A low, spreading cushion of up to 2 cm thickness with a base diameter of up to 12 cm. The sponge surface is hard and rough with a variable number of erect projections bearing a terminal opening (papillae) that may be as tall as 12 cm. The main body mass is pale, usually greyish yellow or more rarely, orangey yellow, and is often buried in sediment with only the papillae visible. The papillae are smooth and clean, creamy white or pale yellow, sometimes translucent, and often a fine network of spicules can be seen on their surface. The exhalant papillae are large and few in number, the inhalant papillae are smaller.May be confused with Polymastia robusta, however the papillae of this closely related species tend to fuse together, which does not occur in Polymastia mamillaris.

Adriatic Sea, Barents Sea, Irish Exclusive economic Zone, North and East Barents Sea, North Sea, Southern Norway, Swedish Exclusive Economic Zone, United Kingdom Exclusive Economic Zone

Prince Edward Island (from the northern tip of Miscou Island, N.B. to Cape Breton Island south of Cheticamp, including the Northumberland Strait and Georges Bay to the Canso Strait causeway), western slope of Newfoundland , including the southern part of the Strait of Belle Isle but excluding the upper 50 m in the area southwest of Newfoundland and the Gulf of St. Lawrence (unspecified region)

infralittoral and circalittoral of the Gulf and estuary

Depth range based on 206 specimens in 1 taxon.
Water temperature and chemistry ranges based on 28 samples.

Environmental ranges
  Depth range (m): 0 - 168
  Temperature range (°C): 10.625 - 12.348
  Nitrate (umol/L): 4.729 - 8.636
  Salinity (PPS): 35.008 - 35.504
  Oxygen (ml/l): 5.712 - 6.346
  Phosphate (umol/l): 0.335 - 0.574
  Silicate (umol/l): 2.311 - 3.727

Graphical representation

Depth range (m): 0 - 168

Temperature range (°C): 10.625 - 12.348

Nitrate (umol/L): 4.729 - 8.636

Salinity (PPS): 35.008 - 35.504

Oxygen (ml/l): 5.712 - 6.346

Phosphate (umol/l): 0.335 - 0.574

Silicate (umol/l): 2.311 - 3.727
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

 Often found at the sediment-bedrock interface on upward facing rock, or growing out of crevices. Prefers silty conditions and is recorded from the littoral to a depth of 2300 m, but usually occurs between 5-15 m.