The type specimen was collected in Juneau Borough, on the west side of Douglas Island.

Here it was found growing on exposed seashore rocks roughly 2 m (6.6 ft) above the high tide line.

Its specific epithet maritimum alludes to its close association with maritime rocks.

[1] The thallus of Spilonema maritimum is made of radiating filaments that form olivaceous brown rosettes measuring 0.5–1.5 cm (0.2–0.6 in) in diameter.

[1] In addition to various locations in Alaska, the lichen has also been recorded from Haida Gwaii and Vancouver Island, British Columbia.