Figure 20. Bottom photograph showing a section of sea floor within Nantucket Sound composed of muddy sand covered by highly concentrated Crepidula fornicata (from Ackerman and others, 2015). The shells produce a continuous high-backscatter zone on the regional acoustic backscatter imagery, despite overlying a substrate primarily composed of muddy sand. See figures 4 and 19 for locations of the photograph and the shell zone from which it was obtained.