Brachiopods
Brachiopods are a phylum of exclusively marine benthic suspension feeding invertebrates. Although superficially similar in appearance to bivalve molluscs (Figure 1), brachiopods are distinct in terms of both their anatomy and functional morphology. Brachiopods are not abundant in modern oceans, however, they were ecologically dominant organisms in shallow marine environments prior to the Permian - Triassic mass extinction event, 252 million years ago (Rudwick, 1970). Physical attachment to the seafloor and subsistence by suspension feeding are two factors that have strongly influenced brachiopod evolution (Rudwick, 1970). All brachiopods possess a feeding organ, called a lophophore, comprised of an array of ciliated tentacles arranged about a pair of symmetrical "arms" that lead to the mouth. Throughout the ontogeny of most brachiopod species, the lophophore changes in shape from a flat, ribbon-shaped structure into a more complex three-dimensional form (Figure 2). This change in lophophore morphology is often accompanied by changes in shell shape that can alter both the orientation of the animal with respect to the seafloor, and the pattern of ambient water flow through the shell (Rudwick, 1970). Passive circulation, or passive flow, of ambient water flow through the shell, and across the arms of the lophophore, is thought to be metabolically advantageous for brachiopods because it can potentially reduce the cost of active pumping via the the lophophore cilia (LaBarbera, 1977).
Rhynchonellide brachiopods, including Hiscobeccus, transition from a two-dimensional, trocholophe lophophore in the juvenile stage to a considerably more complex, three-dimensional spirolophe in adult form (Figure 2). In Hiscobeccus, this change in lophophore geometry is accompanied by a change in shell shape, with both valves becoming visibly more convex as the animal reaches the adult stage. Because brachiopods remain attached to the seafloor throughout most of their ontogeny (with the exception of a free-swimming larval stage), changes in shell shape can potentially have a significant effect on the orientation of different parts of the shell with respect to ambient flow, hence altering the pattern of passive flow over the lophophore.
Research Objectives and Predicted Outcome
The main objective of this research is to identify what features of Hiscobeccus capax contribute most strongly to morphological change throughout the ontogeny of the species, and how these changes relate to patterns of passive flow through the shell. During ontogeny, Hiscobeccus develops a pronounced median deflection, known as a sulcus, on the anterior end of the shell (Figure 3). In many brachiopod genera, the sulcus is thought to function to separate inhalant water flow (containing suspended food particles) from filtered exhalant water and excreta (Rudwick, 1970).
In Hiscobeccus and other sulcate brachiopod genera, the sulcus forms part of the shell commissure - the line along which both valves of the shell meet. Brachiopods feed by opening their shells about the commissure, producing a gape through which water and suspended food particles can flow. Undertaking a shape analysis of H. capax, it is expected that an ontogenetic growth pattern corresponding to increasing convexity of the shell (i.e., increasingly globose shells), and differentiation of the commissure into distinct medial and lateral segments (i.e., development of a pronounced sulcus) will be demonstrated. As is the case in some living brachiopod genera (Rudwick, 1970), it is expected that this change in shape will be associated with changes in the pattern of passive flow through the shell (as represented by 1:1 scale articulated plastic models), potentially causing flow to pass through discrete inhalant and exhalant openings in the shell gape.