''Molgula'', or sea grapes, are very common, globular, individual marine

tunicate Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

s roughly the size of grapes.Sanamyan, K.; Monniot, C. (2012). Molgula Forbes, 1848. In: Noa Shenkar, Arjan Gittenberger, Gretchen Lambert, Marc Rius, Rosana Moreira Da Rocha, Billie J. Swalla, Xavier Turon (2012) Ascidiacea World Database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=103509 on 2012-02-26 ''Molgula'' are a genus of the class

ascidians Ascidiacea, commonly known as the ascidians or sea squirts, is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer test or "tunic" made of the polysaccharid ...

, having many species sized from 20–50 mm and that has a life cycle with a tailed tadpole stage or without a tailed tadpole stage. ''Molgula'' are

sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...

invertebrates that grows on substrates and are always found existing alone. There are over one hundred species of molgulids with a vast majority of them in the Northern and Southern parts of the world with a few along the equator. All species of ''Molgula'' are

suspension feeder Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a spec ...

s and will have the highest population in areas with high amounts of small particles to feed on.

Morphology and development

They are translucent with two protruding siphons, oral and atrial consisting of six and four lobes respectively. They are found

subtidal The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminate ...

ly, attached to slow-moving submerged objects or organisms. All species of ''Molgula'' have a fluid-filled structure called the renal sac. The renal sac contains nitrogenous wastes, solid concretions composed of

weddellite Weddellite ( Ca C2 O4·2 H2O) is a mineral form of calcium oxalate named for occurrences of millimeter-sized crystals found in bottom sediments of the Weddell Sea, off Antarctica. Occasionally, weddellite partially dehydrates to whewellite, formin ...

and

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, and an

apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...

n symbiont called ''

Nephromyces ''Nephromyces'' is a genus of apicomplexans that are symbionts of the ascidian genus '' Molgula'' (sea grapes). Systematics ''Nephromyces'' was first described in 1888 by Alfred Mathieu Giard as a chytrid fungus, because of its filamentous ...

.'' To further expand on the animals structures, they contain a layer of tunic and given the slight variations existing within the species in terms of forms, they have evolved different bodily and structure colorations as well. For instance in the organisms classified as ''Molgula manhattensis'', the majority have adopted shades in between light yellow and green as depicted by the colored images on the right, with a few being a dusty brown color as well. However, when comparing different species, contrast in colors can be observed. Furthermore, to delve into the development of the genus ''Molgula'' has a wide range of variations among in different species. Many of them are

hermaphroditic A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

and can self-fertilize such as ''Molgula pacifica'' which are brooded. They can be

viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...

like ''Molgula citrina'', from which a larva will pop out, or can have an

oviparous Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

egg like ''Molgula oculata''. The way they hatch from the egg can vary such as digestion of the egg membrane or breaking out of it. ''Molgula'' are unique

as they can have a tailed tadpoles larvae like most

or an anural larvae. The tailless larvae have reduced characteristics like

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

and tail muscle cells. ''Molgula'' have holoblastic cleavage which is present In the species ''M. pacifica''. ''Molgula pacifica'' lacks the tadpole stage in its life cycle meaning it has anural development. Most ascidians undergo urodele development meaning that they have a tailed larval stage. The oocytes and fertilized eggs lack a perivitelline space and test cells that differentiate them with those of the urodele species. Embryos are also similar to urodele ascidians in that they have a similar cleavage pattern and also begin gastrulation at the vegetal pole. There is more modification in cell shape and movement during gastrulation. The muscle cells were absent in the posterior region which is the reason for the lack of the larval tail. The ''M. pacifica'' don't have acetylcholinesterase activity on the other hand the urodele species show a high level of acetylcholinesterase activity in the tail muscles. The developmental changes such as modifying the gastrulation and muscle development lead to the elimination of tadpole stage.

Distribution and habitats

In the western

Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

, ''Molgula'' range from the Arctic to

North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...

