External Morphology and Function
The most conspicuous feature of a Scaley Turban (Turbo perspeciosus) is it’s large, often bright
ly coloured shell , that ranges in shape from conical to disc like (Kiel 2001).
|
Figure 1. Image indicating the
area known as the aperture of
a Gastropod.
|
The Turbinid family feature a spirally growing, calcified operculum, as opposed to other Gastropods which instead have a corneous operculum (Williams and Ozawa 2006). The calcification process occurs through deposition of calcium carbonate (CaCO3) onto a corneous operculum, resulting in a rigid structure that completely fills the aperture of the shell (see figure 1), which can be radial or oblique, when the animal’s soft parts are withdrawn (Williams and Ozawa 2006; Vermaij and Williams 2007; Kiel 2001). The unique, calcareous opercula of Turbinids is thought to confer a fitness benefit by limiting water loss, as they seal the apetural opening much more effectively than corneous opercula. This ultimately means the animal is not required to withdraw as deeply into its shell during low tides, of particular benefit to intertidal species (Williams and Ozawa 2006).
Turbinids appear to lack an escape response to predation, even if said predator is very slow moving, and it is for this reason that a calcareous operculum is believed to confer a passive-protective function to its owner (Vermaij and Williams 2007), particularly against predators that are able to break the Turbinid shell around the outer lip, or attempt to enter the shell through its aperture. There is a greater frequency of thick opercula in tropical water Turbinids relative to temperate species, which is thought to be explained by the higher solubility of CaCO3 in cooler waters. This corresponds to a comparatively greater energetic cost associated with the precipitation and formation of CaCO3 operculum under these conditions (Vermaij and Williams 2007).
Turbinidae also possess other passive shell defences to assist in survival and predator avoidance. These may include external spines, apertural teeth, an elongate or small aperture or a slippery outer shell surface, although these features are more common in tropical species than temperate (Vermaij and Williams 2007), and are particularly highly developed in the Indo-West Pacific (IWP). While little literature details the passive defences of the Scaley Turban other than a calcareous operculum, observations made on Heron Island (Invertebrates Field Trip 2011) indicate that there is some evidence of a slippery shell, and of small but rather rough external shell mounds.
Many species of Marmarostoma, a subgenus of Turbinids, lack the siphonal indentations at the front of their shell that are characteristic of many living Gastropods. These organs are used to detect chemical signals associated with food, approaching predators, and locating mates (Vermeij 2007). The species lacking siphonal indentations instead have an extension of the aperture. Unfortunately there is a lack of current literature regarding which of these structures are possessed by T. perspeciosus.
Video 1. Video capture of Turbo perspeciosus moving out of shell, and reacting to the presence of a human by retracting back into the shell cavity. |