Aiptasia occur in two colors – white or a darker, orange brown – depending on the type and density of symbionts present, as illustrated in Figure 1 (Kaplan, 1982; Stephenson and Stephenson, 1959). The spectrum of coloration corresponds to the levels of light exposure organisms are exposed to. Organisms from well-lit habitats with clean water are typically of the brown variety as this colour is caused by zooxanthellae lining the column and coelenteron of tentacles (see Figure 2). Contrastingly, species or organisms with a lighter colouration typically live in areas of low to no light (Garrett et al., 2013). Species of this genus are relatively thin-walled (Figure 2), lending to a translucent appearance regardless of coloration which has them nicknamed ‘glass anemones’ (Fautin and Fitt, 1991; Chen et al., 2008).

The column of Aiptasia polyps, both adult and juvenile, exhibit longitudinal markers of a lighter colour. Additionally, lighter flecks of a green or white colour are commonly observed along the base of tentacles. These flecks are much more common in juvenile organisms than adult specimens. Indeed, juvenile Aiptasia may be covered completely with a 'spotted pattern' (Kaplan, 1999).

Compared to archetypal Anthozoa polyps, Actinaria polyps are relatively larger, between 5cm to 8cm. Adult Aiptasia are smaller than typical sea anemone polyps, ranging in size from a few centimeters up to 5cm (Kaplan, 1982). However, anemones are capable of dramatically changing their size by inflating and deflating. Deflation typically occurs in response to a stressor when the contraction of muscles ejects coelenteric fluid. Inflation occurs either via the uptake of seawater from surroundings or pumping of siphonoglyphs (Ruppert, Barnes and Fox, 2004). As such, size should not be used as the only parameter when identifying sea anemones .

As with typical members of Actinaria, sea anemones from the genus Aiptasia exhibit biradial symmetry. Aiptasia present as single polyps, with a cylindrical column which terminates in a pedal disc on the aboral basis. This pedal disc is adhesive, serving as a point of attachment to substrate. The other end of the cylindrical stalk, the oral end, terminates in an oral disc surrounded by a crown of tentacles (Ruppert, Barnes and Fox, 2004). The oral disc is typically up to 2cm in diameter, this small size contributes to distinct parallel lines of the stalk. Unlike other anemones, Aiptasia columns are perfectly straight and don’t flare out at the oral end (Kaplan, 1999). As such, tentacles bud laterally from the oral disc. These tentacles are larger and fewer compared to other genera within Actinaria. The length of these tentacles varies within organisms, alternating between short and long in a whorl shape (see Figure 3). Additionally, these slender tentacles are elongated, ending in distinct pointed tips (Kaplan, 1982). Central on the oral disc is the manubrium, an elevated slit-shaped mouth, which is expanded on each end. These expansions are the ends of siphonoglyphs, ciliated grooves aligned vertically with the body which confer biradial symmetry (Ruppert, Barnes and Fox, 2004).

Aiptasia are typically located in protected waters: shallow waters along coastlines, rocky intertidal shores, and mangrove forests (Kaplan, 1982). This indicates a preferred benthic habitat with protection from high energy wave action. Indeed, this preference has also been observed in Aiptasia species found in deeper, open waters with strong tidal action (Foo et al., 2020). Despite the tidal action in such locations, wave action is limited. Waves occurring along water-water interfaces are internally generated, based on the movement of water of different densities. However, in the deep water locations where Aiptasia have been found, temperature and salinity differences between layers of the water is limited. As such, the pycnocline is minimized and internal waves are not so easily generated (Bona and Kalisch, 2000). Ultimately, whilst Aiptasia was traditionally considered a shallow-water species located in intertidal or littoral regions, this may not be the case (Grajales and Rodriguez, 2014). Despite this discrepancy, all species of Aiptasia have been observed to live in relatively protected waters with limited wave action in tropical and temperate regions (Chen et al., 2008).

