Aiptasia pulchella (Cnidaria: Anthozoa: Actiniaria: Aiptasiidae: Aiptasia) may seem small and boring, but it possesses an abundance of intriguing characters, it’s not your everyday cnidarian. It is a long-studied, ‘model organism’ in the scientific world due to its many advantageous traits such as pedal laceration which allows it to reproduce ‘in record time’, and its substantial growth rate due to its symbiosis with zooxanthellae. These exact traits however, render it considered a pest species in home aquariums. It’s predatory and defence strategies were of particular interest in this study (Grajales & Rodriguez, 2014; Howe et al, 2012; Rupert, Fox & Barnes, 2004). 

As is the case of most sea anemones (Cnidaria: Anthozoa: Actiniaria), Aiptasia pulchella possesses the polyp body form. Their body column is elongate and smooth, with a mouth-bearing oral disc on one end and an aboral pedal disc on the other. The oral disc is surrounded by thin and pointed stinging tentacles; the plateau-like base of the body column (pedal disc) in A. pulchella (and many other sea anemones) has evolved as a means to attach themselves to hard substrates. Due to their radial symmetry (centred around the oral-aboral axis), in combination with the plentiful branching tentacles, many refer to them as ‘flowers of the sea’ (Rupert, Fox & Barnes, 2004); though don’t be fooled, for A. pulchella is an aggressive predator and a common home aquarium pest (Grajales & Rodriguez, 2014; Rupert, Fox & Barnes, 2004).  

Morphological evolution within the family Aiptasiidae (of which the Aiptasia genus belongs) is significantly homogenous (Grajales & Rodriguez, 2014). Single unique synapomorphies (of the morphological nature) of species within the Aiptasiid family are of little help to delineate taxonomic membership; instead, it is the combination (and unison) of particular morphological factors that has been suggested to make species (or at least genus) circumscription possible (Grajales & Rodriguez, 2014; Grajales and Rodriguez, 2016). These morphological traits are: non-regionated scapus and scapulus/capitulum, a smooth column, cinclides arranged in rows or lines along the column, a mesogleal sphincter, basilar muscles, non-divisible mesenteries, and batitrichs and microbasic p-mastigophores acontia nematocysts (Grajales & Rodriguez, 2014).

Reliable characterisation of species within the Aiptasiidae family is most efficiently done by internalised anatomical qualities, in combination with morphology (Grajales & Rodriguez, 2014) (see Anatomy and Physiology Section). 

Reproduction
Aiptasia pulchella reproduce via asexual (pedal laceration) and sexual means (broadcast spawning). Pedal laceration involves fragmentation of the pedal disc and column junction, resulting in multiple daughter individuals (smaller in size than the parent, but still genetic clones) which then develop into a new polyp within 14 days (Chen et al, 2007; Howe et al, 2012; Rupert, Fox & Barnes, 2004); this often results in founder effects, which involves a population being largely comprised of females (Chen et al, 2007). In regards to the broadcast spawning, dioecious individuals (separate sexes) release gametes into the water column, fertilisation and zygote formation occurs resulting in free-swimming planula larva. Once an appropriate substrate is found, settlement occurs and metamorphosis into the polyp body form is induced (Chen et al, 2007). A. pulchella spawning occurs frequently in a unique lunar cycle of reproduction (Chen et al, 2007; Howe et al, 2012). 

Gonad development in female A. pulchella occurs between mesenteric filaments and the retractor muscles, and has been shown to be influenced by the body size of the individual with larger anemones being more fertile and gonad development occurring sooner (Chen et al, 2007). The oogenesis process of A. pulchella is very similar to many other sea anemones (Chen et al, 2007; Rupert, Fox & Barnes, 2004). However, it is the occurrence of one of the most “primitive types of oviparous- pelagic-planktotrophic development” (characterised by spawning and external fertilisation of small eggs) during sexual reproduction of A. pulchella that is so fascinating; they have smallest mature oocytes of all broadcast-spawning sea anemones, measuring a diameter of ~60-100 um. Yet another unique reproductive factor of A. pulchella is that the gametogenic cycle results in multiple sexual offspring throughout the year because of the frequent monthly spawning events (most sea anemones only produce offspring through sexual means once a year) (Chen et al, 2007). 

