Anthopleura Xanthogrammica Classification Essay

1. Introduction

Cnidarians are simple animals with radial symmetry that contain two layers of cells, ectoderm and endoderm. Mesoglea, a non-cellular matrix, is present between the two layers. Cnidarians are mostly predators but certain species may also scavenge dead animals or obtain nourishment from intracellular, photosynthetic unicellular algae, named zooxanthellae.

At least four toxic living classes of cnidarians are currently recognized by most systematists: Anthozoa, Hydrozoa, Scyphozoa and Cubozoa. Molecular phylogenetic methodologies based on DNA sequencing, allowed to determine that the Anthozoa are the basal group of cnidarians [1] (Figure 1). In fact, Anthozoa has a circular mitochondrial DNA, while Hydrozoa, Scyphozoa and Cubozoa have a linear molecule. Likewise the polyp preceded the medusoid form in the course of evolution [2].

Figure 1. Simplified cladogram of the phylum Cnidaria (adapted from [3]). Photos a, e and f were retrieved from [4].

Figure 1. Simplified cladogram of the phylum Cnidaria (adapted from [3]). Photos a, e and f were retrieved from [4].

There are two main types of life cycles in cnidarians. In Anthozoans, the polyp is the gamete-producing form and the cycle is embryo > larva > polyp. Medusozoans generally have an embryo > larva > polyp > medusa life cycle, in which the medusa is typically the sexual form. Figure 2 shows a typical life cycle of Anthoza [1].

Cnidaria feeding success relies on the presence of specialized poisonous cells, the nematocysts. These organisms have specialized subcellular organelles called cnidae with several structures and functions. Cnidae can be classified into three types: nematocysts, spirocysts, and ptychocysts. Nematocysts deliver the venom through the skin, whereas spirocysts are adhesive and ptychocysts are involved in protection. While Anthozoans have the three types of cnidae, medusozoans (Scyphozoans and Cubozoans) contain only nematocysts. The biological roles of toxins delivered by nematocysts include the capture and killing of prey, digestion, repelling of predators and intraspecies spatial competition [5]. Cnidarians are not just studied by their toxins and venoms, they are a source of marine natural compounds with therapeutically properties, namely antitumor activity [6]. Furthermore, voltage-gated ion channels toxins are studied as an inspiration for drugs design, not only therapeutic but also as insecticides [7].

Figure 2. Schematic representation of a typical life cycle of an Anthozoa.

Figure 2. Schematic representation of a typical life cycle of an Anthozoa.

The composition of cnidarian venoms is not known in detail, but they appear to contain a variety of proteinaceous (peptides, proteins, enzymes and proteinase inhibitors) and non-proteinaceous substances (purines, quaternary ammonium compounds, biogenic amines and betaines) [8]. As an example, palytoxin is a polyether from Palythoa, and caissarone is an iminopurine from Bunodosoma caissarum [9].

The venom is spread all over the body, in a mucous coat, that also protects them from predators, or it is located in the nematocysts. In a recent work, Moran and co-workers, reported that neurotoxin 1 from Nematostella vectensis is confined to ectodermal gland cells. Moreover, in Anthopleura elegantissima this toxin also appears in gland cells, whereas in Anemonia viridis is associated with both nematocytes and ectodermal gland cells [10]. Previously, Honma and co-workers also gave a hint for the same phenomenon when describing that gigantoxins were mostly derived from unknown organelles other than nematocysts [11]. Nematocysts are found mostly on the tentacles, but also exist in other organs such as in acrorhagi and acontia, particularly in certain species of the Actiniidae family, where they are used to fight with nonspecific non-clonemates or for purposes of defence or predation, respectively. Acrorhagi are located in a ring around the base of the tentacles (Figure 3a). Acontia are thin white or color threads attached at one end to the borders of the mesenteries. They can be protruded through the mouth, and in some cases through special pores (cinclides) in the body-wall, for purposes of defence or paralyses of prey (Figure 3b).

The Anthozoa class include sea anemones, and other anemone-like groups with skeletons (such as the “stony” scleractinian corals) and without skeletons (such as tube anemones), as well as sea pens, sea fans, blue corals, and black corals. The word Anthozoa comes from greek anthos, flower + zoon, animal, as sea anemones resemble flowers (Figure 3c).

Figure 3. General aspects of the sea anemone morphology. (a) Acrorhagi, the blue vesicles in Actinia equina, green variety (also called Actinia prasina), are used to fight against space towards other individuals (see arrow); (b) Acontia, the white threads secreted by Calliactis parasitica are used as defensive organs when disturbed (see arrow); (c) Bunodactis verrucosa specimens with tentacles retracted and fully expanded, illustrating the characteristic column with adhesive verrucae and short tentacles.

