Tryblidiida
Encyclopedia
The Tryblidiida are a group of monoplacophora containing the only extant representatives: a total of 29 species are alive today, inhabiting the ocean at depths of between 175 and 6500 metres (531.34 and 21,325.5 feet).
record, ranging from the early Cambrian
to the mid-Devonian
periods (ca. 550 - 380 million years ago). The first captured living monoplacophoran was Veleropilina zografi
in 1896, but it was considered to be an archaeogastropod. This species was revealed as monoplacophoran 87 years later in 1983.
In April 1952, a living specimen was collected from deep depths in the Middle America Trench
off Costa Rica
's Pacific coast. In 1957 that species was described and named Neopilina galatheae by its discoverer, Danish biologist Henning M. Lemche (1904–1977). An expert in the field has called this discovery "one of the greatest sensations in the [twentieth] century." As of 2008, there were 31 living species known, discovered in waters from 200 meters in depth to hadal
depths, or more than 6,000 meters in the deepest ocean trenches.
The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977. Scientists believe that the taxon Monoplacophora is probably polyphyletic and have proposed including all the living members in the order Tryblidiida. An attempt at a common name for Monoplacophores, gastroverm, proved unsuccessful, the term being called "singularly unattractive". By and large, writers since have tacitly agreed; "Monoplacophores" remains the common usage.
In 1989, fossils in Italy from the middle Pleistocene
were described which appear to be identical with the living species Micropilina
minuta.
of the shell is at the anterior end. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism
; indeed, with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation
, which suggested a "missing link
" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary.
Monoplacophorans move on a rounded foot. Their reduced head lacks eyes or tentacles.
The mantle cavity forms a horseshoe-shaped groove running around the muscular foot, in a similar fashion to that of the chiton
s, and contains five or six gills on either side. The mouth opens on the underside between the ends of the groove, while the anus
opens into the hindmost part. Like chitons, monoplacophorans possess a sensory subradula organ, as well as a rasping radula
. A fold of ciliated tissue surrounds the mouth to the front and sides, while a smaller fold, bearing a number of tentacles, lies just behind it. The stomach contains a style
, projecting from a diverticulum, or "style sac".
The mouth has a chevron-shaped lip in front of it, and bears tentacles behind it, which have various shapes and layouts in different species.
The heart is divided into two equal halves, each with its own auricle, ventricle
and aorta
. The left and right aorta fuse shortly after leaving the heart, and supply blood to the open circulatory system. There are six pairs of nephridia
l excretory organs, which empty into the mantle cavity.
The nervous system has small ganglia
around the oesophagus from which two pairs of main nerve cords run through the body; one pair supplying the foot, and the other the visceral organs. As in the chitons, these main nerve cords are connected by a series of lateral nerves, giving the layout of the nervous system an appearance somewhat like a ladder.
There are two pairs of gonad
s, which release gamete
s into the water through one of the pairs of nephridia. The sexes are separate, and fertilisation is external.
. Most are known from deep water (1800 – 6500 meters), although several species are found in shallower waters ranging up to 200 meters.
History of discoveries
Before 1952, these organisms were known only from the fossilFossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
record, ranging from the early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
to the mid-Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
periods (ca. 550 - 380 million years ago). The first captured living monoplacophoran was Veleropilina zografi
Veleropilina zografi
Veleropilina zografi is a recent deep sea species of a monoplacophoran from Atlantic ocean.- Original description :It have been described as Acmaea zografi by Philippe Dautzenberg and H...
in 1896, but it was considered to be an archaeogastropod. This species was revealed as monoplacophoran 87 years later in 1983.
In April 1952, a living specimen was collected from deep depths in the Middle America Trench
Middle America Trench
The Middle America Trench is a major subduction zone, an oceanic trench in the eastern Pacific Ocean off the southwestern coast of Middle America, stretching from central Mexico to Costa Rica...
off Costa Rica
Costa Rica
Costa Rica , officially the Republic of Costa Rica is a multilingual, multiethnic and multicultural country in Central America, bordered by Nicaragua to the north, Panama to the southeast, the Pacific Ocean to the west and the Caribbean Sea to the east....
