The Gills in Detail

The ventral surfaces of horseshoe crabs, in particular the appendages, are covered in a profusion of setae and stout bristles in a range of sizes (top photograph). They appear at their smallest on the surfaces of the gill leaflets.

The branchial warts occur on the endopodites of the gills and are surrounded by spatulate setae, which sample the exhalant water stream from the gills.

The cuticular appendages become very specialized on the branchial warts themselves. The warts are dome-shaped pads that increase in number as the horseshoe crab grows. These warts contain a number of modified setae (see bottom photo), each shaped like a trumpet bell. They account for about 1 million sensory cells (out of about 7 million) that provide neural input to the central nervous system of the horseshoe.

Photographs by W. H. Farenbach