Morphology
The rhizome of H. spinulosa gives rise to erect lateral shoots, up to 15 cm long, bearing compound leaves. Each compound leaf is made up of 10-20 pairs of opposite leaflets, lying in a single plane and obliquely arranged around the common stalk. The leaflets are oblong-linear, 1.5-2.5 cm long and 3-5 mm wide, with a serrated edge distinctly visible to the naked eye. The appearance of H. spinulosa could be described as 'fern-like since the pinnate nature of the leaves bears some resemblance to fern fronds. The rhizomes are thin, sometimes 'woody', and have prominent shoot scars.

Diagnostic features
Compound leaf with serrated leaflets. 10-20 pairs of leaflets per shoot. 'Fern-like'.

Likely to be confused with

Halophila tricostata
Confusion may arise between H.spinulosa and H.tricostata because both have compound leaves i.e. leaves with a number of leaflets, but H.spinulosa has them arranged in definite opposite pairs whereas H.tricostata has 2-3 leaflets per node in a whorled arrangement.

Other Halophila species
The whole intact plant is unlikely to be confused with any other seagrass, except perhaps H.tricostata (see above). However, if examined in isolation, individual leaves may superficially resemble other Halophila species. The very serrated margin and oblong-linear shape of H.spinulosa's leaves should distinguish them from leaves of other species.

Green algae
In the field, there is a real possibility of confusing H.spinulosa with two species of green algae, Caulerpa sertularioides and C.mexicana, but the similarity is largely superficial since the algae lack true leaves, roots and a creeping, underground rhizome system.