Furthermore, typical of sea anemones Aiptasia are benthic and attach to rocks and hard substrates (Kaplan, 1982). These sea anemones often form dense colonies and continuous sheets of polyps, creating the illusion of solid patches (Stephenson and Stephenson, 1952). This has been observed in both natural and artificial habitats.

Aiptasia specimens were observed in aquariums in the University of Queensland, monitored and cultivated by Chris Challen. In these aquariums, organisms were attached to substrate, rocks, corals, and the walls of the tanks themselves. These specimens exhibited the ability to grow on both rough and smooth surfaces, unhindered by algal competition. Indeed, some smaller specimens were observed to be growing on top of corals in the show tanks, presumably they were only small as the tank is regularly cleared of Aiptasia. Additionally, Aisptasia sp. were present in tanks with a variety of light sources, demonstrating their ability to grow and flourish in a wide range of environments. This is reflected in the natural environment where Aiptasia have been observed to grow in a range of high and low light environments (Weis, 1991). Additionally, specimens were observed within cracks of hard substrate. Borruat (2014) presents a labelled image of such a specimen in the collated work describing ‘Aiptasia sp.’ uploaded to the ‘Invertebrates of the Coral Sea’ website, an initiative of the University of Queensland.

Aiptasia species are typically sessile, however, locomotion has been observed in some instances. Aiptasia locomotion is typical of sea anemones, occurring in two stages, firstly, contractions of circular muscles push the pedal disc forward. Subsequent contraction of longitudinal muscles pulls the oral disc behind, thus resulting in a ‘shuffling’ movement (Ruppert, Barnes and Fox, 2004; Robson, 1976).

Since locomotion of this species is typical and sea anemone movement is well documented, this page will focus on new research exploring photo-movement of Aiptasia. Research conducted by Bedgood et al (2020) observed three increased movement behaviours in Aiptasia associated with symbionts in response to nutrient availability. In response to starvation, anemones increased both crawling and detachment movement patterns in order to relocate, presumably in search of food. Additionally, decreasing symbiont density using thermo-shocking techniques, coupled with starvation, resulted in an increase in rates of asexual reproduction. This increase in asexual reproduction led to high production of motile clones (Bedgood et al., 2020). This study supports current understandings of starvation triggering increased movement in sea anemones, as well as implicating symbionts in locomotion.

Foo et al (2020) further observed that aposymbiotic Aiptasia, whether it was due to bleaching or naturally occurring, do not respond to light in the same way. Indeed, Aiptasia with symbionts responded to white light, bending and moving towards the light. Contrastingly, aposymbiotic Aiptasia appeared to move randomly without direction. These observations reveal a relationship between symbionts and host sea anemones in phototaxis and phototropism, with symbionts being required for directed movement and the symbiosis relationship influencing the extent of movement (Foo et al., 2020). This study presents an interesting question for future research into both corals and anemones, do symbionts guide the anemone to light sources or have hosts evolved to move in response to light when symbionts are present? It is possible that symbionts sense the light and thus direct the anemone to move in order to optimize photosynthesis and growth by increasing light absorbance. Alternatively, the anemones themselves may be sensing the light but only when populated by symbionts.

Ultimately, whilst sea anemone locomotion is well characterized, further research is required into reasons for movement beyond starvation. More specifically, what roles do symbionts play in locomotion of symbiont-associated cnidarians.

Feeding mechanisms are widely studied in cnidarians. As such, this page will only briefly summarize feeding modes of Aiptasia and focus on more interesting aspects of this genus. Further information on feeding in this genus can be found in Borruat’s (2014) factsheet on ‘Aiptasia sp.’ uploaded to the ‘Invertebrates of the Coral Sea’ website.

