Aliculastrum debile (Pease, 1860)
|Maximum size: 23 mm
(extrapolated from shell length).
a translucent to opaque-white, spindle-shaped shell.
There are spiral striae that become stronger and "step-like" basally
and apically while the outer lip rises above the apex forming a notched
"spout." Shells from Midway Atoll are broader than those from the main
islands. The animal is translucent-gray variably flecked with opaque
white. White flecks show through the shell in some. It may be
distinguished from Weinkauffia ukulele by its more sharply
"stepped" apical striae, more prominent apical "spout" and lack of
narrow spiral pigment bands in the shell.
Natural history: Aliculastrum debile is one
of the most common sand-dwelling haminoeids in Hawaii. However, it is
seldom seen since it is nocturnal and buries itself during the day.
Even at night, it may spend much of its time crawling just beneath the
surface of the sand leaving short trails reminiscent of those produced
by terebrids. However, it probably emerges to mate and feed. (Note 1) It can be found in protected to moderately
exposed locations and occurs in mixed habitats, open sand and Halimeda kanaloana beds from < 1
to 17 m (< 3 to 56 ft). Dredged shells at
the Bishop Museum extend the depth range to at least 183 m (600 ft).
When threatened by predators such as Philinopsis
speciosa, it can swim strongly by flapping its parapodia. (Note 2) It appears to feed on microbial films of uncertain composition and it
lays a spherical, white egg mass that is anchored in sand. (Note 3) The eggs hatch in about three days in the
Big Island, Maui, Oahu, French Frigate Shoals, Pearl & Hermes Reef
and Midway (also Johnston Atoll): also known from French Polynesia.
This species is listed in Kay, 1979, Kay & Schoenberg-Dole, 1991 and Severns, 2011 as Atys debilis. It was first reported from Hawaii in Pease, 1860. There
possibility that the slightly more inflated shells from
the Leeward Islands might prove to be a separate species. Atys cornuta
Pilsbry, 1917 is a
synonym (Kay, 1979) and it is also listed under that name in Edmondson,
1946, Quirk & Wolfe, 1974, and Tinker, 1958. On-line sources may
also list it as Aliculastrum debilis.
mm: Hekili Point, Maui; Nov. 11, 2004.
Observations and comments:
1: In addition to those found
beneath the sand, we've observed several on the surface at night
including at least one copulating pair. Rarely, we've also seen them on
the surface by day when visibility was reduced by silt or clouds.
In 2006, PF photographed what appeared to be a deposit of fish fecal material that contained large numbers of intact Aliculastrum debile shells. (see photo) It also contained test and spine fragments from the sea urchin Echinothris calamaris
and a few prosobranch snails. The species that produced the fecal
material could not be determined but the contents suggest that a generalized
invertebrate feeder such as a trigger fish may have been responsible.
The large number of intact shells suggests that whatever ate them was
selectively feeding on the species (perhaps due to availability during a
population peak?) and swallowing the animals whole. Perhaps, some fish are significant predators?
In September, 1990 an animal held in a dish with sand was observed
laying eggs. The eggs were extruded from beneath the sand through a
small hole and the mass inflated "balloon-fashion." When done, the
animal crawled off beneath the sand anchoring the mass with its mucous
trail without emerging onto the surface.