semipermanent vertical burrows to reach deeper sediment in which it 

 continually burrows and deposit-feeds. Oxygen exchange occurs near the 

 sediment-water interface using modified posterior segments when the worm 

 surfaces to defecate (Linke, 1939). Paraonis spp. continually burrow to form a 

 deposit-feeding ring within a single stratum of high organic content (Gripp, 

 1927, cited in Schafer, 1962). When the concentric burrowing reaches 8-10 cm 

 in diameter, the worm burrows to a new stratum to begin another feeding ring. 



Partially or non-vagile families such as the Arenicolidae and Chaetopteridae 

 possess U-shaped burrows and irrigate for the dual purposes of feeding and 

 respiratory exchange. Families with the least sediment contact, termed 

 tubicolous polychaetes, include the Sabellidae, Onuphidae and SerpuHdae. 

 These worms develop permanent tubes lined with mucopolysaccharides, shell 

 debris, sand grains or calcite. They generally feed and ventilate above the 

 sediment- water interface. There are many exceptions; for instance, the 

 tubicolous Pectinariidae drag their sand grain tube horizontally through the 

 sediment using it as an irrigation tube to the surface as they deposit-feed 

 below. 



The present investigation provides information on the microhabitat of A'^, 

 incisa by describing the nature of its burrow and examining some of the 

 variables influencing burrowing. Employing laboratory in situ microcosms, 

 coupled with direct observation in the field for verification, this investigation 

 has addressed such topics as the form and make-up of the N. incisa burrow, 

 how it is constructed, what is its horizontal and vertical extent, and how the 

 rate of burrowing is influenced seasonally as the worm is exposed to 

 temperatures which can range from 0-24^C. 



MATERIALS AND METHODS 



Nephtys incisa used in this study were collected from a station north of 

 Conanicut Island, Narragansett Bay, Rhode Island (Figure 20-1). This benthic 

 station is characteristic of a large portion of the Narragansett Bay, where a 

 clayey-silt sediment covers 60-75 percent of the Bay bottom (McMasters, 

 1960). Previous studies (Davis, et al., unpublished) found A', incisa density to 

 drop off rapidly in the sandy sediment toward tlie Bay mouth and decrease 

 gradually toward the northern head of the estuary, the latter possibly due to a 

 pollution gradient (Farrington, et al., 1973). 



A'', incisa were collected by gently sieving Smith-Mclntire grab samples and 

 by SCUBA diver-collected box cores, which were then transported intact to the 

 laboratory and held in flowing seawater. When temperature change was 

 required, it was shifted at a rate of 2*^C per week, which is comparable to the 

 rate of temperature change in the field (Figure 20-2). 



304 