Figure 3. General aspects of the sea anemone morphology. (a) Acrorhagi, the blue vesicles in Actinia equina, green variety (also called Actinia prasina), are used to fight against space towards other individuals (see arrow); (b) Acontia, the white threads secreted by Calliactis parasitica are used as defensive organs when disturbed (see arrow); (c) Bunodactis verrucosa specimens with tentacles retracted and fully expanded, illustrating the characteristic column with adhesive verrucae and short tentacles.

Nematocysts possess a high concentration of polypeptides and proteins that act as neurotoxins, hemolysins and enzymes, which are responsible for a variety of harmful effects to humans. These toxins/venoms are only injected in the prey or predator after a mechanical or chemical stimulation [12]. In humans, toxins cause cardiotoxicity, dermatitis, local itching, swelling, erythema, paralysis, pain and necrosis [8]. In vivo effects of sea anemone toxins include neurotoxicity and cardiotoxicity.

Summarily, the cnidarians venom includes 3.5–6.5 kDa voltage-gated sodium (NaV) channels toxins and 3–5 kDa voltage-gated potassium (KV) channel toxins and ~20 kDa pore-forming toxins. The first type prevents inactivation of NaV channels by stabilizing the open state conformations. This fact is due to the binding of the toxin to neurotoxin receptor site 3 [13]. KV channel toxins reversible blocks potassium current and can block acid-sensing ion channels, which are permeable to several cations. The cardiotoxic effects of toxins includes arrhythmias, triggered by early after depolarizations resulting from incomplete NaV channel inactivation, and systolic arrest due to myocardial cell calcium ion overloading [9].

Besides toxins, there are several other non-toxic proteins from sea anemones that are studied by its biological activities, such as fluorescent properties [14], but they will not be included in this review. However, we will discuss the importance of protease inhibitors as they adopt a structure that inhibits potassium channels.

In this review, we begin with a brief description of the Anthozoa phylogeny, followed by a general characterization of the sea anemone toxins and afterwards we focus on the major groups of toxins. We then refer to the state of the art techniques used for venom extraction. Afterwards we present the structure of the genes involved in toxin production and the three-dimensional (3D) structures of cnidarian toxins described to date. This review will be solely focused in the molecular diversity of sea anemone toxins. Other cnidarian toxins, as those from coral or jellyfish, will not be considered. More comprehensive information is available in a number of specific papers for jellyfish [15,16], cnidarians in general [2,5,17,18] and sea anemones [13,19,20].

2. Phylogenetic Relationships of Anthozoa and Sea Anemone Toxins

Cnidarians are scattered around the world and have around 10,000 estimated species. The majority of the phylogenetic studies classified cnidarians based on morphological characters [21]. At the molecular level, the classification of cnidarians is not yet well established, namely for the order Actiniaria. The phylogeny of Actiniaria is at a suboptimal estimation level [3] and has been retrieved from the sequencing analyses of 12SrRNA, 16SrRNA, 18SrRNA, 28SrRNA and COIII genes [22,23,24,25]. As referred by Turk and Kem [2], the comprehension of the phylogenetic relationships among Anthozoa members will give insights into the evolution of theirs toxins. Thus, a review about sea anemone toxins could not be dissociated from the Anthozoa phylogenetic characterization.

Besides the few studies on the phylogeny of Actiniaria, some other studies have also been done on the population genetics of these animals. Nonetheless, the majority of those works focus on other Orders, especially on corals. Indeed, few studies were done at the intraspecific level on Actiniaria. Population genetics of Actinia spp. assessed with enzyme electrophoresis showed that Actinia nigropunctata from Madeira Island (Portugal) is in fact a different species from all the others in the study, as well as Actinia equina from Africa [26]. Darling and co-workers in 2006 studied the Nematostella vectensis introduced along the Pacific coast of North America and the southeast coast of England, using 10 polymorphic microsatellite loci, and find high variability from Hardy-Weinberg equilibrium as a result of population genetic structure and reproductive plasticity [27].

Considering the molecular markers surveyed in Cnidarians until now, the variation in mitochondrial Citochrome Oxidase I (COI), within and between species, is much lower in Anthozoa compared to Medusozoa. Low identification success and substantial overlap between intra- and interspecific COI distances render the Anthozoa unsuitable for DNA barcoding [28], with COI p-distances among Anthozoa species being equal to 1% [29]. Shearer and co-workers [30] showed that nuclear markers in Anthozoa have much higher substitution rates and therefore should be used instead of mitochondrial genes.

