What are Acantharia, and what is their primary distinguishing feature?

Acantharia are a class of single-celled organisms, specifically a group of radiolarian protozoa. They are primarily distinguished by their skeletons, which are composed of strontium sulfate.

What is the scientific classification of Acantharia within the biological hierarchy?

Acantharia belong to the Domain Eukaryota, Clade Sar, Clade Rhizaria, Phylum Retaria, Subphylum Radiolaria, and are classified as the Class Acantharia. This classification was established by Haeckel in 1881 and emended by Mikrjukov in 2000.

What is the typical size range and habitat of Acantharia?

Acantharians are heterotrophic marine microplankton, meaning they are microscopic organisms that drift in the ocean and obtain nutrients by consuming other organisms. They typically range in size from approximately 200 microns in diameter up to several millimeters.

How do some acantharians obtain their energy, and what term describes this nutritional strategy?

Some acantharians host photosynthetic endosymbionts, which are organisms living within their cells that can convert sunlight into energy. Due to this ability to acquire energy through both consuming other organisms (heterotrophy) and producing their own food (autotrophy), they are considered mixotrophs.

What is the chemical composition of Acantharian skeletons, and what mineral form does it take?

Acantharian skeletons are composed of strontium sulfate (SrSO4), which takes the form of the mineral celestine crystal. The source material includes an image of a celestine crystal, which is known for its delicate blue color.

What are the notable physical properties of celestine, and how do they impact Acantharia?

Celestine is recognized for its delicate blue color and is the heaviest mineral found in the ocean. The high density of celestite ensures that acantharian shells function as mineral ballast, which causes them to settle rapidly to bathypelagic depths, referring to the deep ocean zone.

What observations have been made regarding the sedimentation of acantharian cysts in certain ocean basins?

High settling fluxes of acantharian cysts have been observed in areas such as the Iceland Basin and the Southern Ocean. At times, these fluxes have accounted for as much as half of the total gravitational organic carbon flux, indicating their significant contribution to the transport of organic matter to the deep sea.

How do Acantharians form their strontium sulfate crystals, and what makes this process unique?

The strontium sulfate crystals are secreted by vacuoles that surround each spicule or spine within the acantharian cell. Acantharians are unique among marine organisms for their ability to biomineralize strontium sulfate as the primary component of their skeletons.

Why do Acantharian skeletons not fossilize, unlike those of other radiolarians?

Unlike other radiolarians whose skeletons are made of silica, acantharian skeletons do not fossilize. This is primarily because strontium sulfate is very scarce in seawater, causing the crystals to dissolve after the acantharians die.

How is the precise arrangement of spines on an Acantharian described?

The arrangement of spines on an Acantharian is highly precise and is described by the Müllerian law. This law defines the spines' positions in terms of lines of latitude and longitude, where they lie at the intersections of five lines of latitude (symmetric about an equator) and eight uniformly spaced lines of longitude. Each line of longitude alternately carries either two tropical spines or one equatorial and two polar spines.

What are the two main regions of the Acantharian cell cytoplasm, and what are their primary contents?

The cell cytoplasm of an Acantharian is divided into two regions: the endoplasm and the ectoplasm. The endoplasm, located at the cell's core, contains the main organelles, including multiple nuclei. The ectoplasm consists of cytoplasmic extensions used for capturing prey and also contains food vacuoles for digesting the captured prey.

How is the endoplasm delineated from the ectoplasm, and where are symbionts found in symbiotic species?

The endoplasm is separated from the ectoplasm by a capsular wall, which is made of a microfibril mesh. In symbiotic species, the algal symbionts are maintained within the endoplasm.

What is the structure surrounding the ectoplasm, and what is its connection to the spines?

The ectoplasm is surrounded by a periplasmic cortex, which is also composed of microfibrils but arranged into twenty distinct plates. Each of these plates has a hole through which one spicule projects. This cortex is connected to the spines by contractile myonemes.

What is the function of myonemes in Acantharians?

Contractile myonemes, which link the periplasmic cortex to the spines, assist in buoyancy control. They allow the ectoplasm to expand and contract, thereby increasing and decreasing the total volume of the cell, which helps the organism regulate its position in the water column.

What is a key characteristic used for classifying Acantharians taxonomically?

