Uniqueness of the gastropod accessory boring organ (ABO): Comparative biology, an update

Unique among organs of invertebrate animals, the gastropod accessory boring organ is a compact, histologically, physiologically, biochemically complex mechanism by which boring predatory gastropods penetrate the calcareous armor of live molluscan prey to feed. The present review considers the close similarity of the ABO in naticoidean and muricoidean boring gastropod species, the worldwide, distribution of the ABO, its physiological-biochemical functions, and its possible evolutionary origin. The ABO occurs only in the midventral part of the foot of muricoidean neogastropods and under the tip of the proboscis of naticoidean mesogastropods. Many reports in the literature, and a new study of additional species from temperate and tropical regions by us, further confirm the quite remarkable similarity of the morphology of the ABO in a wide range of species from many different regions of the world. In 29 species and subspecies of live muricoidean snails ranging in shell height from 12.1 to 115.0 mm, the mean diameter of the relaxed ABO ranged from 0.9 to 4.4 mm, and the mean width of the radula, spread over the tip of the odonotophore, from 0.12 to 1.40 mm. The fine structure and physiology of the secretory disc of the ABO is strikingly similar in the one naticoidean and four muricoidean species that have been studied in detail to date. The disc possesses features characteristic of highly active secretory cells. Cytologically, actively boring and inactive ABOs differ conspicuously. Active glands possess a thicker secretory epithelium, longer microvilli, more mitochondria, membrane-bound granules, vesicles, endoplasmic reticulum, and lysosomes, than inactive glands, and a denser concentration of hemocyanin molecules in the intercellular sinuses. Secretory granules probably pass to the surface of the ABO through interstitial ducts in the center of each secretory cell group. Glycogen is abundant in inactive ABOs and sparse in active glands. Shell dissolution in the borehole is primarily a chemical process involving enzymes (not yet identified), an inorganic acid (HCl), and chelating agents (not yet identified). Borehole size and shape mirror the external morphology of the extended ABO. Of the several enzymes that have been identified within the secretory disc, a prominant one is carbonic anhydrase. It plays a pivotal role in shell dissolution, but it does not function as a direct demineralization agent; it catalyzes the hydration of metabolic carbon dioxide produc