Morphology Differences Between Sister Taxa

Classification of the pacific toothed oyster can be considered to be difficult due to the irregularity of shell shape size. Additionally, sister species in the genus Isognomon also do have high amount of irregularity and inconsistency in the shell structure, ultimately making classification of these species just by morphology to be difficult.

Pacific toothed oysters, Isognomon isognomon have high variation of shell structure among individuals despite classified as being the same species. This high variation was hypothesized to be driven by ecological and physiological pressure such as predation and water quality. However, after further investigation over this matter, it was shown that the shell morphology variation is high even among species belonging to Isognomidae family. Crevice-dwelling isognomons are bivalves that attached themselves to the underside of rocky substrates in shallow waters (Kauffman, 1969). As these habitats restrict the amount of space available, these bivalves are forced to alter their final adult size and shape accordingly, resulting from distorted shell shapes (Ubukata, 2003; Tsubaki et al, 2010).  

A study by Ubukata in 2003, demonstrated that the variety of shell shape of the Pterioida family have sets of common prescribed directions. These set of prescribed directions are strongly driven by types of habitat that the individuals are grown in. Shell structure are highly based on the speed and energy of the flow of water current, salinity of the water, predation exposure, types of attached substrates and the amount of materials present for calcification (Ozanna et al, 2002; Ubukata, 2003). Additionally, the growth and shaping of the shells are also highly restricted by ontogenetic growth rules. Due to the ontogenetic rules, restrictions were applied during morphogenesis, causing restrictions on direction of shell margin extension and the direction of mantle extension. Each species has its own rate of ‘polarity’, which causes morphological differences between species in a latter growth stage (Wilson, 1993; Ubukata, 2003).