Cymodoceaceae is a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

flowering plant Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...

s, sometimes known as the "manatee-grass family", which includes only marine species. The 2016

APG IV The APG IV system of flowering plant classification is the fourth version of a modern, mostly molecular-based, system of plant taxonomy for flowering plants (angiosperms) being developed by the Angiosperm Phylogeny Group (APG). It was publishe ...

does recognize Cymodoceaceae and places it in the order

Alismatales The Alismatales (alismatids) are an order of flowering plants including about 4,500 species. Plants assigned to this order are mostly tropical or aquatic. Some grow in fresh water, some in marine habitats. Description The Alismatales compri ...

, in the clade

monocots Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of t ...

. The family includes five genera, totaling 17 species occurring in tropical seas and oceans (so-called

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the orde ...

es). According to th
AP-Website
it is doubtful if the family

Ruppiaceae ''Ruppia'', also known as the widgeonweeds, ditch grasses or widgeon grass, is the only extant genus in the family Ruppiaceae, with eight known species. These are aquatic plants widespread over much of the world. The genus name honours Heinrich ...

is distinct enough to be kept apart. The inclusion of the sole genus ''Ruppia'' in Ruppiaceae in Cymodoceaceae is being considered. The plants in the three families Cymodoceaceae,

Posidoniaceae ''Posidonia'' is a genus of flowering plants. It contains nine species of marine plants ("seagrass"), found in the seas of the Mediterranean and around the south coast of Australia. The APG system (1998) and APG II system (2003) accept this gen ...

and

form a monophyletic group. Its fossil record shows that Cymodoceaceae was established in its current Indo-West Pacific distribution by the early Eocene and perhaps even during the late Paleocene. Fossils of ''Thalassodendron auriculalopris'' den Hartog and ''Cymodocea floridana'' den Hartog (both extant) were also found in west-central Florida and date back to the late middle Eucene. Their age and lack of diversity speaks to an extremely slow rate of evolution within the Cymodoceaceae.

Taxonomy

Marine grasses families:

Zosteraceae Zosteraceae (one of the four seagrasses families, Kubitzki ed. 1998) is a family of marine perennial flowering plants found in temperate and subtropical coastal waters, with the highest diversity located around Korea and Japan. Most seagrasses ...

, Cymodoceaceae,

and

. Related families:

Potamogetonaceae The Potamogetonaceae, commonly referred to as the pondweed family, is an aquatic family of monocotyledonous flowering plants. The roughly 110 known species are divided over six genera. The largest genus in the family by far is ''Potamogeton'', w ...

Zannichelliaceae The Potamogetonaceae, commonly referred to as the pondweed family, is an aquatic family of monocotyledonous flowering plants. The roughly 110 known species are divided over six genera. The largest genus in the family by far is ''Potamogeton'', w ...

(not consistently).

Reproductive strategies

Cymodoceaceae is one of four families to have developed filamentous

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

, along with Ruppiaceae, Zosteraceae, and Posidonaceae. The pollen is assembled as long and thin grains rather than spheres, which increases its surface area when floating on the water. In addition, the pollen can more easily form pollen rafts, enabling distribution over a large surface area of water. The pollen is epihydrophilous (pollen distributed on the surface of the water) or hypohydrophilous (pollen distributed below the surface of the water) depending on the genera. There are three different methods used. Species in ''Halodule'' and ''Cymodocea'' release pollen at low tide, where it floats and assembles into snowflake-like pollen rafts which then hopefully make contact with the stigmas when the tide starts coming back in. ''Amphibolis'' and ''Thalassodendron'' have pollen that is carried up to and then released upon the surface of the water by abscisent male flowers. ''Syringodium'' has pollen grains that are approximately the same density as seawater and form small clumps which move beneath the surface by submarine currents to the stigmas of female flowers. This return to hypohydrophily is interpreted as a reversal to the ancestral state. All species in Cymodoceaceae are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

. Although this occurs in about 75% of the seagrasses, it is a feature found in less than 5% of all angiosperms. There are two leading theories regarding the prevalence of dioecy in Cymodoceaceae. The construction and reception of pollen rafts are bulky operations. To have either perfect flowers or bear both male and female flowers on one plant could interfere with successful fertilization. The other theory is it would ensure cross-pollination in an environment that would make self-pollination much more likely, a process that would limit the gene pool and make plants more susceptible to variable conditions or disease. Two genera have

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...

seedlings. The seeds of ''Amphibolis'' and ''Thalassodendron'' lack seed coats and do not store starch or other important nutrients. They instead latch onto the parent plant immediately after germination. The seedling develops a footing tissue from the hypocotyl, which attaches to the parents through transfer cells. The seedlings develop leafy shoots over the course of 7–12 months before being released. ''Amphibolis'' seedlings develop a grappling apparatus which serves to anchor the seedling to a substrate once released whereas the seedlings of ''Thalassodendron'' are released from an enveloping bract. As the external wall of the footing tissue in the seeds is apoplastic, the seedlings can be considered parasitic on and also cytoplasmically isolated from the maternal tissue.

References

External links

''Monocot families'' (USDA)

(Manatee grass)
links at CSDL, Texas
{{Taxonbar, from=Q131606 Alismatales families Dioecious plants