Key to the species of Halimeda

1	Plants prostrate, decumbent; holdfasts small, multiple, forming at base, along basal parts or as scattered attachments, sometimes inconspicuous	2
1	Plants erect or pendant, holdfast single, basal, sometimes massive or embedded in substrate, often not collected	3
2	Plants prostrate, branching mostly in one plane, branches spread out, not entangled; strictly deepwater	*H. gracilis*
2	Plants decumbent or erect, branching in many planes; bushy, branches difficult to disentangle; more common in NWHI than in main island; intertidal to subtidal	*H. opuntia*
3	Plants brittle, crumbly segments ; decalcified peripheral utricles free, bell-shaped; nodal regions with completely separate filaments	*H. fragilis*
3	Plants with segments otherwise; nodal regions with filaments fused at the nodes, or having in same node both fused and separate filaments	4
4	Terminal segments elongate; cortex of 3 to 5 layers of utricles	*H. incrassata*
4	Terminal segments not elongate, cortex never more than 4 layers	5
5	Plants to 10 cm tall, plants compact, segments close, di-trichotomously branched up to 25 mm wide, distal ones leather, not lobed	*H. discoidea*
5	Plants up to 7 cm tall, plants spreading, segments loosely structured, flexible	6
6	Segments less than 5 mm wide, arranged in rounded rows, distal ones not different from ones below	*H. velasquezii*
6	Segments more than 5 mm wide, not arranged in rounded rows	7
7	Nodal filaments fusing briefly, some filaments separate in the same node; branches spreading, segments irregular in shape	*H. copiosa*
7	Nodal filaments fusing in 2’s , 3’s 4’s, decalcified segments showing polygonal or rounded cells in surface view	8
8	Plants with segments not heavily calcified, 12-125 mm wide; cortex 2 - 4 layers, primary utricles goblet or club-shaped in lateral view	*H. tuna*
8	Plants with segments moderate calcified, tending to be brittle; cortex 2 - 3 layers, primary utricles elongate, occasionally their outer wall ornamented by a central spine	*H. scabra*