''Argyrotheca'' is a genus of very small to minute
lampshells (maximum long). All species share a large pedicel opening (or foramen), one ridge on the inside of the pedunculate valve, pits in a diamond pattern on the inside of both valves, and without radial ridges that end in tubercles. It occurs in depths between 6 and 1300 m. It is known since the latest
Cretaceous.
Description
Some ''Argyrotheca'' species have red stripes radiating from the pedicle opening (or foramen). These species are among a few living brachiopods with color marking on the shell. All other colored brachiopods are from shallow water too. This kind of color patterns may be a form of camouflage.
''Argyrotheca'' is fastened by a very short and thick stalk.
The shell is biconvex with two high and wide ribs radiating close to the midline (a shell-shape called strangulate) and four radiating ribs of declining height and width further lateral (a shell-shape called oppositely multiplicate), smooth or more commonly multiplicate. The inner surface of the shell has many rather coarse pits (or punctae). The larger valve (pedicle valve or ventral valve) has a fairly short stump beak (or subtruncate). The pedicle opening (or foramen) is large (about ⅓× the maximum with of the shell), almost entirely situated in the backfolding part (or delthyrium) of the pedicle valve (submesothyridid to almost hypothyridid), leaving ample space for the small deltidial plates. The pedicle collar is well developed, and is supported by median ridge (or septum).The
lophophore is large.
Differences with related genera
''Argyrotheca'' resembles ''Megathiris'', but ''Argyrotheca'' has one partition in the middle of the inside of the pedicle valve (the medial dorsal septum) and a
lophophore indented in the middle but without lobation of the branches (or schizolophous), while ''Megathiris'' has three septa and a lophophore with lobed branches (or ptycholophous).
''Joania'', which was recently split from ''Argyrotheca'', can be distinguished by the radial ribs on the inside of the peduncle valve that end in tubercles somewhat removed from the margin, and a row of tubercles on the edge of the septum.
Taxonomy
''A. rubrocostata'' does not have a brood pouch and has separate sexes. This cast doubts on the
monophyly of ''Argyrotheca''.
Reassigned species
* ''A. arguta''
= ''Joania arguta''
* ''A. cordata''
= ''Joania cordata''
Ecology
Reproduction
Unique among extant brachiopods, ''Argyrotheca'' and ''Joania'' are
hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
. Another most unusual specialization in both these genera, is that the eggs (or ovae) are retained in the enlarged
nephridia that act as a brood pouch. Fertilization takes place with its own sperm or after sperms have entered with the inhalant water current. Here early larval development takes place. The fertilized ova develop into
ciliated larvae with a feebly free-swimming life of at most a few days before settling and
metamorphosis
Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
into a tiny brachiopod fixed to the substrate.
In ''A. cordata'' and ''A. cistellula'' in the Mediterranean, ripe eggs and larvae are present year round. ''A. cuneata'' however breeds in the autumn. Eggs hatch into
gastrulas, followed by a two- and a three-lobed phase during their stay in the brood pouch. The frontal (or
apical) lobe has a girdle of long cilia, a middle (mantle) lobe carries a band of cilia in the middle of the belly (or
ventral side) and a rear (or pedicle) lobe is without cilia. ''A. cordata'' and ''A. cuneata'' larvae have 4 bundles of bristles (or
setae), while these are lacking in ''A. cistellula''. Extant brachiopods with brood pouches (Megathyridoidea: ''Argyrotheca'' and ''Joania'', Gwynioidea: ''
Gwynia'', and all Thecideoidea) are very small or minute and have short lives (2 years reported for ''A. cuneata'' ). This implies that reproduction needs to be both quick and efficient. Being hermaphrodite allows for quick and efficient self-fertilisation, and the brood pouch would reduce predation of the larvae.
Attachment
Near Bermuda ''Argyrotheca'' is mostly found on the underside of leaf-shaped corals, like ''
Agaricia
''Agaricia'' is a genus of colonial stony corals in the family Agariciidae. These corals are found in shallow waters of the West Indies.
Characteristics
Corals in this genus have several different forms but are usually massive, leaf-like or plat ...
