Among the sponges

Eric recently collected some calcareous sponges (Leucilla nuttingi) in the low intertidal zone for his Invertebrate Biology course.  In the past, we had noticed some isopods living inside these tiny vase-like sponges (see post called "Sponge surprise!" from 12 January 2012).  But last week in class, two students with sharp eyes (Maddy and Michael) noticed some other interesting animals living in association with these sponges:

 

Meet the amphipod Podocerus spongicolus!  This tiny amphipod is only 3-4 mm long.  Notice its very long 2^nd antennae.  The amphipods cling tightly to the outer wall of the sponge with the pointy tips (dactyls) of their thoracic appendages.  In the photo below, you can also see the shiny calcareous skeletal elements (spicules) shining through the body wall of the sponge:

 

This amphipod can be distinguished from other Podocerus species by the fact that article 4 (A4) of the male gnathopod (feeding appendage) is extended forward (see arrow in left figure below).   The photo on the right shows this same feature in the live amphipod:

 

As its name indicates, Podocerus spongicolus is known to be associated with sponges. Otherwise, it appears that little is known about this species.  We have only been able to find a few museum records of this amphipod (from the 1930s and 1960s). 

 

It is interesting to think about its life among the sponges.  Many thanks to Maddy and Michael for spotting these amphipods, Collin for identifying them, and Eric for taking the photographs!