Aiptasia exhibit high levels of plasticity regarding their feeding modes, engaging in both autotrophic and heterotrophic feeding (Leal et al., 2014). By exhibiting a symbiotic relationship with zooxanthellae, as explored in ‘Anatomy and Physiology’, Aiptasia are able to use carbon and other nutrients produced by their symbionts. These nutrients are supplemented by passive suspension feeding and the capture of various planktonic prey using cnidocytes (Bedgood et al., 2020). However, whilst autotrophic feeding occurs year-round in sea anemones, heterotrophic feeding is variable on both geographic and temporal scales (Bedgood et al., 2020; Chintiroglou and Koukouras, 1992). Indeed, heterotrophic feeding appears to be seasonal, potentially due to a dependence on autotrophy from symbiont relationships that may be strained in times of lowered light, such as winter (Chintiroglou and Koukouras, 1992).

The phylum Cnidaria is defined by their cnidocytes, specialized sensory-effector cells used in venom-delivery (Jouiaei et al., 2015). Compared to other orders of Anthozoa, the Actinaria exhibit a wider variety of defensive mechanisms and engage in such behaviour more commonly, presumably due to their mobility (Edmunds et al., 1976). Cnidia, organelles within cnidocytes, define the type of cnidocyte and are divided into three categories: nematocysts, spirocysts, and ptychocysts which are exclusively found in cerianthids (Sunangar et al., 2018). Cnidocytes containing nematocysts are nematocytes whilst those containing spirocysts are spirocytes. Aiptasia possess both nematocysts, common dart-shaped cnidae with hollow tubules and spines used to pierce and deliver venom, and spirocysts which are elastic cnidae unique to anthozoans and used for prey capture (Sunangar et al., 2018; La Spada, Marino and Sorrenti, 2001).

Cnidocytes in Aiptasia are used in defensive, feeding and attachment behaviours. Actonia, white threads bearing nematocytes, are expelled by Aiptasia through the mouth and cinclides (openings on the column) when threatened (La Spada, Marino and Sorrenti, 2001). These nematocytes are hollow cells bearing coiled filaments with spines which evert upon stimulation, thus discharging the barbed filament which is loaded with venom. Nematocytes in Aiptasia have been implicated only in defensive and aggressive behaviours, such as competition (Marí and Tytgat, 2010). Contrastingly, spirocytes function in both prey capture and substrate adhesion. When feeding or upon disturbance,  spirocysts bearing hollow tubules are discharged. These tubules adhere to prey upon solubilization and form a web of microfibrillae which increase the surface area contact to increase adhesive ability (Mariscal, McLean and Hand, 1977). The mechanism of these cnidocytes and the venoms delivered will be expanded upon in ‘Anatomy and Physiology’.

Little is known regarding Aiptasia behaviours due to a lack of research since this genus is typically studied as a model system for examining the impact of climate change on coral-zooxanthellae symbiosis. This is a reductionist approach and limits our understanding of this genus, however, competitive behaviours in sea anemones have been well characterized and are extremely interesting.

Actinaria engage in both intra- and inter-specific competition. Adjacent organisms are typically clonemates due to asexual reproduction, competition between these clonemates is limited and often completely absent. This behaviour has been attributed to a self/non-self recognition system based on external phenotypes extending internal identification of tissues belonging to clonemates. As such, clone mates derived via fission rarely compete for benthic space, and if they do, it is not to the same extent as between different species. Sea anemones, either of different species or the same species but not clones, engage in intense territorial battles using cnidocytes see ‘Anatomy and Physiology’ and ‘Ecology’ (Breed and Moore, 2010).

However, sea anemones of the genus Aiptasia engage in much more aggressive competition compared to other genera. This has often been attributed to rapid reproduction by both asexual and sexual methods, see ‘Reproduction’. Recent literature has characterized a novel neurotoxin, AdE-1, from Aiptasia which has been discussed in ‘Anatomy and Physiology’ in detail. This neurotoxin exhibits an extremely rapid mechanism of action which may contribute to the intense competitive abilities of this genus (Nesher et al., 2013). Additionally, as an extremely thin-walled genus, Aiptasia exhibit minimal diffusion boundary layers relative to other sea anemones. As such, it may be likely that allomones released by these species are at a higher concentration compared to those released by its competition. This may present another reason as to why Aiptasia are rarely overgrown by other sea anemones which have been observed to overgrow other organisms (Fautin and Fitt, 1991; Chen et al., 2008). Whilst this hypothesis is not supported by current literature, there is wide consensus that diffusion, such as that of chemical defences, is more efficient in simpler tissues and/or organisms (Denny, 1993). Furthermore, this has been observed in phytoplankton, whilst such research is not directly applicable, it sets a precedent that is supported by current understandings of cellular limitations and processes (Beardall et al., 2009).

