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Appearance and Identification

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A. serpens is a mid-dark green, pleurocarpous moss which grows in large, extensive patches of slender shoots, either in loosely intertwined wefts or as rough mats which trail across the ground. Shoots are 1-2cm long with small (0.5mm), irregularly branched leaves which stick out from the shoots, giving a soft, feathery appearance (Blockeel, 2010a; Blockeel, 2014; Campisi et al., 2019; Hill et al., 2017; Smith & Smith, 2004). The leaves themselves have an elongated spearhead shape, rounded and wide at the base before suddenly narrowing about half way up and ending in a finely tapered point. They also have a small nerve which runs up to the mid-point of the leaf, although it can be tricky to see in the field even with a hand lens, and the leaf margins can be entire, toothed, or somewhere in between (Blockeel, 2010a; Smith & Smith, 2004). 

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A. serpens is autoicous (Blockeel, 2014), meaning that male and female organs will grow on the same plant but on different branches. With the exception of specimens growing in saltmarsh and dune environments, capsules are very common in this species and can be seen from March-August (Blockeel, 2010a; Hill et al., 2017). Capsules are held horizontal rather than upright, and grow to about 2mm long in a curved, cylindrical shape with conical lids and white calyptra (Blockeel, 2010a).

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Similar Species

• Anomodon longifolius - Overall this moss has a brighter and more yellow colour with longer shoots (60mm), and a nerve which runs the whole length of the leaf. UK specimens of A. longifolius also do not produce capsules, unlike A. serpens where capsules are abundant during the right months (Hill et al., 2017; Lawley, 2010). Additionally, this species is very rare in Great Britain, occuring in just 6 hectads, and only growing below 250m on hard rocks in either broadleaf deciduous woodland or inland cliffs (Hill et al., 2017). 

• Conardia compacta - Leaves are much more irregularly spread which gives the moss a spikier, almost crinkly look. Inspection with a hand lens will also reveal a much longer nerve than found in A. serpens (Blockeel, 2010a; Blockeel; 2010b). The lack of capsules produced by this species in the UK and Ireland can also help to differentiate it from A. serpens (Hill et al., 2017). C. compacta is a lot rarer than A. serpens, it has not been recorded in Wales or Ireland since 1950. It grows almost exclusively in deep shade on inland rock cliffs and outcrops where A. serpens is not found (Hill et al., 2017)

• Cratoneuron filicinum - The most reliable method of differentiation is to view specimens under a microscope and look for patches of orange alar cells, which are present in C. filicinum but not in A. serpens (BIBS, n.d). However, for in the field identification C. filicinum has a more prominent nerve, longer shoots, and only grows capsules in May (A. serpens produces them from March through August) (Blockeel, 2010a; Hill et al., 2017). There are also some habitat differences which can be used to help distinguish these species, C. filicinum but not A. serpens can be found above 580m and in grasslands, bogs and the running waters of streams, while only A. serpens can grow in heathland and horticultural areas (Hill et al., 2017).

• Habrodon perpusillus - Unlike A. serpens, unless they are wet the leaves of H. perpusillus lie overlapping and flat against the stem. They also have no nerve, and are about twice the size of those on A. serpens, making the short, shoots look furry and cylindrical, a bit like a squirrels tail (Blockeel, 2010a; Preston, 2010). This species is also solely epiphytic, restricted to hedgerows and sparsely wooded grassland below 60m elevation, making in very rare in Great Britain and absent in Ireland (Hill, et al., 2017). 

• Heterocladium flaccidum - Its leaves have only a very sharp nerve, and end in a sharp acute point, rather than the long, tapered tip characteristic of A. serpens. Its shoots are also about half the length of those in A. serpens, and it does not produce capsules in the UK and Ireland (Blockeel, 2010a; Bosanquet, 2010; Hill et al., 2017). Additionally, any specimens found outside of broadleaf deciduous woodland or inland cliffs, rocks and pavements cannot be H. flaccidum (Hill et al., 2017).

