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

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