Zosteraceae Dumort.

Haigh, A. (2009). Neotropical Zosteraceae.

Morphology

Description

Herbs perennial , rarely annual , usually submerged, a few species intertidal and periodically exposed to air; rhizomes with monopodial branching. Leaves alternate , distichous , simple , linear . Inflorescences borne on simple or branched, erect spadices, the flowers in two rows, enclosed by spathe . Flowers unisexual ( monoecious or dioecious ), highly reduced; perianth absent. Staminate flowers with one stamen ; anthers with two distinct, bilocular thecae, longitudinally dehiscent , the pollen shed in long, filamentous strands. Pistillate flowers with 1-locular ovary , the stigmas two, long; ovule one. Fruits achenes; seeds without endosperm ; hypocotyl enlarged.

Distribution

Distribution in the Neotropics

A cosmopolitan family of three genera and c. 18 species. All three genera represented in Latin America with c. 3 species.

• Zostera L.:  about 12 species in mainly northern hemisphere waters, but with one species (Z. marina L.) extending into the coast of Mexico and northern Central America.
• Heterozostera den Hartog:  monospecific (H. tasmanica (Martens ex Aschers.) Hartog), native to the coastal waters of southern Australia and northern Chile.
• Phyllospadix Hook.:  five species mainly found in the northern temperate coastal waters of the Pacific ocean, one species (P. torreyi S.Watson) extends to the Neotropics.

General Description

Number of genera

Three Neotropical genera (see above)

General notes

• The family generally occurs in shallow coastal waters of temperate to subtropical regions, all three genera just reaching into the Neotropics.
• Perennial plants can form extensive stands (often called submarine meadows) in shallow brackish and saltwater areas with clear water and minimal wave action.
• Zosteraceae play an important role in stabilization of shallow marine sediments.

Status

Native.

Notes on delimitation

• Zosteraceae is placed in the Alismatales, closest to Potamogetonaceae.

Diagnostic

Useful tips for generic identification

Key to genera of Neotropical Zosteraceae

1. Rhizome thick with condensed internodes; rhizome cortical cells thick-walled, cortical fibre bundles absent; roots short, thick; leaf blade thick, flat to biconvex; normally growing on rock substrate; dioecious — Phyllospadix
1. Rhizome slender, usually with elongate internodes; rhizome cortical cells thin-walled, cortical fibre bundle present; roots long, thin; leaf blade thin, flat; normally growing on sand/mud substrate; monoecious — 2

2. Rhizome internodes with 4-12 vascular bundles, fibre bundles distributed throughout cortex, commonly associated with stele; erect shoots long, wiry, branched and often forming new rhizomes with plantlets; only in older plants erect shoots sometimes developing inflorescences ... Heterozostera
2. Rhizome internodes with 2 cortical vascular bundles, fibre bundles restricted to outer cortex; erect shoots herbaceous not wiry, always developing inflorescences — Zostera

Distinguishing characters (always present)

• Intertidal to marine aquatic herbs.
• Flowers unisexual, aggregated in spadices (spikes), enclosed by a spathe.
• Fruits achenes.

Literature

Important literature

Kuo, J. & McComb, A.J., 1998. Zosteraceae. In: K. Kubitzki (ed.), The families and genera of vascular plants 4: 296-502. Springer Verlag, Berlin.

Stevenson, D.W. 2004. Zosteraceae. p. 496. In: Smith, N., Mori, S.A., Henderson, A., Stevenson, D.W. & Heald, S.V. (eds.). Flowering Plants of the Neotropics. New York Botanical Garden, Princeton University Press, Princeton.