered, resumed normal feeding behaviors, and maintained a 90° gape.

They are sensitive to vibrations and touch, and respond by valve snapping, but are insensitive to light or sodium fluorescein dye. Valve snapping also occurs during feeding without perceptible stimulus. The dye was used to mark water movement during feeding and valve snapping. It shows feeding currents by entering the lophophore and moving diffusely within the filament ring toward the hinge at ~2 mm/s, then crossing to the outer surface of the ring, where it is conveyed forward along the filament ring and exits at ~12 mm/s in a stream at the anterior midline. These brachiopods consistently snap shut within about 0.13 seconds, expelling water at 6 cm/s, but they take 20 seconds or more to fully open. They normally stay shut for 3 s to 15 min, and remain open 2 to 3 min. When repeatedly stimulated, some individuals would only open to 60° and close in rapid succession, managing 3 cycles in 1.5 min.

By analogy, feeding currents of strophomenates were likely too weak to disturb sediments, allowing them to feed close to the sea-floor. The powerful snapping currents might have formed moats or helped the brachiopods to excavate, especially when employed in rapid succession. Further studies of thecidellinids are focused on hinge and muscle configuration, and may be extended to experiments on strength and fatigue limits of muscle, and ability to clear sediment.