Ultimately, the novel neurotoxin AdE-1 may play a large role in the competitive ability of Aiptasia which is a defining characteristic this genus, particularly to aquarists who consider these anemones pests. Further research into this genus, beyond using it as a model system to study symbiosis, is certainly required.

Development of Aiptasia planula larvae is typical of the order Actinaria. Two days following development, larvae become competent to phagocytose symbionts, thus establishing symbiosis with various Symbiodinium strains (Wolfowicz et al., 2016). Since larvae are initially symbiont-free, species of this genus are often used to conduct assays to analyse the dynamics of symbiont establishment (Grawunder et al., 2015). Beyond this, larvae differ amongst different species in the genus – whilst larvae are typically planktotrophic, some species have been observed to produce lecithotrophic larvae (Shick, 1991).

Following fertilization and larval development, settlement and attachment occurs as triggered by environmental cues. The aboral end of the larvae attaches to the substrate from which metamorphosis occurs at around 30 days post-fertilization (Ruppert, Barnes and Fox, 2004). However, beyond the fact that  growth in this genus is indeterminate, not much is known about the metamorphosis of Aiptasia since it has not been observed in laboratory conditions. Metamorphosis is believed to begin with the formation of mesentery, however, only early development has been characterized in Aiptasia (Bucher et al., 2016).

With regards to the presence of toxicity in the typical Aiptasia lifecycle, this has not been characterized yet. However, anemones of the genus Nematostella have been observed to modulate and regulate toxicity throughout their life cycle. More specifically, peptide toxins are expressed in different cell types depending on the life stage of the anemones. This may be due to the differing roles venoms play in sea anemones throughout their lifecycle, ranging from defence to prey capture and disintegration. Indeed, juvenile Nematostella have been observed to sequester venom in gland cells, presumably to create reserves for more effective defence compared to adult anemones which engage in more feeding behaviours utilizing nematocysts than juveniles (Moran et al., 2013). Ultimately, the nematocyst system is extremely complex and its regulation in adult specimens requires further studies, let alone our understanding of how toxins are implicated in the natural history of sea anemones.

The process of feeding begins with tentacles encountering a prey item or solid food, upon the discharge of cnidae the food item becomes adhered to the cnidae. Tentacles subsequently bend in order to clasp the food and bring it to the mouth, where a tubular pharynx brings it into the coelenteric cavity where digestion occurs (Nicol, 1959). The coelenteron, a blind gastrovascular cavity, has been implicated in a variety of systems. With regards to digestion, this cavity is the site of both extracellular digestion and nutrient absorption. The coelenteron is further lined with ciliated canals that enable the distribution of partially digested food (Ruppert, Barnes and Fox, 2004).

Extracellular digestion of proteins occurs in the coelenteron whilst carbohydrate and fat digestion occurs intracellularly (see Figure 6). Food in the coelenteron forms a bolus which is engulfed by mesenteric filaments which, triggered by protein components in the food mass, secrete proteolytic enzymes which begin the process of digestion at the surface of filaments. From here, the absorption of partially digested food occurs (Nicol, 1959). The walls of the coelenteron is divided by septa which secrete hydrolytic enzymes and increase the longitudinal surface area – this ultimately maximizes the efficiency of absorption and thus subsequent intracellular digestion. Small food molecules are phagocyted by gastrodermal and epitheliomuscular cells lining the coelenteric cavity, where intracellular digestion occurs, and absorbed by the gametes (Ruppert, Barnes and Fox, 2004).