Predation and defence against predation
Reactions to the presence of predation in many sea anemones is largely through behavioural, ecological, and morphological manipulation or reaction. Due to the fact that Aiptasia pulchella is not capable of significant movement, a commonly implemented strategy to avoid predation is to retract and make itself smaller, or take refuge in substrate cavities, as a means to protect vulnerable structures (Haag and Dyson, 2014) (see Anatomy and Physiology for a discussion on how this is achieved). 

Such a strategy does have its trade-offs however, for if it is withdrawn for too long then multiple feeding opportunities are missed and future fitness is compromised as a result. The decision of re-emergence (or extension of tentacles back to normal) from the hiding position is influenced by many factors such as conspecific density, body size, previous predation pressure or disturbance, and food availability. Individuals within populations of high conspecific density tend to re-emerge pretty quickly after a predation attack or stimulus because competitive pressure for food is higher, as well as the decreased predation risk that occurs in higher numbers due to the dilution effect. The disc diameter of an individual and the associated hiding time has shown different relationships in varying studies; this is most likely due to the number of environmental factors surrounding different populations, such as the availability of food or the individual’s capacity to regenerate (Haag and Dyson, 2014). 

Observations of my own
The influence of some of the aforementioned factors is illustrated in a small observational study completed by myself throughout two weeks. Results are more of interesting value and should not be considered fact due to the lack of replicates and therefore comparative observations. Each week I would gently rub blunt tweezers along the tentacles of the individual and observe the response. Looking at the A. pulchella individual in the videos below it can be seen that in week 1 it took perhaps as little as 3 stimuli occurrences for the individual to completely (or significantly) retract, whereas in week 2 the response took significantly longer (so much so that it took three videos and 8 stimuli before the video below to get a response). This indicates habituation to aversive stimuli and often occurs in the wild particularly in populations where wave action is significant and common (Haag and Dyson, 2014). Interestingly however, is the fact that the degree of retraction and the time spent withdrawn in week 2 once they did actually respond increased dramatically (approximately 20 seconds in week 1 and around 40 seconds for week 2). This may be due to the fact that the individual had just fed (observed at the beginning of week 2) and therefore could risk the loss in feeding possibilities (Haag and Dyson, 2014). Another observation I noted is the fact that the acontial threads seemed to more abundant and ejected for a longer time in week 2, although I am not too sure as to why this may be. (Apologies for my horrendous filming). 

Week 1 predation pressure response

Week 2 predation pressure response

In terms of predation and aggression strategies, just like all cnidaria, A. pulchella implements the use of cnidocytes (nematocysts in particular) to aid in capturing prey and as a mean of defence. The unique way in which they deliver the nematocysts is rather interesting; acontiarian sea anemones (in which all Aiptasia species belong) have evolved thread-like extensions, termed acontial threads (see pictures below), that are densely surrounded with nematocysts. These are expelled from A. pulchella’s tentacles or cinclides when mechanically or chemically stimulated (see Anatomy and physiology for further information) (Grajales & Rodriguez, 2014; Marino & La Spada, 2007; Rodriguez et al, 2012; Thorington and Hessinger, 1996). In relation to A. pulchella in particular, it has been suggested that the expulsion of acontial threads with large microbasic p-mastigophores (extensions of the mesenterial cavity) from the cinclides is only implemented as a defensive strategy. Aconital threads ejected from the terminal pore of the tentacles however, with medium sized microbasic p-mastigophores, are predominantly implemented as a means to catch prey (Jennings, 2014). 

As mentioned previously (within the Physical Description section), the Aiptasiid family exhibits particular anatomical features, that when in combination, characterise the family. Longitudinal (ectodermal) muscles in the distal column, basilar muscles, non-divisible mesenteries, cinclide rows present in the mid-column, microbasic b-mastigophores within the column, along with acontia (and the microbasic p-mastigophores and basitrichs nematocysts), and a mesogleal sphincter are these aforementioned defined anatomical features that define members within the Aiptasiidae family (Grajales & Rodriguez, 2014; Rodriguez et al, 2012). 

