A scale-worm with a body of uniform width. Dorsal and ventral surfaces smooth.
Prostomium bilobed, without peaks, with a median antenna, a pair of lateral antennae, laterally inserted, and a pair of palps. Two pairs of eyes, the anterior pair near the line of greatest width, the posterior pair near the rear margin.
First segment bearing chaetae and a pair of dorsal and ventral tentacular cirri. Scales overlap, covering the body. The first pair of scales are round, the remainder oval to round, the margins smooth and the surfaces with a patch of tubercles.
Notopodia are small mounds with a ventral acicular ligule and few chaetae.
Notopodial chaetae of 2 kinds: 1) stout with rows of spines and blunt tips; 2) capillaries with rows of spines and fine tips.
Neuropodia are well developed with an anterior bluntly rounded acicular ligule and chaetae.
Neuropodial chaetae spinose on the terminal quarter, with bidentate tips; a few in upper and lower positions with unidentate tips (M. marphysae-detail).
Up to 11 mm for 37 chaetigers.
Pale brown, sometimes with reddish brown markings on the scales.
Littoral and sublittoral, found on muddy sand or in chinks in rock. Also in tubes and burrows of polychaetes such as Marphysa sanguinea , possibly on echinoderms such as the sea cucumber Leptosynapta and the brittle-star Acrocnida brachiata , the sipunculid Golfingia elongata and a crustacean Urothoe grimaldi .
Known from the Thames estuary, western English Channel, Irish Sea, western Scotland, western and southern Ireland.
Pettibone (1993) has re-examined the syntypes of Harmothoe marphysae and described the neuropodia of the mature specimen as having prechaetal lobes with a 'deep acicular notch'. Chambers and Muir (1997) also examined the syntypes and described the acicular lobe as bluntly rounded. In the mature syntype the end of the acicular lobe has bent over to give the appearance of a notch.