Generic descriptions: Melobesioideae

Melobesioideae
(Bizzozero 1885: 109 - as ‘Melobesieae’)

Thallus non-geniculate; some but not all cells of contiguous vegetative filaments normally joined by cell fusions; secondary pit connections apparently absent or comparatively rare; tetrasporangia/ bisporangia bearing apical plugs and borne within multiporate conceptacles.

WARNING: Gametangial characters presented below as part of the generic descriptions are mainly speculative as their state is often not known for the type species of a genus. Please check recent literature before using these characters in defining genera.

Clathromorphum Foslie 1898: 4

• thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction monomerous;
• haustoria absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• subepithallial intitals usually as long as or longer than cells immediately subtending them;
• tetra/bisporangial conceptacles develop from groups of subeithallial initials;
• spermatangial initials formed directly from meristematic cells;
• it is uncertain whether or not spermatangial initials are overlain by a layer of protective cells;
• spermatangial conceptacle roofs formed from vertically-oriented filaments between the spermatangial initials and subepithallial initials;
• spermatangial systems occur on the floor, walls and roof of male conceptacles;
• spermatangial systems simple (i.e. not dendroid) .

COMMENT: In addition to the above characters, most recorded species of Clathromorphum have multicellular epithallial filaments.
ETYMOLOGY:Clathromorphum, probably from the Latin clathro (bearing lattices), and the latinized Greek morphe (form, shape).
KEY FEATURES: All of the above features are needed to confirm identification of Clathromorphum.

Exilicrusta Chamberlain 1992: 185

• Thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction dimerous;
• haustoria absent;
• cell fusions present;
• secondary pit connections absent;
• outermost walls of terminal epithallial cells flared;
• tetrasporangia bearing apical plugs and borne within multiporate conceptacles;
• male conceptacle roofs formed centripetally from groups of peripheral filaments;
• spermatangia confined to the floor of male conceptacles;
• spermatangial systems on the conceptacle floor simple;
• gonimoblast filaments borne on the periphery of a central fusion cell.

COMMENT: Only a single species, E. parva Chamberlain, is known for this genus.
ETYMOLOGY: under development.
KEY FEATURES: Small,thin, dimerous plants with flared epithallial cells and simple spermatangial systems confined to the floor of the male conceptacle.

Kvaleya Adey & Sperapani 1971: 31

• Thallus non-geniculate;
• thallus not arborescent in growth form;
• thallus forming small, pale, pustulent cushions at the surface of other corallines;
• haustoria present;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• epithallial cells commonly absent;
• subepithallial initials as short as or shorter than the cells immediately subtending them, with cortical cell elongation occurring successively down the thallus;
• tetrasporangial conceptacle primordia arising from subepithallial or terminal inititals.

COMMENT: This genus has a single species, K. epilaeve Adey & Sperapani, and it is known only from plants referred to as Leptophytum laeve (Stroömvelt)Adey.
ETYMOLOGY: Kvaleya after the island of Kvaloøy in Norway.
KEY FEATURES: Kvaleya is the only melobesioid genus producing haustoria (i.e. that is parasitic on other corallines).

Leptophytum Adey 1966: 323

• thallus non-geniculate;
• thallus not arborescent in growth form;
• relatively thin thallus lacking protuberances;
• internal construction monomerous;
• haustoria absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• thallus surface, as seen under the SEM, at least partly of Leptophytum-type flat-topped cells, each with a minute central hole, surrounded by a thin calcareous wall;
• epithallial cells very short and domed but not flared;
• subepithallial initials as short as or shorter than the cells immediately subtending them, with cortical cell elongation occurring successively down the thallus;
• tetra/bisporangial conceptacles initiated shalowly within the thallus;
• spermatangial systems borne on floor, walls and roof of the conceptacle chamber;
• spermatangial systems somewhat elaborated in the centre of the conceptacle floor, simple or arcuate elsewhere;
• fusion cell of carposporophyte arachnoid.

COMMENT: There is presently some controversy as to whether Leptophytum is a genus distinct from Phymatolithon.
ETYMOLOGY: fr. Greek leptos(slender, thin) & latinized Greek phyton (plant) - probably referring to the thallus morphology which Adey (1966) describes as 'thin crusts'
KEY FEATURES: The centrally elaborated spermatangial systems, shallow tetra/bisporangial conceptacle initiation, and progressive elongation have been considered recently to diagnose this genus. However, recent studies question whether there is any basis for distinguishing Leptophytum from Phymatolithon.

Lithothamnion Heydrich 1897: 412, nom. cons.

• Thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction monomerous;
• haustoria absent;
• cell fusions present;
• secondary pit connections absent;
• outermost walls of terminal epithallial cells flared;
• actively dividing vegetative initials as long as or longer than their immediate inward derrivatives;
• male conceptacle roofs formed centripetally from groups of peripheral filaments;
• spermatangial initials not overlain by a layer of protective cells;
• spermatangia on the floor, walls, and roof of male conceptacles;
• spermatangial systems on the conceptacle floor dendroid,
• gonimoblast filaments borne across the conceptacle floor.

COMMENT: Lithothamnion is one of the few melobesioid genera that is distinct and clearly separated from all others.
ETYMOLOGY: Lithothamnion, from Greek lithos (stone) and thamnion (little shrub)
KEY FEATURES: The combination of flared epithallial cells, monomerous thallus construction, and dendroid spermatangial systems distinguishes Lithothamnion from all other melobesioid genera. Caution should be applied when looking at specimens with flared epithallial cells, as this character is also present in the Sporolithaceae.