Germ and tissue layers
Cnidarians are diploblastic animals with an ectoderm and endoderm; the epidermis, gastrodermis, and mesoglea are the three main tissue layers. Just as in all cnidarians the mesoglea is solely structural and any sensory-, movement-, digestion-, internal transport-, communication-, germ-, muscular-, nerve-, and reproductive- cells are found in the epithelial layer (Rupert, Fox & Barnes, 2004). 

The coelenteron and the mesoglea are surrounded by epithelia, meaning they have the ability to create extracellular compartments. Such compartmentalisation enables physiological fine-tuning for specific functions. Consequently, because of the shear degree of life’s functions having to occur in one of these layers, the coelenteron and the mesoglea of cnidarian’s are highly multi-functional compartments (Rupert, Fox & Barnes, 2004). 

The coelenteron and mesenteries
The coelenteron within cnidarians is multifunctional: it aids circulation and internal transport, extracellular digestion, excretion, and even reproduction. Within sea anemones the coelenteron is vertically partitioned, forming septa (mesenteries); some of which are complete (span the entire coelenteron and extend to the pharynx), or incomplete septa (Chen et al, 2007; Rupert, Fox & Barnes, 2004). The “inner free margin of each septum” displays a convoluted shape due to the swollen septal filament edge. On a microscopic level, Aiptasia pulchella shows a tri-lobed septal filament; the central lobe (cnidoglandular band) contains cnidocytes (largely the nematocyst type) and enzymatic gland cells. There is one region of the cnidoglandular band of each septum filament, termed aconitum, which lengthens away from its septal attachment to form a long thread armed with cnidae (see below for further information) (Rodriguez et al, 2012; Rupert, Fox & Barnes, 2004). 

Nervous system, movement and musculature
The epidermal smooth muscles of sea anemones are organized in such a way that the longitudinal fibre layers are antagonistic with circular fibre layers (Rupert, Fox & Barnes, 2004). Musculature of epidermal origin occurs mostly in the tentacles and the oral disc, in a longitudinal and radial fashion respectively; any remaining muscle is gastrodermal and consists of a circular muscle around the columnar wall and the pharynx, as well as radial, and longitudinal septal muscles (retractors) (Chen et al, 2007; Rupert, Fox & Barnes, 2004). 

Aiptasia pulchella possess significantly well-developed longitudinal muscle bands within their septa, which has resulted in them having amazing retraction and extension capabilities. Such longitudinal muscle bands stretch from the oral disc and anchor at the pedal disc. The retraction-deflation sequence that A. pulchella exhibit due to predation pressure is made possible by these muscle bands (along with coelenteron activity and the hydrostatic skeleton). This involves significant physiological changes over a very short period; the tentacles and oral disc undergo invagination and are pulled into the body column, the tentacles and body column are then severely deflated and coelenteric fluid expelled through the mouth and cinclides. If you focus on the body column, the expulsion of coelenteric fluid can be faintly seen in the videos below. Furthermore, the oral-wards bend of the tentacles to insert food into the oral cavity, and/or the removal of undesired material is also aided by such muscle bands and coelenteron activity (videos attached below) (Haag and Dyson, 2014; Rupert, Fox & Barnes, 2004). Therefore, the highly developed retractor muscles of sea anemones, such as A. pulchella, enables not only advanced feeding strategies (considering their mostly sessile existence), but also incredibly efficient predator avoidance- all in all greatly increasing their survival probability (Rupert, Fox & Barnes, 2004). (Apologies again for my lack of recording skills, it seems the camera wasn’t attached properly).

Video footage depicting the retraction-deflation sequence of A. pulchella when faced with predation pressure. You can see the retraction of tentacles and oral disc into the column.

A video filmed by myself showing the oral-wards bend of the tentacles made possible by coelenteron activity and the retractor/muscle bands.