One of the primary characteristics used to classify acantharians is the manner in which their spines are joined at the center of the cell. Their skeletons are made up of either ten diametric spicules or twenty radial spicules.

How do diametric and radial spicules differ in Acantharian skeletons?

Diametric spicules are those that cross the center of the cell, extending through it. In contrast, radial spicules terminate at the center of the cell, where they form either a tight or flexible junction, depending on the specific species.

Which Acantharian orders are known for their ability to form cysts?

Acantharians with diametric spicules or loosely attached radial spicules are capable of rearranging or shedding their spicules and forming cysts. Specifically, the Holacanthida and Chaunacanthida orders are capable of encystment.

What are the defining features of the Holacanthida order?

The Holacanthida order is characterized by having 10 diametric spicules that are simply crossed without a central junction. Members of this order are capable of encystment.

What are the defining features of the Chaunacanthida order?

The Chaunacanthida order is characterized by having 20 radial spicules that are loosely attached at the center of the cell. Members of this order are capable of encystment.

What are the defining features of the Symphyacanthida order?

The Symphyacanthida order is characterized by having 20 radial spicules that form a tight central junction within the cell.

What are the defining features of the Arthracanthida order?

The Arthracanthida order is characterized by having 20 radial spines that form a tight central junction within the cell.

How does the morphological classification of Acantharians align with phylogenetic trees?

The morphological classification system of Acantharians generally agrees with phylogenetic trees that are based on the alignment of ribosomal RNA genes. However, it is noted that the groups defined morphologically are mostly polyphyletic, meaning they do not consist of a single common ancestor and all its descendants.

What is the hypothesized evolutionary order of Acantharian clades based on molecular data and skeleton complexity?

Based on molecular data, Holacanthida appears to have evolved first, followed by Chaunacanthida. Arthracanthida and Symphacanthida, which possess the most complex skeletons, are believed to have evolved most recently, suggesting an increase in skeletal complexity over evolutionary time.

Which molecular clades are associated with the Holacanthida order?

The Holacanthida order includes molecular clades A, B, and D.

Which molecular clade is associated with the Chaunacanthida order?

The Chaunacanthida order includes only one molecular clade, clade C.

Which molecular clades are associated with the Arthracanthida and Symphacanthida orders?

The Arthracanthida and Symphacanthida orders constitute molecular clades E and F, respectively.

Which Acantharian clades are known to host single-celled algae within their inner cytoplasm?

Many acantharians host single-celled algae within their inner cytoplasm, specifically some species in clade B (Holacanthida) and all species in clades E and F (Symphiacanthida and Arthracanthida).

What is photosymbiosis, and how does it characterize the relationship between acantharians and their algal partners?

Photosymbiosis is a symbiotic relationship where one organism (the host) benefits from the photosynthetic activity of another organism (the symbiont). In acantharians, this means they acquire energy through both heterotrophy (consuming prey) and autotrophy (from their algal symbionts' photosynthesis), making them mixotrophs.

What are the hypothesized advantages for acantharians engaging in photosymbiosis?

It is hypothesized that photosymbiosis enables acantharians to thrive in low-nutrient oceanic regions. Additionally, the relationship may provide the extra energy required to maintain their intricate strontium sulfate skeletons.

What is the proposed exchange of nutrients between acantharians and their algal symbionts?

It is hypothesized that acantharians provide their algal symbionts with essential nutrients like nitrogen (N) and phosphorus (P), which they obtain by capturing and digesting prey. In return, the algae produce sugars through photosynthesis, which the acantharians then utilize for energy.

Is the symbiotic relationship always mutually beneficial for the algal symbionts?

It is not definitively known whether the algal symbionts consistently benefit from the relationship. There is a possibility that they are simply being exploited by the acantharians, which may digest them after utilizing their photosynthetic products.

What types of symbionts are hosted by symbiotic Holacanthida acantharians?

Symbiotic Holacanthida acantharians host a diverse range of symbiont assemblages. These include several genera of dinoflagellates, such as *Pelagodinium*, *Heterocapsa*, *Scrippsiella*, and *Azadinium*, as well as a haptophyte genus, *Chrysochromulina*.

What does the observed mismatch between internal and external symbiont communities in Clade F acantharians suggest?

The mismatch between the internal symbiont communities of Clade F acantharians and the availability of potential symbionts in the surrounding environment suggests that these acantharians are selective in choosing their symbionts. This also implies that they likely maintain their symbionts for extended periods rather than continuously digesting and recruiting new ones.