, to the center of the United States Eastern Seaboard. The genus ''Molgula'' has a wide distribution with ''Molgula kolaensis'' being found in the Arctic and the majority being found in pacific or Atlantic waters. ''Molgula'' that do not have a tadpole stage and have indirect development are located mostly in northern parts of bodies of water. ''Molgula'' can inhabit sandy environments to which they are unattached or attached; they also attach to hard rock surfaces. The depth at where they are found varies as the ''Molgua pugetiensis'' is found at 15 – 30 meters and ''Molgula pacifica'' can be found at 4 meters deep. The species ''Molgula manhattensis'' and ''Molgula ampulloides'' can be found along shallow water and shorelines where tides can change the water levels.

Evolutionary changes

In ''Molgula occulata'' there is an evolutionary loss of sensory organ associated melanocytes known as melanogenesis. Melanocytes are cells that produce pigments. The Tyrosinase family genes are crucial for melanogenesis. Members of ''Molgula'' occulta have unpigmented and tailless larvae on the other hand members of ''Molgula oculata'' have tails that are pigmented. It is found that the ability for melanogenesis comes down to the ability of the Tyrosinase genes to encode for functional proteins. There is a strong correlation between the pseudogenization of the Tyrosinase genes and the absence of pigmentation in the ''Molgula''. Independent mutations in two of the Tyrosinase family genes which causes them to make inactive enzymes. These inactive enzymes are the reason for the loss of pigmentation even though it has the precursors required. The tail loss has been present in 20 tunicate species of the family Mogulidae and has occurred independently.

Feeding behaviour

As briefly mentioned in the lead, in addition to ''Molgula'' being suspension feeders they have a restricted diet consisting of detritus and phytoplankton. Two important components of their morphology support this behaviour allowing them to gain nutrients in an optimal way: atrial and oral siphons. Their feeding involves using bodily structures such as gills to add an element of purification of the food particles consumed while being suspended in marine environments. Moreover, ''Molgula'' have the structures of siphons to serve the purpose of aiding in both feeding and refuse removal through oral and atrial siphons respectively. Finally, in order to assist in easier breakdown of the food particles, the organisms eaten will be coated in a layer of a gelatinous mixture.

Environmental influences

Deciding how and in what form to spend one’s life is an important decision for all organisms however for ''Molgula'' it is of particular importance as it could heavily impact many of their physiological functions. ''Molgula'' choose to spend their life as sessile organisms increasing their sensitivity to particular aspects of life compared to other organisms. Additionally, as sessile organisms ''Molgula'' can pick what substrate to attach to and sometimes they might choose an organism such as oysters as done by ''Molgula manhattensis.'' Ultimately, ''Molgula'' has a set of optimal environmental conditions under which it will be able to maximize its strengths and efforts and at which its internal processes will perform with the greatest efficiency. To exemplify, a maximum of 35% salinity should be present in the waters in which ''Molgula'' are residing with a minimum of 10% salinity. However, to note exceptions, certain characteristics do not have set minimum or maximum values and can fluctuate between the different levels such as temperature while still maintaining optimization.

Endosymbiotic relationships

''Nephromyces'' is an organism that is fungus-like that is found in the renal sac of ''Molgula manhattensis'' and some of the other molgula tunicates, they both have a symbiotic relationship. The ''Nephromyces'' is obtained from the water surrounding the ''Molgula.'' When the host dies the Nephromyces is released into the water then it can survive in the surrounding water for at least 29 days in which it can infect another ''Molgula''. The ''Nephromyces'' doesn’t need a constant host as it can survive in the environment before finding a host. The role of whether the ''Nephromyces'' is symbiotic, harmless or parasitic is not known yet.

Species

* '' Molgula aidae'' Oka, 1914 * '' Molgula amesophleba'' (Codreanu & Mack-Fira, 1956) * '' Molgula antiborealis'' Millar, 1967 * '' Molgula appendiculata'' Heller, 1877 * '' Molgula arenata'' Stimpson, 1852 * '' Molgula bacca'' (Herdman, 1910) * '' Molgula bathybia'' (Hartmeyer, 1912) * '' Molgula bisinus'' Monniot, 1989 * '' Molgula bleizi'' (Lacaze-Duthiers, 1877) * '' Molgula bourbonis'' Monniot, 1994 * '' Molgula braziliensis'' Millar, 1958 * '' Molgula brieni'' Monniot & Monniot, 1976 * '' Molgula calvata'' Sluiter, 1904 * '' Molgula caminae'' Monniot C. & Monniot F., 1988 * '' Molgula celata'' (Michaelsen, 1914) * '' Molgula celebensis'' Millar, 1975 * '' Molgula celtica'' Monniot, C., 1970 * '' Molgula citrina'' Alder & Hancock, 1848 * '' Molgula coactilis'' Monniot & Monniot, 1977 * '' Molgula complanata'' Alder & Hancock, 1870 * '' Molgula conchata'' Sluiter, 1898 * '' Molgula confluxa'' (Sluiter, 1912) * '' Molgula contorta'' Sluiter, 1898 * '' Molgula cooperi'' (Huntsman, 1912) * '' Molgula crinita'' Sluiter, 1904 * '' Molgula crustosa'' Monniot C. & Monniot F., 1988 * '' Molgula cryptica'' Millar, 1962 * '' Molgula cynthiaeformis'' Hartmeyer, 1903 * '' Molgula davidi'' Monniot, 1972 * '' Molgula delicata'' Monniot & Monniot, 1991 * '' Molgula dextrocarpa'' Monniot C. & Monniot F., 1974 * '' Molgula diaguita'' Monniot & Andrade, 1983 * '' Molgula dicosta'' Millar, 1988 * '' Molgula dione'' (Savigny, 1816) * '' Molgula discogona'' Millar, 1975 * '' Molgula diversa'' Kott, 1972 * '' Molgula dolichentera'' Millar, 1960 * '' Molgula ellistoni'' Kott, 1972 * '' Molgula elva'' Kott, 2008 * '' Molgula enodis'' (Sluiter, 1912) * '' Molgula eobia'' Redikorzev, 1941 * '' Molgula estadosi'' Monniot & Monniot, 1983 * '' Molgula eugyroides'' Traustedt, 1883 * '' Molgula euplicata'' Herdman, 1923 * '' Molgula euprocta'' Drasche, 1884 * '' Molgula falsensis'' Millar, 1955 * '' Molgula ficus'' (Macdonald, 1859) * '' Molgula flagrifera'' Sluiter, 1904 * '' Molgula fortuita'' Monniot & Monniot, 1984 * '' Molgula georgiana'' Michaelsen, 1922 * '' Molgula gigantea'' (Cunningham, 1871) * '' Molgula griffithsii'' (MacLeay, 1825) * '' Molgula habanensis'' Van Name, 1945 * '' Molgula hartmeyeri'' Oka, 1914 * '' Molgula helleri'' Drasche, 1884 * '' Molgula herdmani'' Brewin, 1958 * '' Molgula hirta'' Monniot F. 1965 * '' Molgula hodgsoni'' Herdman, 1910 * '' Molgula hozawai'' Oka, 1932 * '' Molgula impura'' Heller, 1877 * '' Molgula incidata'' Kott, 1985 * '' Molgula japonica'' Hartmeyer, 1906 * '' Molgula karubari'' Monniot & Monniot, 2003 * '' Molgula kerguelenensis'' Kott, 1954 * '' Molgula kiaeri'' Hartmeyer, 1901 * '' Molgula kolaensis'' Ärnbäck-Christie-Linde, 1928 * '' Molgula kophameli'' Michaelsen, 1900 * '' Molgula lapidifera'' Redikorzev, 1941 * '' Molgula longipedata'' Sluiter, 1904 * '' Molgula longitubis'' Monniot, 2002 * '' Molgula longivascula'' Millar, 1982 * '' Molgula lutulenta'' Herdman, 