The sandcastle glue of P. californica
Phragmatopoma californica is a marine tubeworm that literally glues together broken bits of skeletons under water. The worm adventitiously gathers fragments of broken seashells (exoskeletons) and glues them back together into a tough composite tubular shell in which it lives. In addition to seashell bits P . californica glues particles of sand and other minerals onto its tubular home. Particles suspended in the water column are captured and conveyed by cilia along the tentacles of the crown to the building organ, a U-shaped invagination near the mouth. In the building organ, the particles are evaluated for size, shape, and composition. Particles that are about 0.5 mm in diameter are preferred but P. californica does not seem to be particularly fussy about the composition, as long as they are not plastic. After finding a suitable particle, the animal feels around the end of its tube for the best fit. Then a couple dabs of glue from thoracic cement glands are secreted onto the particle, which is pressed into place in a process reminescent of the work of a stone mason.
The shell building strategy of P. californica and related species is unique compared to the strategy of other shell-dwelling marine invertebrates. Most obligate shell-dwellers synthesize the complete mineralized structure of their shell by the controlled precipitation of a supersaturated solution of ions onto a biomacromolecule matrix. By gathering the mineral phase as preformed particles, P. californica, have only to produce a couple dabs of proteinaceous glue to join the minerals into a tough composite shell.
Their stationary, tube-dwelling lifestyles restrict P. californica colonies to turbulent intertidal zones where continuous wave action suspends particles for tube building and repair, supplies food, and removes waste products. The sandcastles, and in particular the mortar, must be sufficiently robust to withstand the surf's constant siege. After 10s or 100s of millions of years of adaptation to bonding wet hard parts to wet hard parts under seawater the P. californica glue may be a perfect model for synthetic medical adhesives targeted at orthopedic applications.
To learn more about the structure, composition, and bonding mechanisms and to watch the sandcastle worm practice its underwater masonry please visit the photo and movie galleries.
