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Tropical Seagrass Identification

from Waycott et al 2004

Seagrasses are a functional group, not a taxonomic group, of angiosperms (flowering plants), i.e. various seagrass families do not necessary have to be closely related.

There continues some debate in the literature and between seagrass taxonomists on the details (particularly below sub class) on the correct classification. For example, the Angiosperm Phylogeny Group published several papers recommending angiosperm classification, and is considered by many to represent the "standard". However many prominent seagrass taxonomists disagree. From the advice of Dr Don Les (University of Connecticut) and Assoc. Prof Michelle Waycott (James Cook University), this is the best we have been able to compile:

Kingdom: Plantae
Division: Magnoliophyta (Angiosperms)
Class: Liliopsida
Sub class: Alismatidae
Order: Alismatales
Family: Hydrocharitaceae
Order: Potamogetonales
Family: Cymodoceaceae, Posidoniaceae, Potamogetonaceae, Ruppiaceae, Zannichelliaceae and Zosteraceae

Please note: This classification is an opinion of Seagrass-Watch HQ based on the best information available. Seagrass-Watch HQ recommends that users exercise their own skill and care with respect to their use of this classification and that users carefully evaluate the accuracy, currency, completeness and relevance of the material on the web site for their purposes.




El Shaffai A. (2011). Field Guide to Seagrasses of the Red Sea. Rouphael, A.and Abdulla, A., eds. First Edition. Gland, Switzerland: IUCN and Courbevoie, France:
Total Foundation. viii + 56pp. [2.8Mb]
To download: Click Here
This guide is also available at the IUCN CLICK HERE
Lanyon J (1986). Guide to the identification of seagrasses in the Great Barrier Reef region. Great Barrier Reef Marine Park Authority Special Publication Series (3) (Nadicprint Services, Townsville). 54pp. [3.3Mb]
To download: Click Here
This guide is also available at the Great Barrier Reef Marine Park Authority CLICK HERE
Waycott M, McMahon K, Mellors J, Calladine A and Kleine D (2004) A guide to tropical seagrasses of the Indo-West Pacific. James Cook University, Townsville. 72pp.
To purchase a copy: Click Here
Cymodocea rotundata
Cymodocea serrulata
Enhalus acoroides
Halodule pinifolia
Halodule uninervis
Halophila capricorni
Halophila decipiens
Halophila ovalis
Halophila minor
Halophila spinulosa
Halophila tricostata
Syringodium isoetifolium
Thalassia hemprichii
Thalassodendron ciliatum
Zostera capricorni

Cymodocea rotundata (CR)

  • Flat, strap-like leaves 2-4mm wide
  • Rounded, smooth leaf tip
  • Smooth rhizome
  • Scars from well developed leaf sheaths form a continuous ring around the stem
  • Found on shallow reef flats

Cymodocea serrulata (CS)

  • Linear strap-like leaves, 5-9mm wide
  • Serrated leaf tip
  • Leaf sheath is broadly triangular with a narrow base
  • Leaf scars do not form a continuous ring around the stem
  • Found on shallow subtidal reef flats and sand banks

Enhalus acoroides (EA)

  • Very long ribbon-like leaves 30-150 cm long
  • Leaves with inrolled leaf margins
  • Thick rhizome with long black bristles and cord-like roots
  • Found on shallow/intertidal sand/mud banks (often adjacent to mangrove forests)

Halodule pinifolia (HP)

  • Fine, delicate leaves up to 20cm long
  • 1 central vein
  • Black central vein splits into two at the rounded leaf tip
  • Usually pale rhizome, with clean black leaf scars
  • Found on intertidal sand banks

Halodule uninervis (HU)

  • Usually larger than Halodule pinifolia
  • Trident leaf tip
  • 1 central longitudinal vein
  • Rhizome usually pale ivory, with clean black leaf scars
  • Dugong preferred food
  • Found on shallow/intertidal sand or mud banks

Halophila capricorni (HC)

  • Small oval leaves that are hairy on one side
  • Central vein on leaf with 9-14 cross veins
  • Usually found deeper than 10m in coral environments proximal to coral reefs
  • Only found in subtidal Australian waters (>10m) proximal to coral reefs

Halophila decipiens (HD)

  • Small oval leaf blade 1-2.5cm long
  • 6-8 cross veins
  • Leaf hairs on both sides
  • Leaves usually longer than wider
  • Found at subtidal depths (>10m)

Halophila ovalis (HO)

  • Oval shaped leaves in pairs
  • 8 or more cross veins
  • No hairs on leaf surface
  • Preferred dugong food
  • Common early colonising species
  • Found from intertidal to subtidal depths

Halophila minor (HM)

  • Less than 8 pairs of cross veins
  • Small oval leaves occurring in pairs
  • Wedge-shaped leaf sheath
  • Found on shallow/intertidal sand flats

Halophila spinulosa (HS)

  • Fern like
  • Leaves arranged in opposite pairs
  • Erect shoot up to 15cm long
  • Found at subtidal depths (>10m)

Halophila tricostata (HT)

  • Erect shoots 8-18cm long
  • Leaves with 3 veins
  • 2-3 leaves at each node
  • Leaves “whorl” around stem
  • Found at subtidal depths (>10m)
  • Endemic to Queensland, Australia

Syringodium isoetifolium (SI)

  • Cylindrical in cross section (spaghetti like)
  • Leaf tip tapers to a point
  • Leaves 7-30cm long
  • Found on shallow subtidal reef flats and sand banks

Thalassia hemprichii (TH)

  • Short black bars of tannin cells in leaf blade
  • Thick rhizome with scars between shoots
  • Hooked/curved shaped leaves
  • Leaves 10-40cm long
  • Common on shallow reef flats

Thalassodendron ciliatum (TC)

  • Cluster of ribbon-like curved leaves at the end of an erect stem
  • Round, serrated leaf tip
  • Tough, woody rhizomes with scars from successive shoots
  • Very coiled, branched roots
  • Typically found in rocky areas with strong reef crests

Zostera capricorni (ZC)

  • Long strap-shaped leaves
  • 5 longitudinal veins
  • Cross veins which form a mesh across leaf blade
  • Rounded leaf tip
  • Leaf grows straight from rhizome ie. no stem
  • Found on shallow and intertidal mud/sand flats

Please note: Revision of Zostera capricorni has resulted in classfication to subspecies. In Queensland, Zostera capricorni has been revised to Zostera muelleri subsp. capricorni (Asch.) S.W.L.Jacobs. See Jacobs, S.W.L., Les, D.H. and Moody, M.L. (2006). New combinations in Australasian Zostera (Zosteraceae). Telopea 11(2): 127-128.
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