Sea anemones do not have defined circulatory, respiratory or excretion systems. Instead, respiration occurs via diffusion of carbon dioxide and oxygen, to and from water in the environment respectively, through cells in the epidermis. This lack of a circulatory or respiratory system manifests itself in limiting thickness of the body wall in order to maximize efficiency of diffusion of dissolved gases. As there is not an excretory system, waste products either diffuse directly from cells in the body wall and tentacles to the environment or into the coelenteric cavity where waste products are expelled through the ‘mouth’ (Hickman et al., 1984; Ruppert, Barnes and Fox, 2004).

Sea anemones engage in both asexual and sexual reproduction, with modes of reproduction in sea anemones are generalized (Bocharova and Kozevich, 2011).

Reproduction modes vary in Aiptasia, sexual reproduction via spawning produces pelagic planula larvae. As a typically gonochoric genus, these anemones either produce eggs (see Figure 7) or sperm which are released within gametes. However, some species have been identified as hermaphrodites (see Figure 8). In either case, gonads are located within the gastrodermis of septa within the coelenteron (Grawunder et al., 2015). Longitudinal gonadal bands are located on the edge of the septa, central to the longitudinal muscles in the column. Gonadal bands are gamete-producing tissue, releasing mature gametes to the coelenteric cavity is the first step to external fertilization, from here, gametes are released through the mouth (Fautin and Mariscal, 1991). Aiptasia engage in year-round gametogenesis, despite environmental conditions (Chen et al., 2008; Schlesinger et al., 2010).

However, increases in asexual reproduction rates has been shown to be triggered by environmental changes, see ‘Natural History’. Aiptasia engage in asexual reproduction via pedal laceration as the dominant mode of reproduction (Hunter, 1984). Buds of tissue, pedal lacerates, on the foot of the polyp are produced. These buds develop into small polyps, producing clonal lines as these polyps are genetically identical to the parent (Grawunder et al., 2015).

Typical of the Actinarians, the base of Aiptasia tentacles each bear a ring of nematocysts. These acorhagi have been implicated in both intra- and inter-specific competition whilst acontia, filaments containing nematocysts are used in defence, see ‘Anatomy and Physiology’ (Jouiaei et al., 2015). In both cases, nematocysts deliver venom. The discharge of nematocysts in order to deliver venom is based on a mechanical trigger, that is, the cell must be touched in order to expel the barbed thread which occurs in less than 3ms (Holstein and Tardent, 1984). This discharge is based on a combination of tensional forces initially generated during the formation of the nematocyst cells and supplemented by the high osmotic pressure within these hollow cells. Once triggered, the operculum of the nematocyst open, thus rapidly increasing the hydrostatic pressure within the cell as water rushes in due to the osmotic difference. As a result, this pressure expels the nematocyst thread in a tumbling motion wherein the extreme force of the pressure causes the barbed thread to turn inside out, exposing hollow spines filled with venom, as illustrated in Figure 9 (Hidaka, 1993; Hickman, Roberts and Larson, 1997).

In the case of Aiptasia, the mechanism of nematocyst discharge remains contested. Whilst the role of tensional forces has been widely accepted in barb discharge, the specific mechanism is relatively controversial. In either case, free calcium ions (Ca²⁺) have been implicated in ensuring the stability of nematocysts in resting states – thus in-directly contributing to nematocyst discharge by maintaining the osmotic pressure which expels the barbed thread following a mechanical trigger (Salleo, la Spada and Denaro, 1988). Santoro and Salleo (1991) further observed that in the absence of Ca²⁺ nematocyst discharge was completely inhibited. This may implicate a role for Ca²⁺ in nematocyst discharge beyond simply maintaining the resting state of nematocysts. Santoro and Salleo (1991) postulated that Ca²⁺ conductance in either the nematocyst itself or supporting cells may trigger discharge, further reporting that cyclic AMP (cAMP) did not appear to be involved in this process. Whilst more recent studies, such as that of Ozacmak et al (2001) clearly demonstrate the role of cAMP in nematocyst discharge, our understanding of this complex process is continuously changing and further research is most certainly required.