Cnidocytes and acontia
Nematocysts (toxin injecting cnidocytes) are the most complex biological weapons in the animal kingdom, many argue that sea anemones possess the most sophisticated nematocysts due to the extreme complexity of the biologically active compounds within their nematocysts (Sanchez-Rodríguez et al, 2006). 

The cnidae-armed threads, as mentioned previously when looking at the coelenteron anatomy, are termed acontial threads. These sensory and secretory cells are used in prey capture and prey digestion via extracorporeal means, as well as a defence strategy. Basitrichs and microbasic p-amastigophores are the nematocysts found on A. pulchella’s acontial threads (Marino & La Spada, 2007; Rodriguez et al, 2012). 

Aiptasia pulchella have been recorded extensively throughout subtidal and intertidal rocky shores in the shallow waters of the Pacific Ocean (Howe et al, 2012). However, if recent taxonomical changes are applied (see Evolution and Systematics) such as the one suggested in Grajales and Rodriguez (2014; 2016) and Voolstra (2013), then its biogeographic distribution could be global, throughout both temperate and tropical oceans. 

The Aiptasia genus has long been of interest in many phylogenetic, pest expansion probability, and cnidarian-dinoflagellate studies. Aiptasia is said to comprise of 16 valid species, most of which are distributed throughout the Atlantic Ocean as well as the Mediterranean Sea. Aiptasia pulchella and A. californica were both described by Carlgren in 1943 and 1952, respectively; both of which have been said to only occur in the Pacific Ocean. However, a recent taxonomic study of the Aiptasiidae family, resulted in significant nomenclature and membership changes due to detailed morphological character observations; one of the most significant changes being that of the Aiptasia genus being split into two distinct genera, Aiptasia (only A. mutabilis and A. couchii) and Exaiptasia (all remaining Aiptasia species). This was done largely because of the differences seen in reproductive strategies and cnidae structure. Furthermore, due to the lack of morphological differences seen in the species comprising the new cryptic Exaiptasia genus, all such species were synonymised as a single and widespread species, Exaiptasia pallida. Therefore, meaning that the individual I studied, could perhaps be E. pallida. The separation of Aiptasia and Exaiptasia resulted in the latter genus having pedal laceration as one of their defining features, whereas Aiptasia species are now said to lack the ability to fragment from their pedal disc, and also exhibit increased cnidae size (Grajales & Rodriguez, 2014; Grajales and Rodriguez, 2016). 

This reclassification of most Aiptasia species into E. pallida means that instead of A. pulchella being confined to the Pacific Ocean, they could perhaps have a significantly cosmopolitan distribution (Grajales & Rodriguez, 2014; Grajales and Rodriguez, 2016). 

Author's comments
Taking into consideration the multiple studies that I have looked at in regards to A. pulchella, if this classification needed to be confirmed in any way, future studies could look at whether all the individuals that were previously Aiptasia (but now labelled as the cosmopolitan Exaiptasia pallida) have the ability to be aposymbiotic. If such a study gives mixed results (or if none possess this ability), then this could suggest that the classification needs reconsidering because A. pulchella are able to be asymbiotic and therefore may not be E. pallida at all. In addition, the size of the mature oocytes in the individuals now classified as the E. pallida species could also be looked at; if they were any bigger than ~60-100 um then this could also indicate that revision may be required. The presence of pedal laceration and a comparative study of cnidae size may be also be beneficial to confirm the new separation of Aiptasia into two genera, and the unison of A. pulchella and all other previous Aiptasia species (bar A. mutabilis and A. couchii). 

A. pulchella sea anemones are very ‘hardy’ creatures. They reproduce rapidly through both sexual and asexual means and possess many other qualities which allows their populations to expand in a significantly short amount of time. This enables Aiptasia pulchella to escape any significant conservation threat. While the loss of zooxanthellae due to global warming is threatening many marine organisms who form vital symbioses with these dinoflagellates, even this appears to be of little threat to A. pulchella due to their ability to be asymbiotic. It could even be argued that perhaps they pose threats to other organisms due to their remarkable competitive abilities (Grajales & Rodriguez, 2014; Grajales and Rodriguez, 2016; Voolstra, 2013). 

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