What is a characteristic feature of adult Acantharians in terms of their cellular structure?

Adult Acantharians are typically multinucleated, meaning their cells contain multiple nuclei.

What role do cysts play in the life cycle of earlier diverging Acantharian clades?

Earlier diverging Acantharian clades are capable of shedding their spines and forming cysts, which are often referred to as reproductive cysts. These cysts are believed to be involved in the reproductive process.

How is reproduction generally thought to occur in Acantharians?

Reproduction in Acantharians is thought to occur through the formation of swarmer cells, which were previously known as 'spores'. Cysts have been observed to release these swarmers, and non-encysted cells have also been seen releasing swarmers under laboratory conditions.

Have all stages of the Acantharian life cycle been fully observed and understood?

No, not all life cycle stages of Acantharians have been observed. Specifically, the fusion of swarmer cells to produce a new acantharian has not yet been witnessed, indicating gaps in the understanding of their complete reproductive cycle.

What is the significance of acantharian cysts being found in sediment traps?

The frequent discovery of acantharian cysts in sediment traps suggests that these cysts play a crucial role in helping acantharians sink into deep water, contributing to the vertical transport of biomass in the ocean.

What is hypothesized about the reproductive strategy of non-cyst-forming acantharians?

Genetic data and some imaging studies suggest that non-cyst-forming acantharians may also sink to deep water to release their swarmer cells, similar to the function of cysts in other clades.

What potential benefit does releasing swarmer cells in deeper water offer to Acantharian juveniles?

Releasing swarmer cells in deeper water is believed to improve the survival chances of Acantharian juveniles, possibly by providing a more stable or protected environment away from surface predators or harsh conditions.

What is the primary obstacle hindering the comprehensive study of Acantharian life cycles?

The main challenge that has hampered the study of Acantharian organisms is the inability to 'close the lifecycle' and successfully maintain these organisms in culture through successive generations, making it difficult to observe their full developmental stages.

Who initially classified the Class Acantharia, and when was it emended?

The Class Acantharia was initially classified by Haeckel in 1881 and was later emended by Mikrjukov in 2000.

What is the common name for the mineral form of strontium sulfate found in Acantharian skeletons?

The mineral form of strontium sulfate (SrSO4) that constitutes Acantharian skeletons is called celestine.

What is the purpose of the microfibril mesh capsular wall in Acantharians?

The microfibril mesh capsular wall delineates the endoplasm, which contains the main organelles and nuclei, from the ectoplasm, effectively dividing the cell's cytoplasm into two distinct regions.

What are cytoplasmic extensions in the ectoplasm used for?

The cytoplasmic extensions found in the ectoplasm of Acantharians are utilized for prey capture, allowing the organism to actively acquire food from its marine environment.

What is the function of food vacuoles located in the ectoplasm?

Food vacuoles, located within the ectoplasm, are responsible for the digestion of captured prey, processing the nutrients for the acantharian.

How many plates make up the periplasmic cortex, and what is their structural feature?

The periplasmic cortex, which surrounds the ectoplasm, is made up of twenty plates. Each of these plates features a hole through which one of the acantharian's spicules projects.

What is the relationship between the complexity of Acantharian skeletons and their evolutionary timeline?

Acantharian clades with more complex skeletons, specifically Arthracanthida and Symphacanthida, are believed to have evolved more recently compared to those with simpler skeletons like Holacanthida and Chaunacanthida.

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Acantharia: An Overview

Microscopic Marine Wonders

Acantharia represent a distinct class of radiolarian protozoa, single-celled organisms that are a vital component of marine microplankton. Their most striking feature is their intricate internal skeleton, uniquely composed of strontium sulfate. These organisms typically range in size from approximately 200 micrometers to several millimeters, making them visible under a microscope and, in some cases, to the naked eye.

Heterotrophs and Mixotrophs

Primarily, Acantharia are heterotrophic, meaning they obtain nutrients by consuming other organisms. However, a significant number of acantharian species engage in a fascinating symbiotic relationship with photosynthetic endosymbionts. This allows them to function as mixotrophs, deriving energy from both predation and photosynthesis, a strategy that is particularly advantageous in nutrient-poor oceanic environments.