Furthermore, it has been widely speculated that nematocyst are not only the site of delivery of toxins, but also that site of venom production in cnidarians. This theory has been widely accepted based on experiments chromatographically fractioning nematocysts from a variety of cnidarian species and observing the immunolocalization of both pore-forming toxins (which are typically neurotoxins) and phospholipases (Moran et al., 2012). However, recent studies have observed the localization of neurotoxins in Nematostella to ectodermal gland cells (Moran et al., 2011). Further research into this has demonstrated that the localization of venoms differs between various sea anemones species. As such, historical views on the exclusive localization of venoms to nematocysts in cnidarians appear to be outdated. Indeed, Moran et al (2011) suggest that a toxin-secretion system wherein venoms were secreted directly into the surrounding medium by gland cells may be the ancestral state of venom delivery with nematocysts evolving later, see ‘Evolution and systematics’.

The most interesting aspect of nematocysts in this genus, is the novel neurotoxin AdE-1. This neurotoxin exhibits an extremely high interference efficiency by targeting both voltage-gated sodium (Na_v) channels and voltage-gated potassium (K_v) channels (Nesher et al., 2014). As a result, AdE-1 inhibits the generation of rapid upstrokes of action potentials, reduces nerve impulse transmission and compromises the overall excitability of nerves (Campbell et al., 2017; Liao et al., 2019).

Whilst the delivery of this neurotoxin is not novel, its action on animals’ nervous system is. AdE-1 inhibited Na+ inactivation in crabs which ultimately increased the intracellular Na+ concentration. This is typical of sea anemone neurotoxins target receptor site III. By inhibiting the Na_v channel’s transition to the inactivated state peak amplitude is increased by the neurotoxin (Nesher et al., 2014; Campbell et al., 2017). However, AdE-1 increases voltage-dependency which directly contradicts our previous understanding of such neurotoxins, indeed, this indicates that binding of the neurotoxin increases at positive membrane potentials. Additionally, most sea anemone KTxs slow the activation and inactivation kinetics however AdE-1 increases the speed of activation to an extent to which the ion channel is unable to release potassium ions (Rodríguez et al., 2014). Ultimately, AdE-1 exhibits both unique interactions with Na_v channels and a novel effect on K_v channels whilst behaving in a manner typical of potassium-channel targeting (Nesher et al., 2014).

The evolution of and within the phylum Cnidaria is aptly described by Borruat (2014) in the collated work describing ‘Aiptasia sp.’ uploaded to the ‘Invertebrates of the Coral Sea’ website, an initiative of the University of Queensland. In acknowledgement of this work, this discussion of Cnidarian evolution will focus on: firstly, the evolution of Cnidarian neurotoxins and secondly, evolution of the novel toxin AdE-1 found in Aiptasia sp.

Cnidaria’s characterizing nematocysts evolved to deliver toxins which were in turn evolved and conserved due to strong pressures of negative selection (Jouiaei et al., 2015). These toxins have historically been classified as either pore-forming cytolysins, neurotoxins acting on sodium ion channels or neurotoxins acting on potassium ion channels (Marí and Tytgat, 2010). This review will focus on neurotoxins.

In species from the order Actiniaria, commonly referred to as sea anemones, neurotoxins comprise the majority portion of peptides in sea anemone venoms (Jouiaei et al., 2015). Neurotoxins present a great evolutionary advantage, particularly for sessile organisms such as sea anemones, which are dependent on defensive mechanisms as they are unable to flee. Additionally, the lack of mobility presents a need for advanced predatory mechanisms in sea anemones. Furthermore, these neurotoxins have been implicated in intra- and inter-species competition, allowing sea anemones to monopolize their chosen habitats (Jouiaei et al., 2015). As such, it is hypothesized that the neurotoxins evolved by Cnidaria present an evolutionary advantage by targeting universal ion channels, allowing for activity against a broad range of phyla (Liao et al., 2019). Whilst these ion channels are not present in a variety of phyla, it is crucial to note that those which have these channels are the organisms which would either be potential prey for heterotrophic sea anemones or potential predators. Hereby, neurotoxins present an evolutionary advantage by contributing to both feeding and defensive mechanisms.