The reduce knowledge on sea anemones phylogeny make it difficult a direct comparison with the toxin genes phylogeny. While previous studies showed a reduced level of congruence between species phylogeny and the toxin gene phylogeny, further research is needed to better clarify this pattern. Such findings may not be unusual due to distinct patterns of toxin gene evolution (e.g., gene duplication/gene loss, horizontal gene transfer, and lineage sorting and diversification). However, future studies are needed to better elucidate the phenomena behind the acquisition and evolution of the toxin genes in Anthozoa.

Concerning the phylogeny of toxins, we assessed the phylogenetic relationships of NaV channel and KV channel toxins. In order to systematize the information, we have assessed a phylogenetic tree of cytolysins using only Actinoporins with evidence at transcript level and with full-length sequences. A multiple sequence alignment of amino acids with 533 sites, was made with WebPrank [31] followed by an analysis to choose the best fit model for protein evolution with ProtTest [31], that gave WAG model. A Maximum Likelihood tree reconstruction was made in Mega 5 [32] using 100 bootstrap inferences. A discrete Gamma distribution was used to model evolutionary rate differences among sites (4 categories), Figure 4. (The alignment is available upon request to the corresponding author.)

Figure 4. Maximum likelihood tree of Cytolysins with 100 bootstrap replicates (only bootstrap values > 50 are shown). I—proteins without the MACPF domain, II—proteins with the MACPF domain, III—toxins from Actiniidae family members, IV—toxins from Stichodactilidae family members and Oulactis orientalis (Actiniidae ), V—toxins from Sagartiidae and and Alisiidae family members. Toxins are also referred on the Cytolysins chapter.

Figure 4. Maximum likelihood tree of Cytolysins with 100 bootstrap replicates (only bootstrap values > 50 are shown). I—proteins without the MACPF domain, II—proteins with the MACPF domain, III—toxins from Actiniidae family members, IV—toxins from Stichodactilidae family members and Oulactis orientalis (Actiniidae ), V—toxins from Sagartiidae and and Alisiidae family members. Toxins are also referred on the Cytolysins chapter.

Considering the phylogenetic tree of cytolysis, two major groups can be defined; one including the proteins without the MACPF domain (I) and the other comprehending those with the MACPF domain (II). Within the major group “I” three clusters can be identified (III to V). Toxins from Actiniidae family members are clustered in group III. In Group IV cluster toxins from Stichodactilidae family members and toxins from Oulactis orientalis, (Actiniidae). In fact, toxins from Oulactis are more closely related to Stichodactilidae than to Actiniidae toxins. As mentioned previously, Or-A and Or-G and RTX-S-II and RTX-A from Hecteractis crispa have in common (albeit others characteristics), the substitution of a Trp by a Leu in the position Trp112 of Equinatoxin-II. Moreover, the conserved RGD sequence that occurs in Sticholysin-II, RTX-A and Equinatoxin-II, in the toxins from Oulactis is replaced by the GGD sequence. The cluster V includes the Src-I and the toxins from Alisiidae family. The only member of Sagartiidae family (Src-I), has the EGD sequence instead of the RGD motif. Toxins of Alisiidae family members, share a similar gene organization with three exons (two introns). In addition the RGD motif is replaced by the KPS tripeptides in PsTX-20A and Avt.

Regarding the sea anemones phospholipases toxins, the study of Romero and co-workers [33] comparing PLA2 from Condylactis gigantea (Actiniidae family member), CgPLA2, with the other PLA2s from five animal phyla, suggested that sea anemones PLA2s form a monophyletic group. Within this group, CgPLA2 showed to be closer to the Adamsia carcinoapados (Hormathiidae family member) PLA2, AcPLA2, than to others of Nematostella vectensis, suggesting a significant divergence from the latter.


Anthopleura xanthogrammica, or the giant green anemone, is a species of intertidalsea anemone of the familyActiniidae.

Other common names for this anemone include green surf anemone, giant green sea anemone, green anemone, giant tidepool anemone, anemone, and rough anemone. [2]

Description[edit]

The column width and height can reach a maximum of 17.5[3] and 30 cm, respectively.[4] The crown of tentacles can be as wide as 25 cm in diameter,[4] while the column, itself, tends to be widest at the base in order to offer a more stable connection to the rocks.[5]

It has a broad, flat oral disk surface[6] and no striping, banding, or other markings.[5]

Coloration[edit]

If A. xanthogrammica is exposed to proper amounts of sunlight, it can appear bright green[5] when submerged under water.

