Lethasterias nanimensis has five long flexible arms with numerous, black-tipped spines wreathed with crossed pedicellariae. The background colour on the upper surface is yellowish-brown. In alcohol, the black colour in the spines may be lost. The arms up to 30 cm long and the arm-to-disc ratio is 8. The aboral spines form a regular carinal series, but other spines are less regular. The tips of the spines tend to be fluted like a drill bit. The crossed pedicellariae have teeth of equal size. Occasional straight pedicellariae occur on the oral surface near the mouth plates. The marginals consist of a row of superomarginals with one spine (usually blackish) and a row of inferomarginals with two spines, the latter usually gouge shaped and bearing a cluster of pedicellariae on the distal side. Small clusters of up to ten papulae occur between the spines, especially near the marginals. The smaller adambulacrals (33 plates per 10 inferomarginals) each bear two spines, the proximal one smaller. The mouth plates are proximal to a short adoral carina of two fused adambulacrals and they each bear two marginal spines, one longer than the other, and one or two suborals.
The subspecies L. n. chelifera has more numerous straight pedicellariae on all parts of the body, with two or three longer curved interlocking fingers. The aboral spines are less regular and smaller.
Similar SpeciesLethasterias nanimensis might be confused with Orthasterias koehleri or Stylasterias forreri, but the living colours are quite distinct. O. koehleri has a row of oral intermediates and two rows of inferomarginals. The c