Cnidarian neurotoxins typically target either voltage-gated sodium (Nav) channels, or voltage-gated potassium (Kv) channels, both of which are present in most animals with a functioning nervous system (Liao et al., 2019). These neurotoxins present high potency and high specificity to these ion channels, prompting hypotheses as to their evolutionary origin. Studies have demonstrated that protein families which are originally non-toxic are regularly, via gene duplication, being expressed in toxins as a result of differential gene expression (Fry, Roelants and Norman, 2009). This suggests that neurotoxins have evolved from proteins, specifically, that this process was dependent on the expression of these protein families in different contexts (Fry, Roelants and Norman, 2009). For Cnidarians specifically, 70% of the most common proteins identified in their neurotoxins lack homologs (Moran et al., 2013). As such, whilst the specific pathways of Cnidarian neurotoxin evolution are unknown, genomic analysis has identified extreme conservation of genes at the nucleotide level (Moran et al., 2008). It is likely that these neurotoxins evolved from protein families due strong selection pressures that increased the rate of evolutionary turnover in response to the predator-prey evolutionary “arms race” (Fry, Roelants and Norman, 2009; Moran et al., 2013). This is unusual relative to toxins of other animals as it is currently theorized that diversifying selection drove the evolution of their toxins, this is a potential explanation for the wide range of genetic diversity and differences in toxin action (Moran et al., 2008). In contrast to this, Cnidarian toxins are believed to have evolved in concert as toxin genes of sea anemones are typically similar and differ highly from other animals such as some species of scorpions and snakes whose toxin encoding genes are similar (Moran et al., 2008; Rodríguez de la Vega and Giraud, 2016).

Beyond looking at the evolution of neurotoxins, it is crucial to consider that of neurotoxin delivery mechanisms which have been discussed in more detail in ‘Anatomy and Physiology’. Recent literature posits that our understanding of nematocysts being the only mechanism of venom delivery is reductionist. Moran et al (2013) suggest that ectodermal gland cells are the ancestral evolutionary state of toxin delivery mechanisms in sea anemones. In various sea anemone species, studies observed the immunolocalization of venoms to ectodermal gland cells rather than nematocysts (Moran et al., 2013). This delivery mechanism of Type I sodium channel neurotoxins has been observed to be notably less efficient in quickly delivering toxins (Moran et al., 2012). However, it is postulated that the gland cell strategy may allow for the delivery of much higher concentrations of toxins as opposed to nematocyst delivery (Moran et al., 2012). Whilst it is acknowledged that this may simply be a secondary mechanism of toxin delivery, it is hypothesised that nematocysts evolved as specialised structures from this ancestral state due to strong selective pressures on venom delivery systems (Moran et al., 2013). Ultimately, further research is required to completely understand the evolution of structures used to deliver toxins in Cnidarians.

AdE-1 presents a highly unusual structure and mechanism of action compared to typical sea anemone neurotoxins, acting on both Nav and Kv channels (Nesher et al., 2014). This in itself is unusual as Nav and Kv ion channels exhibit dramatically different regulation mechanisms. Nav channels regulate membrane permeability in the action potential phase of nerve action whilst Kv channels operate in the resting potential phase by maintaining the concentration gradient across membranes (Stevens, Peigneur and Tytgat, 2011). Neurotoxins are typically adapted to target one of the two voltage-gated ion channels, increasing interference efficiency (Liao et al., 2019).