Ecological Significance

As marine microplankton, Acantharia play a role in the oceanic food web and biogeochemical cycles. Their unique strontium sulfate skeletons, while not fossilizing, contribute to the vertical flux of organic carbon in the ocean. The rapid sedimentation of their cysts, particularly observed in regions like the Iceland Basin and the Southern Ocean, highlights their contribution to the deep ocean's particle flux.

Intricate Morphology

The Strontium Sulfate Skeleton

The defining characteristic of Acantharia is their skeleton, meticulously constructed from strontium sulfate (SrSO₄) in the form of celestine crystals. This biomineralization of strontium sulfate is a unique capability among marine organisms. Celestine, known for its delicate blue hue and significant density, acts as a mineral ballast, facilitating the rapid sinking of acantharian remains and cysts to bathypelagic depths upon the organism's demise. Unlike the silica skeletons of other radiolarians, acantharian skeletons do not fossilize due to the scarcity of strontium sulfate in seawater and the subsequent dissolution of the crystals post-mortem.

Precise Spine Architecture

The arrangement of the acantharian spines is remarkably precise, adhering to a geometric principle known as the Müllerian law. This law describes the spines as lying at the intersections of five lines of latitude, symmetrically positioned around an equator, and eight lines of longitude, uniformly spaced. Each longitudinal line alternately bears either two "tropical" spines or one "equatorial" and two "polar" spines, creating a highly ordered and aesthetically complex skeletal framework.

Dual Cytoplasmic Regions

The cellular structure of Acantharia is distinctly divided into two primary regions: the endoplasm and the ectoplasm. The endoplasm, forming the central core of the cell, houses the main organelles and numerous nuclei. It is separated from the ectoplasm by a capsular wall, a mesh-like structure composed of microfibrils. In species that host symbiotic algae, these algal partners are typically found within the endoplasm. The ectoplasm, extending outwards, is crucial for prey capture and contains food vacuoles for digestion. Surrounding the ectoplasm is a periplasmic cortex, also made of microfibrils, arranged into twenty distinct plates, each perforated to allow a spicule to project through. Contractile myonemes connect this cortex to the spines, enabling the ectoplasm to expand and contract, thereby regulating the cell's buoyancy.

Classification & Phylogeny

Spine Junctions as Classifiers

The primary method for classifying Acantharia hinges on the manner in which their skeletal spines converge at the cell's center. Skeletons are composed of either ten diametric spicules, which traverse the cell's core, or twenty radial spicules, which terminate at the center. The nature of this central junction—whether tight or flexible—is a key distinguishing feature. Acantharians possessing diametric spicules or loosely attached radial spicules exhibit the remarkable ability to rearrange or even shed their spicules, a mechanism often associated with the formation of protective cysts.

Orders of Acantharia

The class Acantharia is traditionally divided into several orders based on these skeletal characteristics. While morphological classifications generally align with phylogenetic trees derived from ribosomal RNA gene sequences, it is noted that many of these groups are polyphyletic, indicating complex evolutionary pathways.

The primary orders of Acantharia include:

Holacanthida: Characterized by 10 diametric spicules that simply cross without a central junction. These species are capable of encystment and are considered to have evolved earlier, encompassing molecular clades A, B, and D.
Chaunacanthida: Possess 20 radial spicules that are loosely attached at the cell's center. Like Holacanthida, they can form cysts and represent an intermediate evolutionary stage, corresponding to molecular clade C.
Symphyacanthida: Distinguished by 20 radial spicules that form a tight central junction. These are among the more recently evolved groups, constituting parts of molecular clades E and F, and typically feature more complex skeletons.
Arthracanthida: Also exhibit 20 radial spines with a tight central junction, sharing the more recent evolutionary trajectory and complex skeletal structures with Symphyacanthida, belonging to molecular clades E and F.

This classification system provides a framework for understanding the diversity within Acantharia, reflecting both their structural variations and evolutionary relationships.

The Symbiotic Life

Photosymbiosis in the Endoplasm

A remarkable feature of many acantharians is their engagement in photosymbiosis, hosting single-celled algae within their inner cytoplasm, the endoplasm. This includes certain species within clade B (Holacanthida) and all species in clades E and F (Symphiacanthida and Arthracanthida). This symbiotic relationship transforms these acantharians into mixotrophs, enabling them to harness energy through both heterotrophic feeding and autotrophic photosynthesis. This dual energy acquisition strategy is hypothesized to be crucial for their prevalence in the oligotrophic (low-nutrient) regions of the oceans and may also provide the substantial energy required to maintain their elaborate strontium sulfate skeletons.

