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