As early as 1968, Blanquet (1968) observed that compounds of typically found in nematocyst toxins, such as hydroxyindoles, were absent from Aiptasia species. Subsequent research has demonstrated that this is likely due to evolution of a novel neurotoxin, AdE-1, that is unique to Aiptasia (Nesher et al., 2013). Whilst targeting both types of ion channels is not unique to AdE-1, this is highly unusual and is theorized to have required a novel evolutionary pathway (Nesher et al., 2014). AdE-1 shares similar post-translational modifications to other sea anemone neurotoxins, specifically, the formation of three disulfide bridges – however, the mechanism of action and replacement of residues renders AdE-1 unlike any other sea anemone neurotoxin that has been sequences (Nesher et al., 2013). Additionally, AdE-1 targets Nav channel site III, unlike all other described neurotoxins isolated from sea anemones which target either site I or site II (Nesher et al., 2013). This is likely due to the differences in conserved and essential sites in AdE-1 compared to other toxins, to date, the homology of AdE-1 to other toxins is less than 36% (Nesher et al., 2013). However, AdE-1 exhibits the same cysteine residue arrangement as other sea anemone toxins, demonstrating that in the evolution of this novel neurotoxins this crucial alpha-sequence has been conserved (Nesher et al., 2013).

However, it is the replacement of key residues, such as N16, that presents the question as to whether AdE-1’s mechanism of interaction with sodium channels differs from that of typical sea anemone neurotoxins (Nesher et al., 2013). As such, questions regarding the evolutionary history of this neurotoxin have been raised as AdE-1 exhibits both an unusual structure and mechanism of action relative to other sea anemone toxins (Nesher et al., 2013). This is crucial as it was previously believed that there was a high degree of structure–function universality in sea anemone neurotoxins, particularly those targeting sodium ion channels (Smith and Blumenthal, 2007; Nesher et al., 2013).

The below figure explores evolutionary relationships between AdE-1 and typical sea anemone neurotoxins that target sodium channels (Figure 10). Type II neurotoxins are delineated in blue whilst type I is in magenta. The distances between neurotoxins are to scale and have been determined based on the number of amino acid substitutions per site. AdE-1 diverges from both lineages and exhibits the longest distance, demonstrating it is evolutionarily the most different. Notably, AdE-1 exhibits no polar residues, specifically N16 which has been conserved by all characterised sea anemone Na+ channel type 1 and type 2 toxins.

Sea anemones belonging to the genus Aiptasia are widely used as model systems for studies of symbioses and coral bleaching, however, the systematics of this group is not widely studied (Sawyer and Muscatine, 2001; Grajales and Rodriguez, 2014). This lack of studies is enhanced by a number of species complexes within this genus that all contribute to difficulties in identifying organisms to the species level and in determining relevant phylogenetic relationships. Indeed, recently Rodriguez et al (2012; 2014) came to the conclusion that the genus is not monophyletic as previously thought. As a result, many originally described species have been reviewed and subsequently reclassified. One such example is that of the formerly-named Aiptasia pallida (Agassiz in Verrill, 1864) (now Exaiptasia pallida comb. nov.) which was placed in a new genus, Exaiptasia gen. nov., erected by Grajales and Rodriguez (2014). Grajales and Rodriguez (2014) further reviewed a number of other originally accepted Aiptasia species, using morphological evidence to split them up further and to reassign them to other families, still within the genus. Ultimately, this single paper has highlighted firstly, the need for further reviews of systematics within this genus and secondly, the need for attentiveness when consulting old literature that has not been updated.

Phylum: Cnidaria (Hatschek, 1888)

Class: Anthozoa (Ehrenberg, 1834)

Subclass: Hexacorallia (Haeckel, 1896)

Order: Actinaria (Hertwig, 1882)

Suborder: Enthemonae (Rodríguez & Daly in Rodríguez et al., 2014)

Superfamily: Metridioidea (Carlgren, 1893)

Family: Aiptasiidae (Carlgren, 1924)

Genus: Aiptasia (Gosse, 1858)