A Reciprocal Exchange?

The symbiotic interaction is thought to involve a reciprocal exchange of resources. It is hypothesized that the acantharians supply their algal partners with essential nutrients, such as nitrogen and phosphorus, which they obtain through the capture and digestion of prey. In return, the algae provide the acantharians with sugars produced during photosynthesis. However, the precise nature of the benefit to the algal symbionts remains an area of active research; it is not definitively known whether the algae truly benefit from the relationship or if they are primarily exploited and subsequently digested by their acantharian hosts.

Diverse Symbiont Communities

The diversity of algal symbionts varies across different acantharian clades. Symbiotic Holacanthida acantharians are known to host a wide array of symbiont assemblages, including several genera of dinoflagellates (e.g., Pelagodinium, Heterocapsa, Scrippsiella, Azadinium) and a haptophyte (Chrysochromulina). In contrast, acantharians from clades E and F exhibit a more specific symbiosis, predominantly hosting symbionts from the haptophyte genus Phaeocystis, though Chrysochromulina symbionts are occasionally observed. Intriguingly, clade F acantharians can simultaneously host multiple species and strains of Phaeocystis. The observed discrepancy between the internal symbiont community and the external availability of potential symbionts suggests that acantharians are selective in their choice of partners and likely maintain these symbionts for extended periods rather than continuously digesting and recruiting new ones.

The Enigmatic Lifecycle

Reproduction and Encystment

Acantharian adults are typically multinucleated organisms. A key aspect of their reproductive strategy, particularly in earlier diverging clades, involves the ability to shed their characteristic spines and form cysts. These structures are often referred to as reproductive cysts, indicating their role in the organism's life cycle. Reproduction is believed to occur through the formation of "swarmer cells," which are often flagellate. Observations in laboratory settings have shown both cysts and non-encysted cells releasing these swarmers. However, a complete understanding of the acantharian life cycle remains elusive, as the fusion of swarmers to produce a new acantharian has yet to be directly observed.

Deep-Water Dispersal Strategy

Acantharian cysts are frequently detected in sediment traps, leading to the hypothesis that these cysts play a crucial role in enabling acantharians to sink into the deep ocean. This deep-water dispersal strategy is thought to enhance the survival prospects of juvenile acantharians by releasing swarmer cells in environments potentially more conducive to their development. Furthermore, genetic data combined with imaging studies suggest that even non-cyst-forming acantharians may undertake a similar descent to deeper waters for the release of their swarmers. The inability to sustain these organisms through successive generations in laboratory cultures has significantly hindered comprehensive research into their full life cycle.

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Ocean's Crystalline Architects

💡 Dive in with Flashcard Learning! 💡

Acantharia: An Overview 🌊

🔬 Microscopic Marine Wonders

🍽️ Heterotrophs and Mixotrophs

🌍 Ecological Significance

Intricate Morphology 🐚

💎 The Strontium Sulfate Skeleton

📐 Precise Spine Architecture

🧬 Dual Cytoplasmic Regions

Classification & Phylogeny 🌳

🔗 Spine Junctions as Classifiers

📜 Orders of Acantharia

The Symbiotic Life 🤝

🌿 Photosymbiosis in the Endoplasm

🔄 A Reciprocal Exchange?

🐠 Diverse Symbiont Communities

The Enigmatic Lifecycle 🔄

🌱 Reproduction and Encystment

⬇️ Deep-Water Dispersal Strategy

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Dive in with Flashcard Learning!

Acantharia: An Overview

Microscopic Marine Wonders

Heterotrophs and Mixotrophs

Ecological Significance

Intricate Morphology

The Strontium Sulfate Skeleton

Precise Spine Architecture

Dual Cytoplasmic Regions

Classification & Phylogeny

Spine Junctions as Classifiers

Orders of Acantharia

The Symbiotic Life

Photosymbiosis in the Endoplasm

A Reciprocal Exchange?

Diverse Symbiont Communities

The Enigmatic Lifecycle

Reproduction and Encystment

Deep-Water Dispersal Strategy

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice