Molgula, or sea grapes, are very common, globular, individual marine tunicates 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 Molgula Forbes, 1848 on 2012-02-26 Molgula are a genus of the class ascidians, 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 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 feeders 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 subtidally, 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 and calcite, and an apicomplexan symbiont called Nephromyces. 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 and can self-fertilize such as Molgula pacifica which are brooded. They can be viviparous like Molgula citrina, from which a larva will pop out, or can have an oviparous 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 ascidians as they can have a tailed tadpoles larvae like most ascidians or an anural larvae. The tailless larvae have reduced characteristics like notochord 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, Molgula range from the Arctic to North Carolina, 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