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Eleanor Marjorie Anne Pease 2018
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Summary | |
In 2018, a colony of small transparent ascidians was observed growing on the wall of a tank in the Goddard aquaria at the University of Queensland. This species was identified as Perophora multiclathrata, a colonial ascidian found throughout the tropics and subtropics of the world. A sessile filter-feeder, P. multiclathrata has a cryptic habit, and can undergo both sexual and asexual reproduction to form extensive colonies of hundreds of individuals. It is known to bioaccumulate vanadium, a toxic heavy metal, from the environment, for reasons which remain unclear.
Following several weeks of observation, the following webpage was constructed to describe the major features Perophora multiclathrata's anatomy, life history, ecology, distribution and evolution. A small project was also conducted into the cellular basis of vanadium accumulation.
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Physical Description | |
Perophora multiclathrata is a small, rounded, transparent ascidian (Figure 1). Individual zooids are clear to yellowish in colour, reaching 2-3mm in length. At the bottom of each zooid is a small stem, leading to a network of threadlike basal stolons. Zooids are spaced at intervals of 5-10mm along this network, forming colonies which may number up to hundreds of individuals (Figure 2) (Fukumoto 1971).
The most obvious feature of the zooids is two siphons, each rimmed by small lobes. The organs are visible through the transparent tunic, and include a basket-like pharynx with 5 rows of stigmata, an ovular stomach, and a small oval-shaped testis lying within a straight gut loop (Kott 1985) (Figure 1).
P. multiclathrata looks superficially similar to a number of other Perophora or Ecteinascidia species,and is often misidentified as P. viridis or P. bermudiensis (Goodbody 1994; Da Roche and Kremer 2005).However, it bears a number of distinguishing anatomical features. The pharynx always contains exactly 5 rows of stigmata; and a distinctive band of muscles passes both above and below the atrial siphon. This produces a characteristic pucker-shape when contracting that is not observed in other species (Goodbody 1994) (Figure 3). P. multiclathrata colonies also grow differently to colonies of P. viridis and P. sagamiensis. In these other species, stolons can fuse to form a complex, fine net; whereas in P. multiclathrata, stolon-pieces are longer and more loosely spaced (Figure 2) (Mukai et al. 1983).
Some authors (e.g. Kott 1985) have suggested that P. multiclathrata can be identified based on the 7 lobes present around each siphon. Observations suggested that this trait is variable, however; zooids were observed with 6-9 lobes, with the number sometimes even differing between the two siphons of a single individual. This suggests that this is not a useful character for identification.
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| Figure 1 |
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| Figure 2 |
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| Figure 3 |
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Ecology | |
Habitat
P. multiclathrata is a sessile epibenthic species which inhabits shallow-water environments. Colonies grow both horizontally and vertically over hard surfaces, including reef rocks (Goodbody 1994), mangrove roots (Farnsworth and Ellison 1996), and manmade pilings and harbours (Izquierdo-Munoz 2009). Compared to related species like P. bermudiensis, P. multiclathrata tends to prefer gentle slow-moving currents, favouring the undersides of rocks (Goodbody 1994) and the leeward sides of mangrove cays (Farnsworth and Ellison 1996). In Shimoda, Japan, an area where it occurs in high abundance, it has also been observed in the intertidal zone, where it appears to tolerate some tide action and exposure at low tide (Fukumoto 1971).
Biofouling
P. multiclathrata forms part of the biofouling community. It is adept at colonising manmade substrates, growing over ship-hulls, pilings in harbours, and plastic settlement plates (Izquierdo-Munoz 2009; Lezzi et al. 2018). This tendency was observed firsthand in the Goddard aquaria, where colonies grew on the plastic tank walls but were not observed on the natural-reef rock (Figure 2). As yet the literature contains no reports of the species fouling aquaculture stock, so it is not considered a serious economic pest.
Biofiltration
P. multiclathrata feeds by filtering minute food particles out from the water column. Among filter feeders, ascidians are particularly adept at extracting tiny suspended particles (Conley et al. 2018), removing pollutants and pathogens that other species fail to capture (Rosa etal. 2013). This ecosystem service has been shown to improve water quality and mitigate outbreaksof disease (Burge et al. 2016). P. multiclathrata also extracts vanadium, a toxic heavy metal, from the water (see “Vanadium accumulation”).
