SHIFTING SANDS AROUND OAHU AND THE SPARSE FAUNA OF SHALLOW-WATER INFAUNAL BIVALVES

When one of us moved to Honolulu, he was stunned to see virtually no seashells at the seashore. Nearly all bivalve occurrences on beaches are of very small, thin shells that occasionally occur in windrows, having been washed up at high tide. We hypothesized that nearshore benthic environments (few of which are in protected areas) experience such strong water movements that the instability of sands fatally destabilizes bivalves, presumably usually at their postlarval stage, although it would appear that few free-living species of larger size could survive in these environments.

Most non-rocky seafloors that border Oahu consist of thin blankets of sand resting on rock, and the sand is often in motion, not just in saltation but also at substantial velocities well above the sediment-water interface. We have filmed these motions, and also the visible migration of sizeable ripples, which are ubiquitous. Additional evidence here is the heavy concentration of species in offshore environments and the differing modes of of life of shallow and deep-water species. Only 4 burrowing species occupy shallow muddy environments, which are uncommon. For shallow-water sands, the absence of slow-burrowers and near-absence of endobyssate species is quite striking. Instead there is a predominance of rapid burrowers among infauna and a great abundance of cryptic species. These patterns and the high frequency of endobyssate species and slow burrowers in deep environments support the unstable substratum hypothesis. Also supporting this hypothesis is a general comparison with the tropical Eastern Pacific shallow-water bivalve fauna, for which the ratio of infauna to epifauna plus nestlers is 3.6. In Oahu, burrowers are relatively rare: this ratio is only 0.39.

The exclusion of marine life by strong water movements around Oahu may provide insights into the nature of the shallow seafloors of ancient epicontinental seas because of the long fetches for winds sweeping toward their shores.