Crofton Lake: Freshwater Sponges

CROFTON LAKE CE2This beautiful small lake sits on the northeast slope of Mount Richards, not far from the small oceanside mill town of Crofton.  Accessible only by foot, it was the water supply for Crofton until 2001, and now sits quiet and relatively unspoiled.  The trail to the northeastern water intake at the dam passes through forest, a replanted zone, and marshy areas, then opens onto a lake vista and an easy water access ideal for collecting.  LAKE CROFTON 01 JULY 2015_Richtone(HDR)

In addition to the more usual fauna, this lake has one species that is either not found or is overlooked in other local water bodies: freshwater sponges.  These are easy to miss, appearing as unobtrusive 3-4 mm diameter branching strands that blend with similar green stems in shallow water.  Massed sponge formations are not visible but may occur elsewhere:

Sponges in Shallow Water - Easy to miss
Sponges in Shallow Water – Easy to miss

Of the thousands of known sponges, only one genus, Spongilla, with over 200 species, populares fresh water; this specimen is probably the branching Spongilla lacustris, a common species with a worldwide distribution (see Altinsach et al).  Unlike marine species, which are typically fixed to rocks or coral, Spongilla can flourish on floating debris, muddy bottoms, or rocks.

Turning over a nearby rock shows a new strand of sponge originating from translucent nodules, possibly the gemmules that allow the sponge to survive winter conditions:

Underside of Rock Showing Clear, Nodular Gemmules and emerging mature sponge branch
Underside of rock showing clear, nodular gemmules and emerging mature sponge branch

The mature sponge branches tend to be quite fragile and the soft parts somewhat hard to examine.  The spicules are, however, easily visualized:

SPICULES 03

The pointed tips of intact spicules are also consistent with Spongilla lacustris:

SPONGE SPICULES PPL
Pointed spicule of Spongilla lacustris, partially polarized light

The spicules lend Spongilla specimens a distinctly spiky appearance when viewed at close range:
SPONGE CE

 

References:

Altinsach, S. et al.  “On occurrence of Spongilla lacustris (Linnaeus,1759) from Lake Saklıgöl, Kandıra District, Turkey.”  Journal of Entomology and Zoology Studies 3:68-73.  http://www.entomoljournal.com/vol2Issue3/21.1.html

Bird, Johnathan (YouTube Video).  Johnathan Bird’s Blue World: Sponges.”  https://www.youtube.com/watch?v=m8a0oNsDEx8

Skelton, J. and Strand, M.  “Trophic ecology of a freshwater sponge (Spongilla lacustris) revealed by stable isotope analysis.”  Hydrobiologia, Springer Science+Business Media Dordrecht 2013, Published online with no journal reference 25 January 2013.  https://www.nmu.edu/biology/sites/DrupalBiology/files/UserFiles/Files/Pre-Drupal/SiteSections/Faculty/Bios/Pubpdfs/Skelton_and_Strand_2013.pdf

Wisconsin Department of Natural Resources.  “Catalog of North American State and Regional Freshwater Sponge References.”  http://dnr.wi.gov/files/PDF/pubs/ss/SS1040.pdf

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Crofton Lake: Freshwater Sponges