Mastophoropsis Woelkerling 1978: 210

• Thallus non-geniculate;
• thallus arborescent (tree-like) and flabelliform (fan-shaped) in growth form, and anchored to the substratum by a distinct holdfast and stipe;
• internal construction monomerous;
• haustoria absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• gametangial characters uncertain;
• tetrasporangial conceptacle primordia arising from subepithallial initials.

COMMENT: Only a single species, M. caniliculata (Harvey in J.D. Hooker) Woelkerling, has been described and it is known only from southeastern Australia.
ETYMOLOGY: Mastophoropsis, from the generic name Mastophora and the Greek word opsis (appearance).
KEY FEATURES:Mastophoropsis is the only genus of melobesioid in which the thallus is aborescent, fan-like, and anchored to the substratum by a distinct holdfast and stipe.

Melobesia Lamououx 1812: 186

• Thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction dimerous;
• thallus thin, erect filaments up to 8 cells long;
• haustoria absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• dividing vegetative initials usually as long as, or longer than, cells immediately subtending them
• spermatangial systems occur on the floor, walls and roof of male conceptacles;
• spermatangial systems somewhat dendroid;
• fusion cell apparently lacking, but gonimoblast filaments developing only from peripheral supporting cells;
• tetrasporangial primordia developing dorsally on basal filaments, cells of erect filaments above the primordium being shed during development.

COMMENT: Once species of Melobesia, M. van-heurckii (Heydrich) Cabioch, is apparently uncalcified.
ETYMOLOGY:Melobesia, probably after Melobosis, a daughter of Oceanus in Greek mythology.
KEY FEATURES:Melobesia is the only genus of melobesioid in which the thallus is dimerous and the epithallial cells are not flared. Species of Melobesia are commonly epiphytic on other algae, sometimes epilithic or on shells

Mesophyllum Lemoine 1928: 251

• Thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction monomerous;
• haustoria absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• vegetative intitals usually as long as or longer than cells immediately subtending them;
• spermatangial initials formed directly from meristematic cells;
• spermatangial initials overlain by a layer of protective cells;
• spermatangial conceptacle roofs formed centripetally from groups of peripheral filaments;
• spermatangial systems occur on the floor, walls and roof of male conceptacles;
• spermatangial systems simple.

COMMENT: Originally, a coaxial arrangement of medullary filaments was considered as diagnostic of Mesophyllum, but this is no longer the case. However, most species that have been studied recently have at least some degree of coaxial medulla.
ETYMOLOGY: Mesophyllum, from the Greek mesos (middle) and the latinized Greek word phyllon (leaf). Presumably this refers to Lemoine's hypothesis that species of Mesophyllum were intermediate in morphology between Lithophyllum and Lithothamnion.
KEY FEATURES: Mesophyllum is the only melobesioid to have: outermost walls of terminal epithallial cells rounded or flattened but not flared; vegetative intitals usually as long as or longer than cells immediately subtending them; spermatangial initials overlain by a layer of protective cells; simple spermatangial systems all around the inner surface of the male conceptacle.

Phymatolithon Foslie 1898: 4, nom. cons.

• thallus non-geniculate;
• thallus not arborescent in growth form;
• internal construction monomerous;
• haustoria absent;
• cell fusions present;
• secondary pit connections absent;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• actively dividing vegetative initials as short as or shorter than than their immediate inward derrivatives;
• tetra/bisporangial conceptacle initials adventitious; often shedding disc of cells;
• male conceptacle roofs formed centripetally from groups of peripheral filaments;
• spermatangial initials not overlain by a layer of protective cells
• spermatangial systems on the conceptacle floor are dendroid (branched).

COMMENT: There is presently some controversy as to whether Leptophytum is a genus distinct from Phymatolithon.
ETYMOLOGY: Greek, phyma = swelling, tubercle; lithos = stone.
KEY FEATURES: Phymatolithon is the only melobesioid to have: outermost walls of terminal epithallial cells rounded or flattened but not flared; vegetative intitals usually as short as or shorter than cells immediately subtending them; spermatangial initials not overlain by a layer of protective cells; and branched spermatangial systems on the floor of the male conceptacle (those on the roof are usually unbranched or weakly branched). Phymatolithon is also considered to have adventitious, "deep" tetra/bisporangial conceptacle initiation.

Synarthrophyton Townsend 1979

• plants non-taeniform;
• plants non-taeniform;
• thallus monomerous;
• dividing subepithallial initials usually as long as or longer than cells immediately subtending them;
• outermost walls of terminal epithallial cells rounded or flattened but not flared;
• spermatangial initials overlain by protective cells;
• spermatangial branches dendroid or both dendroid & simple (see also second image);
• tetra/bisporangial conceptacles develop from subepithallial initials ??????

COMMENT: This genus is based on characters that appear to be continuous with characters used to delimit Mesophyllum (spermatangial systems) and Leptophytum/Phymatolithon.
ETYMOLOGY: Greek, syn = together, arthron = jointed, phyton = plant. R. Townsend - the genus was considered to be at the point in the branch of the phylogenetic tree where Lithothamnion and Mesophyllum meet.
KEY FEATURES: The presence of branched spermatangial systems, and elongate subemithallial initials are considered to be the main features distinguishing this genus.

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Derek Keats,
updated 02/01/01