's Pacific coast. In 1957 that species was described and named Neopilina galatheae by its discoverer, Danish biologist Henning M. Lemche (1904–1977). An expert in the field has called this discovery "one of the greatest sensations in the [twentieth] century." As of 2008, there were 31 living species known, discovered in waters from 200 meters in depth to hadal
Hadal zone
The hadal zone , also known as the hadopelagic zone and trench zone, is the delineation for the deepest trenches in the ocean...
depths, or more than 6,000 meters in the deepest ocean trenches.
The first specimen photographed alive was Vema hyalina, at a depth of 400 meters off Catalina Island, California, in 1977. Scientists believe that the taxon Monoplacophora is probably polyphyletic and have proposed including all the living members in the order Tryblidiida. An attempt at a common name for Monoplacophores, gastroverm, proved unsuccessful, the term being called "singularly unattractive". By and large, writers since have tacitly agreed; "Monoplacophores" remains the common usage.
In 1989, fossils in Italy from the middle Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
were described which appear to be identical with the living species Micropilina
Micropilina
Micropilina is a genus of monoplacophoran; in addition to a number of living, deep-sea species, it also includes a shallow water fossil from the middle Pleistocene of Italy - and as such the only fossil representative of this lineage after the Devonian....
minuta.
Anatomy
Little is known about the monoplacophora. They have a single, flat, rounded bilateral shell that is often thin and fragile; it ranges in size from 3 to 30 millimetres (in recent species). The apexApex (mollusc)
Apex is an anatomical term for the tip of the mollusc shell of a gastropod, scaphopod, or cephalopod mollusc.-Gastropods:The word "apex" is most often used to mean the tip of the spire of the shell of a gastropod...
of the shell is at the anterior end. The fossil shells exhibit a series of muscular attachment scars on the inner side, suggesting metamerism
Metamerism
Metamerism has at least three meanings:*Metamerism is the property of having repeated segments, as in annelids; a concern of zoology and developmental biology...
; indeed, with living Monoplacophora to study, it can be seen that their body segments exhibit a serial repetition of kidneys, gills and reproductive structure. This used to be interpreted as a true segmentation
Segmentation (biology)
Segmentation in biology refers to either a type of gastrointestinal motility or the division of some animal and plant body plans into a series of repetitive segments. This article will focus on the segmentation of animal body plans, specifically using the examples of the phyla Arthropoda,...
, which suggested a "missing link
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
" between mollusks and annelids. More recent studies have shown that the repetition of these organs is secondary.
Monoplacophorans move on a rounded foot. Their reduced head lacks eyes or tentacles.
The mantle cavity forms a horseshoe-shaped groove running around the muscular foot, in a similar fashion to that of the chiton
Chiton
Chitons are small to large, primitive marine molluscs in the class Polyplacophora.There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura....
s, and contains five or six gills on either side. The mouth opens on the underside between the ends of the groove, while the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
opens into the hindmost part. Like chitons, monoplacophorans possess a sensory subradula organ, as well as a rasping radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
. A fold of ciliated tissue surrounds the mouth to the front and sides, while a smaller fold, bearing a number of tentacles, lies just behind it. The stomach contains a style
Digestive system of gastropods
The digestive system of gastropods has evolved to suit almost every kind of diet and feeding behavior. Gastropods as the largest taxonomic class of the mollusca are very diverse indeed: the group includes carnivores, herbivores, scavengers, filter feeders, and even parasites.In particular, the...
, projecting from a diverticulum, or "style sac".
The mouth has a chevron-shaped lip in front of it, and bears tentacles behind it, which have various shapes and layouts in different species.
The heart is divided into two equal halves, each with its own auricle, ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
and aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
. The left and right aorta fuse shortly after leaving the heart, and supply blood to the open circulatory system. There are six pairs of nephridia
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...
l excretory organs, which empty into the mantle cavity.