When not submerged, it appears dark green or brown. This is because the anemone tends to close up and "droop" and its now exposed column is actually dark green and slightly brown, but the hidden tentacles and oral disk are bright green.[3]

Tentacles[edit]

The tentacles, which are short and conical,[3] are arranged in six or more rows surrounding the oral disk[4][7] and can be pointed or blunt at the tips.[5]

Distribution[edit]

Generally, A. xanthogrammica is found along the low to mid intertidal zones of the Pacific Ocean, from Alaska to southern California and sometimes downwards to Panama, where cold water swells can occur.[4][5][7][8]

Habitat[edit]

A. xanthogrammica prefers to inhabit sandy or rocky shorelines, where water remains for most of the day.[4] They can generally be found in tide pools up to 15 m deep.[3] Occasionally A. xanthogrammica can also be found in deep channels of more exposed rocky shores and concrete pilings in bays and harbors.[5]

Biology and natural history[edit]

Photosynthetic algae, zoochlorellae, and the dinoflagellates, zooxanthellae, live in the epidermis and tissue of the gut of A. xanthogrammica. In this symbiotic relationship, the zoochlorellae and zooxanthellae provide nutrients to the anemone via photosynthesis and contribute to the bright green color of the anemone's oral disk and tentacles.[4][7] The bright green color is also due to pigmentation.[5]

Anthopleura xanthogrammica anemones living in caves and shady zones have reduced or no natural symbionts and tend to be less colorful.[3][4][5][7]

Behavior[edit]

These anemones tend to live a solitary life, but can be occasionally seen as groups with no more than 14 individuals per square meter.[4][5][7] They can move slowly using their basal disks, but usually stay sessile.[4][7] Like other anemones, A. xanthogrammica can use stinging cells located in the tentacles as protection from predators and a mechanism to capture prey.[4][7]

Reproduction[edit]

Anthopleura xanthogrammica reproduce sexually via external fertilization of sperm and eggs in the late fall. Newly formed pelagic, planktotrophic larvae float in the water until dispersing and settling in mussel beds.[3][4][7]

Feeding[edit]

Nematocysts found in the tentacles assist A. xanthogrammica to catch and paralyze prey.[3][4][7] After feeding and digestion is complete,the anemone excretes its waste back through the mouth opening.[4][7]

Predators and prey[edit]

Main predators of A. xanthogrammica include: the leather seastar Dermasterias imbricata,[5] the nudibranch Aeolidia papillosa and the snail Epitonium tinctum (both feed on the tentacles), and the snails Opalia chacei and Opalia funiculata and the sea spider Pycnogonum stearnsi (that feed on the column).[4][7]

The anemone feeds on sea urchins, small fish, and crabs, but detached mussels seem to be the main food source.[5][7] There are rare instances where the giant green anemone has consumed seabirds.[9] It is not known whether the birds were alive or dead when engulfed by the anemone.

Similar species[edit]

Occasionally, A. xanthogrammica can be confused with large individuals of A. elegantissima or A. sola, but both of these other anemones have (usually) pink-tipped tentacles and a striped oral disk, unlike A.xanthogrammica. The anemone pictured bottom-right appears to an example of a sunburst anemone (Anthopleura sola) with its distinct radial stripes and is often confused with the giant green given its color.[5]

References[edit]

  1. ^Anthopleura xanthogrammica (Brandt, 1835) World Register of Marine Species. Retrieved 2011-11-22.
  2. ^Lamb, A and B Handy. 2005. Marine Life of the Pacific Northwest. Harbour Publishing, British Columbia: 85.
  3. ^ abcdefgLaroche, C. (2005). Anthopleura xanthogrammica (on-line), Race Rocks.com. Accessed May 10, 2010 at http://www.racerocks.com/racerock/eco/taxalab/2005/anthopleurax/anthopleurax.htm
  4. ^ abcdefghijklmnSkiles, M. 2001. Anthopleura xanthogrammica (On-line), Animal Diversity Web. Accessed May 11, 2010 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Anthopleura_xanthogrammica.html
  5. ^ abcdefghijklWhite, B. (2004). Anthopleura xanthogrammica (on-line), Walla Walla University. Accessed May 10, 2010 at http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Cnidaria/Class-Anthozoa/Subclass_Zoantharia/Order_Actiniaria/Anthopleura_xanthogrammica.html
  6. ^Kozloff, E. (1973). Seashore Life of the Northern Pacific Coast. University of Washington Press, Seattle, 166-167.
  7. ^ abcdefghijklEncyclopedia of Life. (2010). Anthopleura xanthogrammica (on-line), EOL.org. Accessed May 10, 2010 at http://www.eol.org/pages/704306
  8. ^Gotshall, D. (2005). Guide to Marine Invertebrates. Shoreline Press, Santa Barbara:30.
  9. ^http://www.marineornithology.org/PDF/42_1/42_1_1-2.pdf
Spectacular lineup of Giant greens at Hazard Reef, Montana de Oro State Park. Channel is about a foot (30cm) wide. It drains a large tidepool and must have lots of good food.

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