'', ''
Mycetophyllia'' or ''
Montastraea
''Montastraea'' is a genus of colonial Scleractinia, stony coral found in the Caribbean seas. It is the only genus in the monotypic family Montastraeidae and contains a single species, ''Montastraea cavernosa'', known as great star coral. It form ...
'' or in between branches of corals, such as ''
Porites'', ''
Mussa'' and ''
Madracis'', up to about 75 m. Further down, sponges like ''
Agelas'' and ''
Plakorthis'', and concretions dominante as substrate.
''A. cuneata'' in deeper waters near Brazil is commonly found attached to shell fragments like those of bivalves.
Elsewhere undersea caves provide shelter in shallow water.
Predation
One study showed that 3,8% of the empty shells of recent ''A. cuneata'' were predated. Three types of hole were distinguished, neat regular borings by a mollusc
radula
The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...
, large irregular ones presumably made by crabs, and bowl shaped hollows with a small opening at the bottom, characteristic for
foraminifers. Another study of fossil ''A. cuneata'' showed a large difference between two nearby (2 km) and almost contemporary (Middle Miocene,
Serravallian) locations in Poland. At
Węglin, 23% of the shells had holes, but only 2% at Weglinek. Here two types of holes were distinguished: conical, assigned to
naticid molluscs, and cylindrical holes, that are assumed to be made by
muricate sea snails. ''Agyrotheca'' shakes forcefully when disturbed by animals swimming by and when disposing of particles.
Distribution
''Argyrotheca'' is known from the
Upper Cretaceous in Europe and North America, from the
Eocene in Europe, North and Latin America and the West Indies, from the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
in Europe and Mexico, from the
Miocene in Europe, the Russian Federation, North America, the West Indies and New Zealand, and from the
Pliocene in England and Italy. It can currently be found in the Atlantic (60-1280m), Pacific (160m), Mediterranean (6-400m),
the Red Sea and the Persian Gulf.
''A. cistellula'', ''A. cordata'' and ''A. cuneata'' are the only species present in the Mediterranean.
Species
''A. ageriana''
''A. ageriana'', has been described from the lower Pleistocene of Italy (Castro Marina, Lecce, Apulia). It lived in a temperate to warm-temperate sea, 75–100 m deep.
''A. cistellula''
''A. cistellula'' is a minute brachiopod, dirty white to yellow or grey in color, and has a maximum size of 2 x 3 mm. The brachial valve is rounded rectangular, a bit wider than long, or square in outline. This valve is slightly bilobed. The hinge line is straight and spans the entire width of the shell. Externally the surfaces are smooth or show faint concentric growth lines. The inner surfaces of the shell have small pits (or is endopunctate). The peduncle valve is moderately convex and has a short drop-shape, with a straight beak (or umbo), and a straight margin opposite to the hinge. The opening for the peduncle (or foramen) is large, ¼-⅓ of the total width and the triangular plates that bridge the distance between the dorsal and ventral surfaces of the shell (or deltidial plates) do not touch. This species attaches itself to hard substrates, in water between 20–100 m. The current known distribution of ''A. cistellula'' is Norway, the United Kingdom and Ireland (north-east Scotland, English Channel, west coast of the British Isles), and Italy (Sardinia and Sicily).
''A. cuneata''
Living specimens of ''A. cuneata'' have been reported from a submarine cave in Cyprus (Cape Greco, 6 m deep, with two other brachiopod species: ''Novocrania turbinata'' and ''Megathyris detruncata''), and the Cape Verde Islands (Tarrafal, northwestern coast of Sao Tiago Island, 15 m deep).
The species is also reported from the outer shelf (100–200 m deep), off the coast of Brazil (Rio de Janeiro, São Paulo, and Parana states), on carbonate rich sediment, where they are attached to shell fragments like those of bivalves.
''A. jacksoni''
''A. jacksoni'' is known from the Red Sea (shallow reef cave at Ras Muhammad, southernmost Sinai Peninsula, Gulf of Aqaba and around Port Sudan) and the Persian Gulf (Karan Island, 27°43’N 49°48.8’E; Jana Island, 27°22.4’N 49°54’E) from a depth between 5 and 16 m. It is close in size and shape to ''A. cuneata'', which differs by having a pink-red wash between the costae.
References
{{Taxonbar, from=Q3542820
Brachiopod genera
Extant Maastrichtian first appearances
Terebratulida