Rhabdus rectius (Carpenter, 1864) in situ. This species burrows through the sediment and seldom extends its dorsal aperture out of the sediment at the surface. Note the feeding cavity constructed by the foot. c = captacula. fo = foraminiferan, f= foot, w = water, sed = sediment, pp = pavilion. Scale bar = 1 cm.
Scaphopods are infaunal predatory mollusks with a tubular and, generally, curved shell having openings at both ends. They are morphologically well-known (See Shimek & Steiner, 1997, for a review). The large aperture is functionally anterior, the concave portion of the shell is dorsal, and the convex portion of the shell is ventral. These animals are bilaterally symmetrical, and completely surrounded by the shell, which encloses the mantle cavity. The body is suspended from dorsal part of the shell by the mantle and is surrounded laterally and ventrally by the mantle cavity. The characteristic molluscan gills, or ctenidia, are lacking, but the mantle cavity typically bears prominent ciliated ridges presumed to aid in respiration.
The two scaphopod orders, the Dentaliida, and the Gadilida differ significantly in shell shape and soft-part morphology (Palmer 1974; Starobogatov, 1974; Steiner, 1992a; Shimek & Steiner, 1997). Generally, the Dentaliids have a longitudinally-ribbed or smooth "unpolished" shell which is largest at the anterior aperture. Gadilids typically lack sculpture and have a polished shell with the widest portion of the shell some distance behind the aperture (Shimek 1989). The radula or grinding organ differs significantly between the two orders, both in shape and usage. The image on the left below shows the radulae from Antalis pretiosum (a Dentaliid) and Gadila aberrans (a Gadilid). The teeth are labeled: L = laterals, M = Marginals, C = Central. The dentaliid radula is a ratchet or, occasionally, a grinding organ. It is rigid and relatively inflexible and the large central tooth holds the lateral teeth erect, where they can function to pull material into the gut. The gadilid radula is flexible and asymmetric and can close rather like a zipper. This radula functions to cut and crush hard prey. The left central figure below shows a partially eaten foraminiferan ( Elphidiella hanni ) and the radula of Gadila aberrans . The foram is at the action plane of the radula and the damage caused by the radula is evident. The figure on the right central below shows a series of partially eaten Elphidiella hanni recovered from the buccal pouches of several Gadila aberrans; the scale bars are 100 µm. When eating the animal holds the prey in the buccal sphincter and uses the radula to knock pieces off rotating it between radular strokes. Gadila aberrans reaches lengths of about a centimeter and is illustrated on right along with the other scaphopods from the Barkley Sound region.
The foot extends from the anterior aperture for burrowing and retracts by bending into the shell in the Dentaliids, and by introverting within itself in the Gadilids (Shimek & Steiner, 1997). While the Dentaliid foot bears a pair of lateral lobes slightly proximal to the end of the foot, the Gadilid foot terminates in a fringed disk (Steiner 1992b). Several hundred specialized feeding tentacles, captacula, originate lateral to the proboscis or buccal tube base (Morton, 1959; Dinamani, 1964; Gainey, 1972; Poon, 1987, Shimek, 1988; Shimek & Steiner, 1997).
The illustration at the left below is a functionally dorsal view of Pulsellum salishorum with the mantle removed to show the mouth, buccal tube, captacula and foot. Pulsellum salishorum is a gadilid; note the dimple created when the foot introverts. The illustration to the right is a Scanning Electron Micrograph of the radula of an individual of this species showing the variation in the mineralization of the radula. Pulsellum salishorum reaches lengths of about 6 mm and is shown above.
Most illustrations depict scaphopods with the apical aperture extending from the sediment. Although this posture occasionally occurs, particularly in spawning animals, it is not the typical posture. Most scaphopods spend most of their lives buried into sediment and below the surface of sediments. An exposed scaphopod is crab or fish food. Gadila aberrans and Rhabdus rectius may both be found commonly at least 30 cm under the sediment-water interface (Shimek, 1990). The natural history and ecological interactions of some Northeastern Pacific scaphopods are relatively well-known. Some, such as Rhabdus rectius, are generalist predators, while most species are selective predators on specific foraminiferans (Poon, 1987; Shimek, 1990).
About 15 scaphopod species may be found in the sediments off the United States temperate West Coast, with more in Alaskan waters (Pilsbry & Sharp, 1897-1898; Raymond, 1904; Emerson, 1978; Baxter, 1987; Shimek & Moreno, 1996; Shimek, 1997, 1998). A number of these are from very deep water, and unlikely to be collected at depths less than 1000 m. One of the more interesting of the deep-water forms is Fissidentalium actiniophorum , Shimek, 1997, which typically bears a sea anemone living on the functionally dorsal part of the shell. This species was described from specimens collected from 4100 m off the California coast, and is illustrated in the left image below.
Most of the other west coast scaphopods are potentially collectable in depths of 30 m or less, and some may be found in quite shallow waters (Keen, 1971). Some are habitat specialists but others, such as Rhabdus rectius, are true habitat generalists. None of these species are likely to be found intertidally. Shells may be occasionally found in the intertidal zone as beach drift or shells with hermit crabs in them. See the right image above, which shows an Antalis pretiosum shell occupied by the straight hermit crab, Orthopagurus minimus . Scaphopods are preyed upon by fishes and crabs, and may be important components of their diets in some localities (Shimek, 1990).
Until recently scaphopod nomenclature was simple, most Dentaliids were placed in Dentalium , and most Gadilids in Cadulus , and really, that was that (Pilsbry & Sharp, 1897-1898; Grant & Gale, 1931, Emerson, 1952, 1962). Recent morphological, systematic and taxonomic advances have resulted in many necessary and reasonable nomenclatural changes, which are reflected here (Steiner 1992a; Shimek, 1998).