1923 * '' Molgula macquariensis'' Kott, 1954 * '' Molgula malvinensis'' Arnback, 1938 * '' Molgula manhattensis'' (De Kay, 1843) * '' Molgula marioni'' Millar, 1960 * '' Molgula millari'' Kott, 1971 * '' Molgula mira'' (Ärnbäck, 1931) * '' Molgula mollis'' Herdman, 1899 * '' Molgula monodi'' Peres, 1949 * '' Molgula mortenseni'' (Michaelsen, 1922) * '' Molgula napiformis'' Lambert, 1993 * '' Molgula novaeselandiae'' (Michaelsen, 1912) * '' Molgula occidentalis'' Traustedt, 1883 * '' Molgula occulta'' Kupffer, 1875 * '' Molgula oculata'' Forbes, 1848 * '' Molgula oligostriata'' Tokioka, 1949 * '' Molgula oregonia'' Ritter, 1913 * '' Molgula pacifica'' (Huntsman, 1912) * '' Molgula pedunculata'' Herdman, 1881 * '' Molgula phytophila'' Monniot, 1970 * '' Molgula pigafettae'' Monniot & Monniot, 1983 * '' Molgula pila'' Monniot & Monniot, 1985 * '' Molgula plana'' Monniot, C., 1971 * '' Molgula platana'' Van Name, 1945 * '' Molgula platei'' Hartmeyer, 1914 * '' Molgula platybranchia'' (Monniot, 1970) * '' Molgula primitiva'' Redikorzev, 1941 * '' Molgula provisionalis'' Van Name, 1945 * '' Molgula pugetiensis'' Herdman, 1898 * '' Molgula pulchra'' Michaelsen, 1900 * '' Molgula pumila'' Monniot F. & Monniot C., 1976 * '' Molgula pyriformis'' Herdman, 1881 * '' Molgula redikorzevi'' Oka, 1914 * '' Molgula regularis'' Ritter, 1907 * '' Molgula retortiformis'' Verrill, 1871 * '' Molgula rheophila'' (Pérès, 1956) * '' Molgula riddlei'' F. Monniot, 2011 * '' Molgula ridgewayi'' (Herdman, 1906) * '' Molgula rima'' Kott, 1972 * '' Molgula robini'' Monniot & Monniot, 1983 * '' Molgula robusta'' (Van Name, 1912) * '' Molgula romeri'' Hartmeyer, 1903 * '' Molgula rotunda'' Oka, 1914 * '' Molgula roulei'' Monniot C., 1969 * '' Molgula sabulosa'' (Quoy & Gaimard, 1834) * '' Molgula salvadori'' Monniot, 1970 * '' Molgula satyrus'' Monniot C. & Monniot F., 1993 * '' Molgula scutata'' Millar, 1955 * '' Molgula setigera'' Arnback, 1938 * '' Molgula shimodensis'' Nishikawa, 1982 * '' Molgula simplex'' Alder & Hancock, 1870 * '' Molgula siphonalis'' Kiaer, 1896 * '' Molgula siphonata'' Alder, 1850 * '' Molgula sluiteri'' (Michaelsen, 1922) * '' Molgula socialis'' Alder, 1863 * '' Molgula solenata'' (Lacaze-Duthiers, 1877) * '' Molgula somaliensis'' Millar, 1988 * '' Molgula sphaera'' Kott, 1972 * '' Molgula spiralis'' Kott, 1954 * '' Molgula susana'' Monniot & Monniot, 1976 * '' Molgula tagi'' Michaelsen, 1923 * '' Molgula taprobane'' Herdman, 1906 * '' Molgula tectiformis'' Nishikawa, 1991 * '' Molgula tethys'' Monniot F. & Monniot C., 1974 * '' Molgula topata'' Monniot & Monniot, 1987 * '' Molgula tubifera'' (Orstedt, 1844) * '' Molgula tzetlini'' Sanamyan, 1993 * '' Molgula undulata'' (Tokioka, 1949) * '' Molgula vara'' Monniot C. & Monniot F., 1979 * '' Molgula variazizi'' Monniot, 1978 * '' Molgula verrilli'' (Van Name, 1912) * '' Molgula verrucifera'' Ritter & Forsyth, 1917 * '' Molgula xenophora'' Oka, 1914