Living off the Limulus

As a crab ages and its growth rate slows, it sheds less frequently and begins to display a striking variety of hitchhikers.
An abundance of smaller creatures live on (and off) the horseshoe crab in a symbiotic relationship.

Sponges and other fouling Porifera occasionally become established on the posterior of horseshoe crabs; probably when the water is cool and the crab is half buried in a dormant stage.

The Limulus leech (Bdelloura) is a flatworm that is found around the book gills and leg joints of crabs, especially on older females that have not shed for a long time.
The leech lays its eggs in the "pages" of the crab's book gills and these are visible as little dark spots.
It may also use the cuticle of the gills as a substrate for chemical activity.

Several crustaceans are regular companions of the horseshoe crab, including mud crabs and sand shrimp.
Tiny juvenile spider and rock crabs also find a home in the crevices inside Limulus molts.

The mollusks are better represented on the horseshoe than any other phylum.
Several species of bivalves become attached to the crabs, and a number of snails are also regularly found on them.
Mussels usually attach themselves near the hinge where water is circulated to the gills by the resting crab.
Three species of slipper shell--the common, convex and flat-- are regularly found attached to the underside of the crab.

Snails are well represented on horseshoe crabs, including periwinkles, basket and mud snails and the drill.
Snails also lay eggs on the back of the crab and, in the spring, horseshoes can be carpeted from head to tail with drill and mud snail egg capsules.

The Horseshoe Crab ~ http://www.horseshoecrab.org/nh/eco.html