Interspecific interactions
It is unknown whether P. multiclathrata has any natural predators, as predation on the species has never been observed. Over the course of the semester, P. multiclathrata samples were exposed to two crab species and Pseudoceros indicus, a flatworm species known to prey on colonial tunicates (Sreeraj and Raghunathan 2013). None of these species showed any interest in the zooids. It is possible that P. multiclathrata can deter these predators using a natural chemical defense mechanism: some authors have suggested that vanadium protects against predation by making the animal unpalatable to consumers (Fattorini et al. 2012). Solid evidence for this is scarce, however (see “Vanadium accumulation”), and it is possible that natural predators of this species do exist.
Aside from predators, P. multiclathrata were observed interacting with a number ofepizootic, possibly parasitic, organisms. These included tunic-fouling algaes,small crustaceans and nematodes embedded in the tunic, and a xenacoel-like wormwhich was observed swimming through the branchial basket of one individual. It is unknown whether these represented parasitic or commensal interactions;however, they do suggest that if a chemical defense mechanism exists it is noteffective against all organisms.
Larval interactions
In the Goddard aquaria, a number of smaller colonies of P. multiclathrata were observed growing over the surface of a species of red coralline algae (Figure 4,5). Certain species of algae and other benthic organisms can induce or inhibit the settlement of ascidian larvae, providing them with an ideal substrate to settle and begin growth (Degnan and Johnson 1999). The fact that this algae seemed to be the substrate for colonies at an early stage of growth suggested that this interaction may be occurring here. Interestingly the same correlation was not observed for a colony of Ecteinascidia sp., suggesting the interaction is species- or at least genus-specific. However, larval settlement experiments would be necessary to determine whether there is indeed an interaction or whether this correlation is simply coincidental.
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| Figure 4 |
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| Figure 5 |
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Life History and Behaviour |
Behaviour | |
Contraction and pumping
Given their benthic and sessile habit, ascidian behaviour is relatively simple. The main behaviour that can be observed in P. multiclathrata is a periodic pumping. Every so often, zooids close their buccal aperture and contract their transverse muscles, forcing water out the atrial siphon. This accelerates the flow of fresh water through the body, and flushes water and waste products out of the atrium (Brusca et al. 2016). Zooids also contract when poked, presumaby as a predator-defense strategy.
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Video 1: Light microscope video of Perophora multiclathrata contracting in response to mechanical stimulation. |
Two distinct behavioural patterns were observed in P. multiclathrata. In some zooids, pumping was frequent, the zooids remained open for extended periods of time, and the body of the animal regained its original shape very quickly after being poked. The second group of zooids was much less resilient to mechanical damage, seemed to keep their siphons permanently closed, and almost never pumped. This group also seemed much more severely infested by parasites and tunic-fouling algae (Figure 6). These zooids almost invariably possessed mature gonads and egg cells, while zooids without reproductive organs almost always fell into the first group. This pattern was consistent across 17 zooids, with only one egg-brooding, pumping individual deviating from the norm. All zooids were taken from the same colony and often a pumping and non-pumping zooid would be situated immediately beside each other.
Similar behaviours have been observed in P. sagamiensis, but never in P. multiclathrata (Mukai et al. 1983). In this related species, siphons close and pumping ceases once brooding begins. Organs begin to regress during brooding, and after larvae have been released the mother zooid regresses fully back into the stolon. The observations made in this project suggest P. multiclathrata undergoes a similar process (c.f. Mukai et al. 1983).
The fact that the zooids were not synchronised in thisbehaviour, with some retaining non-reproductive pumping behaviour while adjacent zooids reproduced, suggests that the colony might be undergoing some sort of functional division of labour. In this scenario, zooids are divided into feeders and reproducers. Feeding individuals continue to collect nutrients which they transfer to their reproductive neighbours via the basal network of stolons. This allows reproducers to shut down all physiological functions and divert all their energy into reproduction. Even immune function appears to be suppressed, leaving these zooids vulnerable to the parasitic infestations observed here.
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| Figure 6 |
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Life History | |
P. multiclathrata is a hermaphroditic species which can undergo both sexual and asexual reproduction (Figure 7) (Kott 1985). After initial settlement, the animal begins to proliferate asexually. Stolons extend out from the first zooid (or "oozoid"), and lymphocytes and mesenchymous septum cells in the stolon form an endoblastic vesicle (Fukumoto 1971). This develops into a bud, and eventually a new functional zooid. Perophora colonies can consist of hundreds of functional individuals, all genetically identical to each other. New colonies can also develop from isolated pieces of stolon (Goodbody 1971), allowing the animal to regenerate and spread if broken.
Observations in this investigation suggest that Perophora multiclathrata undergo asexual reproduction for some time before sexually reproducing (Shenkar and Swalla 2011. In the first few weeks of this study, none of the zooids observed had reproductive tissue. Gonads and egg cells only appeared after a few weeks, after which point the colony had reached its maximum size and began to regress.
Like most colonial ascidians, Perophora multiclathrata broods its young and releases lecithotrophic, i.e. short-lived, larvae . After fertilisation, egg cells are transferred across the body to a brood pouch on the right side of the pharynx. Embryos are arranged serially in the brood pouch, with the most advanced at the back. When development is complete, the larva swims out an opening in the back of the brood pouch into the water column.
The ascidian larva resembles a small tadpole, and exhibits many of the features of the chordate groundplan including a hollow dorsal nerve cord (Tsagkogeorga 2009). It also possesses sophisticated sensory abilities, which allow it to quickly identify a suitable substrate for growth in the short time it can remain in the water column (Aldred and Clare 2014). Upon settlement, the larvae adheres to the substrate using its oral papillae and begins metamorphosis, resorbing its larval features and replacing them with functional adult organs (Brusca et al. 2016).
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| Figure 7 |
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Anatomy and Physiology |
Body wall and tunic | |
The tunic of P. multiclathrata is relatively simple, being clear and free of spicules or conspicuous pigments. It receives its own blood supply from an internal network of blood vessels. The body wall also contains the major musculature, which is embedded in the mantle (Brusca et al. 2016). Like most other ascidians, P. multiclathrata possesses rings of circular muscles around each of the apertures (Goodbody 1994), as well as the species-specific pattern of transverse muscles originating above and below the atrial siphon. The circular muscles serve as sphincters, controlling the flow of water entering or exiting the siphons (Brusca et al. 2016), while the transverse muscles allow the zooid to contract its entire body, pouching the atrial siphon inward (Figure 3). This action is used to facilitate pumping in feeding, as well as predator-response behaviour (see “Behaviour). The dramatic double-puckering of the atrial siphon is a feature of this species (Goodbody 1994).
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Digestive system | |
The digestive system in P. multiclathrata lies on the left-hand side of the body, alongside the branchial sac. A narrow oesophagus feeds into an oval-shaped stomach, a short duodenal area, and a relatively short intestine that forms a straight loop in the body and opens into the atrium (Kott 1985). The process of feeding follows the generalised ascidian system: water is pumped through the buccal siphon and passes through the pharynx, where suspended food particles are caught on a mucous net secreted by the endostyle. A dorsal lamina at the opposite end of the pharynx passes the mucous-food complex into the oesophagus. Waste water and faeces pass into the atrium, where they are flushed out via the atrial siphon (Figure ) (Brusca et al. 2016).
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| Figure 8 |
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Circulatory system | |
The circulatory system of P. multiclathrata consists of a tubular heart on the right side of the body, feeding a network of blood vessels throughout the tunic and viscera. Circulation is clearly visible through the clear tunic when blood cells are stained (video 2).
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Video 2: Light microscope video of blood cells flowing between a P. multiclathrata zooid and its attached stolon. The pumping heart is visible in the bottom right hand corner of the image. Cells are stained with 2,2-bipyridine, which stains vanadium-carrying blood cells purple. |
Zooids within a colony are connected by vascular tissue, and blood cells move freely from zooid to zooid along the stolons. Though the colony shares blood and nutrients, there does not seem to be any obvious coordination in heart contraction between adjacent zooids. Groups of interconnected zooids were observed beneath a microscope for some time, and never synchronised in either the rhythm or direction of the blood flow.
Ascidian circulation is unique in that the heart can regularly reverse its flow (Heron 1975). Every few minutes, the blood flow of P. multiclathrata pauses briefly, then resumes again in the opposite direction (Video 3).
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| Video 3: Light microscope video of P.multiclathrata heart, showing pauses and change in flow direction. The heart is the tubular organ at the bottom of the image. Blood cells are stained with 2,2-bipyridine, which stains vanadium-carrying cells purple. |
The cause for this is uncertain, but one possibility is that the organs in the ascidian blood circuit are arranged in a series, rather than parallel to each other (Heron 1975). The first organs in the series receive the richest blood coming directly from the heart, but by the time the blood reaches the end of the circuit most nutrients and oxygen have been depleted. Reversing the flow of blood ensures that all organs receive a steady nutrient supply.
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Reproductive system | |
The testis and ovary of P. multiclathrata lie within the gut loop on the left side of the body. Unlike the related species P. viridis and P. sagamiensis (Mukai et al. 1983), P. multiclathrata possesses an oval-shaped non-lobed testis, and the ovary is located some distance away from it along the gonoduct (Figure 7). On the opposite side of the body is a brood pouch, where eggs and larvae are incubated. This pouch can contain a maximum of 10 young; relatively few for the genus (Mukai et al. 1983). A narrow oviduct connects the brood pouch to the ovary, while the gonoduct travels up alongside the intestine so that sperm can be released into the atrium (Kott 1985).
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Nervous system | |
The nervous system and sensory structures of adult tunicates are highly reduced. At metamorphosis, the larva loses its sophisticated sensory capabilities and dorsal nerve cord, replacing it with a simple system consisting mainly of a dorsal ganglion and some mechanoreceptors (Brusca et al. 2016). In P. multiclathrata, the dorsal ganglion is known to innervate the heart and control the rate and direction of contraction (Ebara 1970). Observations also confirm that there is sensory apparatus in the tunic and neural control of the muscles, since the zooid was very sensitive to being handled and responded by contracting its entire body (Video 1). However, neither mechanical (this study) nor electrical (Ebara 1970) stimulation of the stolon seems to have any effect on the behaviour of the zooid. This suggests that neural tissue is present in the zooids only, and not their interconnecting stolons.
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Mini-project: Vanadium accumulation | |
Vanadium is a heavy metal present in low concentrations in seawater. Certain ascidians are known to accumulate vanadium in their blood cells, at concentrations of up to 350nM, or 107 times ambient levels (Michibata 1996). Vanadium has useful applications in biomedicine (e.g. Fukunaga and Bhuiyan 2012), reducing pollution (Romaidi and Ueki 2016), and for efficient storage of electricity (Doetsch and Burfeind 2016). Determining how ascidians concentrate and stabilise such high quantities of vanadium could assist in developing technologies to better exploit these benefits.
Despite this, the current literature on vanadium accumulation suffers some serious shortcomings. Efforts to quantify the amount of vanadium in ascidian tissues have produced wildly different results, partly because concentration differs between species (Swineheart et al. 1994) and times ofyear (Stacey 2009), but also due to the lack of a standard preparation method for quantification (Michibata 1993). Though some authors suggest some vanadium is stored in solid tissues, such as the mantle (Michibata et al. 1986), most now agree that it is concentrated in specialised blood cells known as “vanadocytes”;but the identity of the vanadocytes remains disputed. Vanadium is certainly stored in signet ring cells (Michibata 1996), but other authors also suggest morula cells (Pirie and Bell 1984), compartment cells (Anderson and Swineheart 1991), granular amoebocytes (Michibata 1996; Pirie and Bell 1984; Rowley 1982), vacuolar hyaline amoebocytes (Rowley 1982), and pigment cells (Anderson and Swineheart 1991) as possible vanadocytes.
At an even finer-grained level, there seems to be disagreement regarding how to distinguish these cell types from one another. Classification criteria vary and are often poorly-defined (e.g. compare Michibata 1993, Mukai et al. 1983 and Nette et al. 2004), and even with in a species the number of identified cell types can vary (e.g. George 1926 c.f. Overton 1966). Some authors have proposed that certain groups of cells represent different phases in a single developmental lineage (Endean 1960; Nette et al. 1998), but disagree on which are related to each other and their order of development.
Perophora multiclathrata is an ideal species in which to clarify some of these issues.Members of the family Perophoridae are known to accumulate vanadium in the +3 oxidation state (Overton 1966; Hawkins et al. 1983); and the small size and transparent tunic of P. multiclathrata allows vanadium localisation to be viewed throughout the whole body at once. The species is also interesting because it has been suggested to lack morula cells (Mukai et al. 1983). This would render it unique, since morula cells typically account for about 30% of an ascidian’s haemocytes (Nette et al. 2004), and are the terminal cell in the vanadocyte lineage proposed by Endean (1960).
Methods
In this investigation, whole P. multiclathrata zooids were stained with 2,2-bipyridine, which stains purple in the presence of vanadium. Zooids were viewed under adissecting microscope, then their tunics were pierced and samples of their blood extracted so that vanadium localisation could be viewed at both a whole-body and cellular level. The same process was conducted for a species of Ecteinascidia, the sister-genus to Perophora which also collects vanadium, to compare differences in the two species.
Vanadium at a whole-body level
Staining with bipyridine successfully confirmed the presenceof vanadium in both Perophora multiclathrata and Ecteinascidia sp. In both species, no stain was observed on any tissues or organs, apart from the blood cells. This supports the current consensus that vanadium is only stored in mobile blood-borne vanadocytes. Also consistent with the literature, only a subset of cells took up the stain (Figure 9). This suggests that vanadium-storage is a specialised function not undertaken by all cells. These vanadocytes seemed to congregate in three specific regions of the animal: in rings around the atrial and buccal siphons, and around the oesophagus near its junction with the stomach (Figure 10).
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| Video 4: Light microscope video of 2,2-biypridine stained cells circulating through the body of live P. multiclathrata. Vanadium-containing cells are visible as the moving dark dots. |
The cause of these patterns cannot be precisely determined from this study. The patterns did seem to arise several minutes after the stain was applied, so one possibility is that these accumulations are simply cells that have become engorged with bipyridine becoming lodged in narrow vessels.Alternatively, the delayed appearance may be a result of the slow action of the stain, as it often took several minutes for all cells to change colour. This would suggest that the patterns do represent natural accumulations. They might imply an antimicrobial function for vanadium, as suggested by Stoecker (1978): the rings around the siphons deter bacteria entering from the external environment, while the accumulation at the oesophagus exterminates harmful bacteria before they enter the gut. Why this function could not simply be undertaken by digestive chemicals in the stomach, however, and whether the thin rings of vanadium would even be effective at deterring bacterial entry, requires further investigation.
Another suggested function of vanadium is as chemical antipredator defense (Stoeker 1980). Certain polychaete worms are known to accumulate vanadium in their lophophores to protect these exposed tissues from predators (Fattorini and Regoli 2012). If this were the case in tunicates, we would expect to see accumulations of vanadium within the tunic. This pattern was not observed here, providing little support for the chemical-defense hypothesis.
Vanadium at a cellular level
Cells types were identified using the criteria described by Endean (1960; criteria are summarised in Figure 11). Cell sections revealed the presence of 5 different cell types for P. multiclathrata and 6 for Ecteinascidia sp. Common cell types were signet ring cells, morula cells and compartment cells (Figures 12-15). P. multiclathrata also possessed pigment cells (Figure 12a) and pluri-vacuolated cells (Figure 13d), while Ecteinascidia sp. possessed phagocytes,lymphocytes and vacuolated amoebocytes (Figure 14b). No amoebocytes of any type were observed in P. multiclathrata.
As expected, signet ring cells stained positive for vanadium(Figure 12a, 13a, 14a, 15a). Other vanadocytes identified were pluri-vacuolated cells (Figure 13d) and compartment cells (Figure 12c, 12d, 14c and 14d). Only some compartment cells appeared to take up the stain; the majority remained colourless (Figure 12a,c,e,f, 13a, 13c, 15b). This supports Nette et al. (2004)’s suggestion that compartment cells are divided into two types: a classical type which does not store vanadium, and type II which does.
Some of the so-called “compartment” cells identified appeared very similar to signet ring cells, staining purple but containing multiple vacuoles instead of one large one (e.g. 12e, 14b, 15d). These may represent transitional forms, and lend support to the idea that compartment cells and signet ring cells form part of the same developmental lineage.
Under certain light conditions, there also appeared to be a faint region of purple within the morula cells (Fig. 13b, 15a, 15b). This might suggest that these cells contain some vanadium, as suggested by several authors (e.g. Pirie and Bell 1984; Swineheart 1991). However, the colouration is paler than that of fully-stained vanadocytes (e.g. compare the cells in Figure 4a), and wasonly visible under certain optical conditions (compare Figures 12, 14 with Figures 13, 15). As such this probably represents an optical effect, and there is no evidence that morula cells contain any vanadium.
Morula cells were observed in P. multiclathrata (Figure 12a-d; Figure 13b-d); in fact, they were noticeably more abundant than in Ecteinascidia. Mukai et al.’s (1983) claim that morula cells are absent in this species is probably due to different identification criteria. According to Endean’s (1960)system, morula cells are the cells which appear green under light microscopy(e.g. Figure 13 and 15). Mukai et al., however, defines “green cells” as an entirely separate cell type. Presumably the cells they identify as “morula cells” represent a different type again. This highlights the need for a standard system of cell classification. Much of the debate over which cells contain vanadium likely arises because there is no consensus on how to identifyeach cell type.
As well as this, the current classification system probably needs to be extended. Several cell types were identified that did not fiteasily within Endean’s criteria. Many of these appeared to be type I compartment cells, but possessed much smaller vacuoles and larger regions of empty cytoplasm than the typical compartment cell (e.g. Figure 12d, 12f; compare with 12c,12e, 13c, and compartment cells identified in Nette et al. 2004). These may represent a new cell type, or suggest that compartment cells have much more variability than is captured by the current classification system.
Additionally, the assemblage of cells varied considerably between the two species. Perophora multiclathrata had considerably more signet ring and morula cells, while Ecteinascidia possessed more amoebocytes and type I compartment cells, particularly those with the unusual morphology described above. This suggests that future classification systems should not only get a fuller inventory of all cell types, but should examine the variability across multiple species.
Conclusions
In summary, the investigation showed that vanadium is present in the signet ring, pluri-vacuolated, and type II compartment cells of Perophora multiclathrata and Ecteinascidia sp. These cells tend to concentrate around the oesophagus and siphons, though many remain mobile in the blood. In contrast with the claims made by Mukai et al. (1983), P. multiclathrata clearly possesses morula cells. This calls for more standardised and detailed systems of ascidian blood cell classification, which account for variability among cell morphologies and between different ascidian species.
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| Figure 9 |
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| Figure 10 |
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| Figure 11 |
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| Figure 12 |
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| Figure 13 |
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| Figure 14 |
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| Figure 15 |
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Biogeographic Distribution | |
P. multiclathrata is a pan-tropical species, spread throughout shallow waters off the tropical and subtropical coasts of all major continents and many of the Pacific Islands (Figure 16) (Akram et al. 2017; Da Rocha and Kremer 2005). Populations are also known from the Mediterranean, though these are believed to be invasive and only recently-established (Izquierdo-Munoz et al. 2009; Marchini et al. 2015; Lezzi et al. 2018). This range is relatively typical among colonial ascidians, which tend to predominate in tropical environments (Shenkar and Swalla 2011).
It is unclear which of these regions P. multiclathrata is native to, as the species' cryptic habit makes it difficult to identify and most populations have only been identified in the last 20 years (Da Rocha et al. 2013). It is thus regarded as "cryptogenic" in origin, and much of its range may represent recent invasions (Da Rocha and Kremer 2005).
