Halichondria bowerbanki Burton, 1930
This sponge is native to the Atlantic ocean and was first seen in San Francisco Bay in the early 1950's. It is believed to have arrived in either shipments of Atlantic oysters or as fouling organisms on ships. (Adapted from Cohen and Carlton, 1995)
The second image shows a microscopic view of a cross section taken from a specimen of H. bowerbanki. It illustrates the difficulty associated with properly identifying sponges. This particular species is defined by having multispicular tracts (spicules are tiny skeletal elements that make up the structure of sponges) running parrallel to one another with the area between the tracts being further subdivided by a pattern of overlapping individual spicules.