• Hygroamblystegium humile - Leaves are sparser and stick out more from the stems, making H. humile look sharp and spiky, in contrast to the soft and feathery appearance of A. serpens, it also rarely produces capsules. H. humile frequents well-lit, waterlogged areas, while A. serpens prefers to be partially shaded with moist but not waterlogged substrate (Blockeel, 2010c, Hill et. al., 2017). Additionally, while A. serpens is widespread, H. humile is absent from most of Northern England, Wales, Ireland, and Scotland (Blockeel, 2010a; Blockeel, 2010c).

• Hygroamblystegium varium - The leaves are larger and wider, with a thicker nerve which extends all the way to the tip of the leaf, and unlike A. serpens it rarely produces capsules. H. varium grows in much less intricate and less dense patches than A. serpens, and can also tolerate waterlogged soils, which A. serpens cannot (Blockeel, 2010a; Blockeel, 2010d; Hill et al., 2017). 

• Kandea elodes - At 70mm long the shoots are much loarger, while the leaves are narrower with a longer, more prominent nerve that runs all the way to the leaf tip. It also rarely produces capsules (Blockeel, 2010a; Hill et al., 2017). K. elodes is not only much rarer than A. serpens, but it also grows in quite different habitats. It favours well-lit, waterlogged environments, but cannot grow on rocky substrate or above 365m elevation (Hill et al., 2017). 

• Leskea polycarpa - Has sharply pointed leaves which lack the characteristic long, tapered tips of A. serpens leaves. Similarly to A. serpens it frequently produces capsules, although only between May and August, and they are visually quite different, growing upright and straight rather than horizontal and curved (Blockeel, 2010a: Hill et al., 2017).

• Microeurhynchium pumilum - Leaves are shorter, with a more acute, less drawn out tip, and both leaves and branches spread out more than in A. serpens giving it an overall softer, neater, and fluffier look. The differences in leaf shape as easiest to see when examined under a microscope (Blockeel, 2010a; Smith & Smith, 2004). Another way to tell them apart in the field is to look at the timing of when mature capsules are present, for A. serpens they will be visible from March-August, but for M. pumilum they will be visible from November to March (Hill et al., 2017). 

• Platydictya jungermannioides - The leaves are nerveless and taper to an even finer point than those of A. serpens, they also sometimes grow purple rhizoids, which do not occur in A. serpens (Blockeel, 2010a; Blockeel, 2010b; Sleath, 2010). Their distribution is much more restricted than A. serpens, they are mostly absent from the South East, South West, and Anglia, and only grow in broadleaf deciduous woodland and inland rocky environments (Hill et al., 2017; Sleath, 2010). 

• Rhynchostegiella curvieseta - Leaves have a much narrower base and a less drawn out tip, somewhat resembling a pine needle, in contrast to the wide egg-shaped base and long tip of A. serpens (Blockeel, 2010a). Additionally, R. curvieseta inhabits quite different environments to A. serpens, requiring deep shade and permanently waterlogged substrate below 200m elevation, it is also very rare in the UK (Hill et al., 2017). 

• Rhynchostegiella tenella - Has very narrow leaves, which gives the overall moss a distinctive silky appearance. It also frequently produces capsules, but at different times of the year to A. serpens, with mature capsules being most abundant from October to April (Blockeel, 2010a; Hill et al., 2017). Unlike A. serpens, R. tenella needs full shade, and relatively dry, well drained soils in order to grow (Hill et al., 2017). 

• Rhynchostegiella teneriffae - Leaves have a much narrower base and a less drawn out tip, somewhat resembling a pine needle, in contrast to the wide egg-shaped base and long tip of A. serpens (Blockeel, 2010a). Also R. teneriffae can only be found near inland water sources, growing in areas which are deeply shaded and permanently water-logged e.g. in streams and bogs (Hill et al., 2017). 

• Serpoleskea confervoides -Shoots are generally much shorter, growing to only 10mm, and the leaves do not have any nerves. S. confervoides is also much less common and widespread, it grows specifically on hard rock in deciduous woodland, or occasionally on inland cliffs, and nowhere else (Blockeel, 2010a; Hill et al., 2017). 

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