sediment preference may be related to its equally unique deposit-feeding habit 
since silt-clay sediments are typically rich in organic matter. Motility of N. 
incisa in fine sediment can best be described as an extended period of open 
burrow habitation followed by its extension of the burrow developing another 
temporary burrow. This habit is different from infauna that constantly burrow 
as predators, or continual deposit-feeders, or those which reburrow only 
because of complete burrow destruction. Whatever the purpose(s) for high 
burrowing rates by N. incisa , reported densities of 600 — 1200 per rrT may 
significantly perforate the top 10 — 15 cm of sediment during warmer months 
(such as suggested by model #1 above). This perforation probably results in a 
large increase in the surface area of the sediment-water interface (model #2). 
Since both present and some past burrows are irrigated, this expansion of 
surface area may result in a general biological model for sediment-seawater 
exchange, assuming a gradient as in dissolved oxygen (model #3). 
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