Monitoring at MacMillan Wharf, Provincetown, MA

Monitoring at MacMillan Wharf, Provincetown, MA
Marine Invasive Species (MIS) Monitoring at MacMillan Wharf, Provincetown, MA.

Saturday, January 21, 2012

Diplosoma Ascidian Up Close

Diplosoma listerianum is a cryptogenic species of almost cosmopolitan distribution that is found from the low-tide mark until about 80 m deep and is a close relative of Didemnum vexillum.  Both species have rapidly spread throughout New England waters and have become dominant species in many localities.  They are members of the same ascidian family, the Didemnidae, and and have a similar zooid body structure but differ primarily by the presence of stellate spicules and a coiled sperm duct in Didemnum.  The lack of spicules gives Diplosoma a smooth, gelatinous feel to it, whereas Didemnum has a tougher, slightly calcareous surface.  By eye, the two species would rarely be confused because they differ dramatically in color.  Dilosoma is an inconspicuous green-grey whereas Didemnum is a bright tan or cream color.  The feel of Diplosoma comes into play when trying to determine the identity of an amorphous green gelatinous mass, particularly when feeling the out-of-view but within-reach under-surfaces of docks. 

Diplosoma listerianum Colony showing Distinctive Green-Grey Color.
Numerous, small openings in the surface of the colony are incurrent branchial siphons of individual zooids.  Less frequent, large openings are common, excurrent apertures.  
Diving photograph by Rokus Groeneveld,

During the monitoring season, Diplosoma was seen throughout the summer in Salem Sound but did not make its first appearance at the Provincetown Docks until August and continued to spread through September. I never saw it on the sides of docks in Wellfleet.  Earlier in the season, most of the Diplosoma specimens I brought to the lab were not very photogenic because of their dark color, lack of contrast, or poor viability in standing sea water.  However, in September, I collected a colony growing on red algae that I kept in replenished sea water until it was examined in the lab the next morning.  This colony showed beautiful light-blue structural features and white pigmented cells in the surface that glowed like stars in the sky.  The zooids, which are only about 2 mm long, afforded great views of the branchial siphons and stigmata in the branchial sac.   

Diplosoma listerianum on Red Alga Glows Light Blue by Stereomicroscopy
This view is from above, looking almost directly down on the branchial siphons, which have 6 lobes and up to 24 thin branchial tentacles of variable size.  The branchial sac bears 4 rows of stigmata, which can be clearly seen through the opened branchial siphon arranged in a ring around the opening. 

Diplosoma  listerianum Zooids showing Rows of Light Blue Stigmata
 This view is looking at a slight angle through the branchial opening showing some stigmata from the side.  The upper rows of stigmata are in clear focus.  The digestive tract, ovary, and testis are localed deeper in the colony below the plane of focus.