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| Figure 16 |
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Evolution and Systematics | |
Phylogeny
The scarce fossil record available for P. multiclathrata, like most ascidian taxa (Brookfield 1988), makes their evolutionary history somewhat obscure. Despite this, the species' phylogeny has been reliably confirmed using both morphological (Kott 1985) and genetic (Tsagkogeorga et al. 2009) data (Figure 17).
| Systematics |
| Phylum |
Chordata |
| Subphylum |
Urochordata |
| Class |
Ascidiacea |
| Order |
Enterogona |
| Suborder |
Phlebobranchia |
| Family |
Perophoridae |
| Genus |
Perophora |
| Species |
Perophora multiclathrata (Sluiter 1904) |
| Synonyms |
| Ecteinascidia euphues (Sluiter 1904)
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| Ecteinascidia multiclathrata (Sluiter 1904)
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| Perophora formosana (Oka 1931)
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| Ecteinascidia formosana (Oka 1931)
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| Perophora orientalis (Ärnbäck 1935)
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Perophora africana (Miller 1953)
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Relationships with Ecteinascidia
Traditionally, the family Perophoridae has been split into two genera. This split was based on morphological differences: perophorans tend to be under 5mm in length, with 4-5 rows of stigmata and a horizontal intestine, whereas ecteinascidians are larger with over 11 rows of stigmata and an intestine that angles sharply upwards (Rocha et al. 2012). Recent genetic data has disputed this view, suggesting that some ecteinascidians actually nest within Perophora (Tsagkogeorga et al. 2009).
Observations from this study support the earlier, morphological view. Specimens of Ecteinascidia sp. were also growing in the Goddard aquaria at the time of the study, and comparison between these and P. multiclathrata confirmed that the two clearly vary in their diagnostic features (Figure 18 and 19). Such obvious morphological difference suggests that there is a phylogenetic division between the two taxa. More detailed genetic studies will be needed to confirm this, however.
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| Figure 17 |
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| Figure 18 |
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| Figure 19 |
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Conservation and Threats | |
Conservation status
The conservation status of P. multiclathrata has not currently been assessed (IUCN, 2017); however, it is unlikely that it is facing serious ecological threat. Conversely, in fact, the range P. multiclathrata seems to be increasing (see “Geographical Distribution”).
Invasive potential
Like many other biofouling ascidians, P. multiclathrata is known to have a high invasive potential. Its ability to colonise artificial surfaces allows it to be easily-transported by human boat traffic and thrive in human-modified environments far out of its native range (Lezzi et al. 2018). P. multiclathrata is already known to have invaded the Mediterranean, which it likely entered via the Strait of Gibraltar by attaching to ship-hulls (Izquierdo-Munoz et al. 2009). Introductions such as this can be particularly damaging to native communities, causing the displacement of native organisms and drastic shifts in species interactions and community structure (Lezzi etal. 2018). In fact, along with anthropogenic habitat modification, invasive species like P. multiclathrata are considered the most significant drivers of coastal biodiversity loss (Katsanevakis 2014). To address this problem, management strategies should target the problem of biofouling specifically. Most transport of P. multiclathrata is facilitated by attachment to ship-hulls and other artificial surfaces; thus, improving systems for treating these surfaces against biofouling should help curb the spread of P. multiclathrata and other marine invaders (Aldred and Clare 2014).
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References | |
References
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Aldred, N. and Clare, A.S. (2014). Mini-review: Impact and dynamics of surface fouling by solitary and compound ascidians. Biofouling: The Journal of Bioadhesion and Biofilm Research 30(3), 259-270.
Anderson, D.H. and Swineheart, J.H. (1991). The distributionof vanadium and sulfur in the blood cells, and the nature of vanadium in theblood cells and plasma of the ascidian, Ascidiaceratodes. Comparative Biochemistryand Physiology 99A(4), 585-592.
Brookfield, M.E. (1988). Where are all the fossil sea squirts? Micropalaeontology 34, 277-283.
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