The nervous system has small ganglia
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
around the oesophagus from which two pairs of main nerve cords run through the body; one pair supplying the foot, and the other the visceral organs. As in the chitons, these main nerve cords are connected by a series of lateral nerves, giving the layout of the nervous system an appearance somewhat like a ladder.
There are two pairs of gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s, which release gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s into the water through one of the pairs of nephridia. The sexes are separate, and fertilisation is external.
Habitat
Monoplacophora are a geographically widespread component of the benthosBenthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
. Most are known from deep water (1800 – 6500 meters), although several species are found in shallower waters ranging up to 200 meters.
Feeding habits
It is presumed that they graze on microscopic organisms in mud or bottom detritus.Taxonomy of extant species
Order Tryblidiida- Family Laevipilinidae
- Genus Laevipilina J. H. McLean, 1979
- Species Laevipilina antarctica Warén & Hain, 1992
- Species Laevipilina cachuchensis Urgorri, García-Alvarez & Luque, 2005
- Species Laevipilina hyalina J. H. McLean, 1979
- Species Laevipilina rolani Warén & Bouchet, 1990
- Species Laevipilina theresae Schrödl, 2006
- Genus Laevipilina J. H. McLean, 1979
- Family Micropilinidae
- Genus MicropilinaMicropilinaMicropilina is a genus of monoplacophoran; in addition to a number of living, deep-sea species, it also includes a shallow water fossil from the middle Pleistocene of Italy - and as such the only fossil representative of this lineage after the Devonian....
Warén, 1989- Species Micropilina arntzi Warén & Hain, 1992
- Species Micropilina minuta Warén, 1989
- Species Micropilina rakiura Marshall, 1998
- Species Micropilina reingi Marshall, 2006
- Species Micropilina tangaroa Marshall, 1992
- Species Micropilina wareni Marshall, 2006
- Genus Micropilina
- Family Monoplacophoridae
- Genus Monoplacophorus Moskalev, Starobogatov & Filatova, 1983
- Species Monoplacophorus zenkevitchi Moskalev, Starobogatov & Filatova, 1983
- Genus Monoplacophorus Moskalev, Starobogatov & Filatova, 1983
- Family Neopilinidae
- Genus Adenopilina Starobogatov & Moskalev, 1987
- Species Adenopilina adenensis (Tebble, 1967)
- Genus NeopilinaNeopilinaNeopilina is a highly derived genus of modern monoplacophoran.However, molecular methods show that they fall within the polyplacophoran clade, and fossil and morphological data show that they are rather derived and bear very little resemblance to an 'ancestral mollusc'.- Anatomy :Its anatomy led...
H. Lemche, 1957- Species Neopilina bruuni Menzies, 1968
- Species Neopilina galatheae Lemche, 1957
- Species Neopilina rebainsi Moskalev, Starobogatov & Filatova, 1983
- Genus Rokopella Starobogatov & Moskalev, 1987
- Species Rokopella brummeri Goud & Gittenberger, 1993
- Species Rokopella capulus Marshall, 2006
- Species Rokopella euglypta (Dautzenberg & Fischer, 1897)
- Species Rokopella goesi (Warén, 1988)
- Species Rokopella oligotropha (Rokop, 1972)
- Species Rokopella segonzaci Warén & Bouchet, 2001
- Genus Veleropilina Starobogatov & Moskalev, 1987
- Species Veleropilina reticulata (Seguenza, 1876)
- Species Veleropilina veleronis (Menzies & Layton, 1963)
- Species Veleropilina zografiVeleropilina zografiVeleropilina zografi is a recent deep sea species of a monoplacophoran from Atlantic ocean.- Original description :It have been described as Acmaea zografi by Philippe Dautzenberg and H...
(Dautzenberg & Fischer, 1896)
- Genus VemaVema (genus)Vema is a genus of monoplacophoran mollusc....
(Clarke & Menzies, 1959)- Species Vema bacescui (Menzies, 1968)
- Species Vema ewingi (Clarke & Menzies, 1959)
- Species Vema hyalina
- Species Vema levinae Warén, 1996
- Species Vema occidua Marshall, 2006
- Genus Adenopilina Starobogatov & Moskalev, 1987