Biology

Lifecycle

Large Hippochrenes amplus shells may be 25cm long, 15cm wide and comprise about 17 whorls. 

The shell was secreted during three successive life stages:

Larval stage

Compared to the adult, the larval shell is very small with a tiny smooth nucleus followed by four conical whorls, the last with a rounded outline.The larval shell is very small, suggesting that the comparatively huge adult produced a vast number of eggs, but few of the young reached maturity.

Juvenile stage

The immature shell has a fusiform (spindle-shaped) outline, with a tall pointed spire and a shorter, straight, pointed rostrum. Spire whorls have flat sides and a base covered with fine spiral grooves. The shallow suture is marked by two fine spiral grooves

Adult stage

Subadult and adult whorls are typically broader than earlier ones, giving the spire a concave (coeloconoid) outline.

At adulthood the outer lip of the aperture is extended into a large, horizontal, flattened semi-circular ‘wing’ with a truncated anterior (base).

Grooves at the suture become hidden by the succeeding whorl later in life. On the spire whorls growth increments are seen as very narrow lines that are slightly backwards bowed (opisthocyrt). They are grouped into clusters separated by a slight groove. These grooves are most clearly seen on the wing of the adult where they parallel the margin.

The shell surface is glossy and the base of the shell (parietal wall and ventral part of the last whorl) is covered by a smooth callus layer when the wing has formed. Callus is shell that is secreted in adulthood over earlier deposited shell. It varies from a thin glaze through to a massive layer of shell.

In Hippochrenes amplus the callus layer is thin except toward the apex where it forms a thick ridge. Between this ridge and the inner surface of the wing a narrow channel is developed in the callus. This extends over the apex and is then recurved abapically (away from the apex). This channel would have protected a filamentous extension of the mantle that may have had a sensory function.

Growth is determinate throughout the superfamily Stromboidea, meaning that the shell stops growing in size at adulthood, though it may continue to thicken. These features formed during the last period of growth are termed adult modifications.

In this species they are comprised of a thickened, wing-like outer lip and callus secretions. The wing continued to increase in thickness, perhaps throughout the animal’s lifespan. This process is indicated by the presence of numerous thin layers of callus on the inner surface of the wing.