Thalassaphorura (formerly Protaphorura) debilis is confined to the littoral zone. There are records from salt marshes, and under stones below the high tide level. In some localities it is found together with Thalassaphorura halophila with which it has sometimes been confused. I have examined all the available specimens of Thalassaphorura evansi (Bagnall, 1935) and Thalassaphorura thalassophila (Bagnall, 1937) in the NHML collection; it is clear that these two species are junior synonyms of Protaphorura debilis (the latter already noted by Fjellberg 1998).
     Distinctive characters of Thalassaphorura debilis include the complete absence of any trace of a furca, a broadly oval post-antennal organ with a maximum of 20 vesicles (Fig. 1), a short empodial appendage about a third of the length of the claw with a distinct lamella (Fig. 2), and the arrangement of pseudocelli (PSO) on the dorsal side of the abdomen (Fig. 3). The typical PSO formula is 32/233/33343. There are two anal spines (Fig. 4). Also illustrated is the male genital plate on the ventral side of the fifth abdominal segment (abd5).

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Fig. 1 (above): Post-antennal organ (PAO) of Thalassaphorura debilis collected from Stoke Gabriel, River Dart in June 1937 by Bagnall.

Fig. 2 (above): Foot of the third leg of the same specimen of Thalassaphorura debilis shown in Fig. 1.

Fig. 3 (above): Dorsal side of the first (abd1) and second (abd2) abdominal segments of Thalassaphorura debilis collected from the Alnmouth Estuary in March 1937 by Bagnall. Note that there are three pseudocelli (1 to 3) on each side of abd2. VT, ventral tube on the ventral side (visible through the dorsal cuticle).

Fig. 4 (above): Anal spines (*) of the same specimen of Thalassaphorura debilis shown in Fig. 1.

Fig. 5 (above): Ventral side of the posterior abdomen of the same specimen of Thalassaphorura debilis shown in Fig. 1. The male genital plate (mgp) is clearly visible on the fifth abdominal segment (abd5).