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Secernentea
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Secernentea
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Nematoda
Class: Secernentea
Subclasses

Diplogasteria (disputed)
Rhabditia (paraphyletic?)
Spiruria
Tylenchia (disputed)
and see text

Secernentea was a class of nematodes in the Classical Phylogeny System (Chitwood, 1958) and is no longer in use.[1][2] This morphological-based classification system has been replaced by the Modern Phylogeny system, where taxonomy assignment is based on small subunit ribosomal DNA (SSU rDNA).[3][4]

Description

[edit]

Amphid apertures of Secernenteas are pore or slit-like.

Some Secernenteas have deirids which are located near nerve ring.

Secernenteas have posterior phasmids.

The excretory system of a Secernentea is tubular.

Cuticles of Secernenteas are striated in two to four layers. Lateral field is present.

Secernenteas have three esophageal glands. Their esophageal structure varies.

Male Secernenteas generally have one testis.

Caudal alae are common in Secernenteas.

Sensory papillae of Secernenteas are cephalic only. It might be caudal papillae in male Secernenteas.

Secernenteas are mostly found in land but they are rarely found in marine water or freshwater.

Taxonomy

[edit]
See also: List of nematode families

Subclasses and orders of Secernentea are:[5]

Some families traditionally considered to be Rhabditida seem to be closer to the Tylenchida. If the Tylenchia are to be maintained as separate, they probably will be included therein.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Secernentea

View on Grokipedia
from Grokipedia
Secernentea is a major lineage within the phylum Nematoda, traditionally recognized as a class in classical and currently classified as a subclass under the class Chromadorea in modern phylogenetic systems. Characterized primarily by the presence of phasmids—paired, unicellular chemosensory organs located posteriorly on the body—these nematodes also feature a tubular with lateral canals and pore-like or slit-like amphid apertures positioned labially. This group encompasses a diverse array of free-living and parasitic species, predominantly terrestrial but occasionally found in freshwater or marine environments, and includes economically and medically significant forms such as plant-parasitic root-knot nematodes and human pathogens like hookworms and roundworms. In the classical classification system established by Chitwood in 1958, Secernentea (also known as Phasmidia) formed one of two primary classes of nematodes, contrasted with Adenophorea (Aphasmidia) based on morphological traits like the absence of caudal glands and the presence of phasmids. However, , including analyses of small subunit (SSU rDNA), have revealed Secernentea as a monophyletic nested within , with accelerated evolutionary rates in crown groups linked to parasitic lifestyles and short generation times. This modern framework, supported by comprehensive genomic and developmental data, emphasizes synapomorphies such as three esophageal glands, a single testis in males, and common caudal alae, distinguishing Secernentea from basal nematode s like . Secernentea comprises over 15,000 described species across 12 orders, including (free-living microbivores like Caenorhabditis elegans, a key in ), Tylenchida (major plant parasites causing agricultural damage), Strongylida (vertebrate parasites such as hookworms), and Ascaridida (intestinal parasites of s and animals). Subdivisions historically included subclasses like Rhabditia, Spiruria, and Diplogastria, reflecting ecological roles from soil bacterivores to filarial worms transmitted by insects. These nematodes play critical roles in ecosystems as decomposers and parasites, influencing , , and , with ongoing research highlighting their evolutionary adaptations for host exploitation.

Introduction

Definition and Historical Context

Secernentea represents an obsolete class within the phylum Nematoda, encompassing nematodes distinguished by the presence of phasmids—paired chemosensory organs situated laterally near the posterior end—and a tubular featuring lateral canals that facilitates and waste elimination. This emphasized morphological traits such as the shape of amphids (anterior sensory organs) and caudal structures to delineate evolutionary relationships among groups. The taxonomic framework for Secernentea originated from early 20th-century efforts to organize the diverse based on observable , building on N.A. Cobb's foundational 1919 proposal for the phylum Nemata, which introduced a hierarchical system of orders and classes influenced by buccal cavity and overall body structure. Cobb's work laid the groundwork for subsequent refinements, particularly in grouping forms with prominent sensory features, though the specific term "Phasmidia" emerged later to denote nematodes bearing phasmids, as formalized by B.G. Chitwood and M.B. Chitwood in 1933. Chitwood further developed these ideas, initially proposing "Secernentea" alongside "Adenophorea" in 1937 to reflect functional differences in glandular and secretory systems, but administrative constraints led to the temporary use of "Phasmidia" and "Aphasmidia"; he rectified this in his seminal 1958 publication, establishing Secernentea as the preferred nomenclature within the classical system. This binary division into Secernentea and Adenophorea (the latter lacking phasmids) was instituted to streamline phylogeny by prioritizing the presence or absence of phasmids as a primary diagnostic criterion, thereby separating secretory-active forms from those reliant on glandular excretory mechanisms. Key publications spanning to , including Cobb's structural analyses and Chitwood's iterative classifications, solidified this morphology-driven approach as the cornerstone of mid-20th-century . Although influential, the Secernentea class has since been superseded by , rendering it obsolete in contemporary .

Significance in Nematode Biology

Secernentea represents a major portion of described nematode species, comprising over 15,000 described and encompassing a diverse array of free-living, parasitic, and symbiotic forms that play crucial roles in ecosystems and host-parasite interactions. With estimates indicating that s as a whole number around 4.4 × 10^{20} individuals in the Earth's alone, Secernentea contributes significantly to this ubiquity, facilitating cycling and processes essential for terrestrial habitats. These nematodes, including bacterivores and fungivores, act as primary decomposers in food webs, transforming into forms available for plant uptake and influencing microbial communities. The evolutionary significance of Secernentea lies in its into terrestrial environments, where innovations like the dauer larva stage enabled survival in fluctuating conditions and facilitated transitions to . This radiation, occurring within the Chromadoria (corresponding to Secernentea in classical ), underscores the group's success in diverse niches, from free-living soil dwellers to obligate parasites of and animals. A prominent example is from the order, which exemplifies how Secernentea species have colonized decaying , highlighting evolutionary adaptations for rapid and environmental resilience. Secernentea species have profoundly influenced biological research, serving as key model organisms for studies in , development, and aging. C. elegans, with its fully sequenced published in , has enabled groundbreaking discoveries, including the elucidation of over 19,000 genes and their roles in cellular processes. This work paved the way for the 2006 Nobel Prize in Physiology or Medicine awarded to and for discovering (RNAi) through experiments on C. elegans, a mechanism that revolutionized techniques across . Beyond research, Secernentea impacts and by modeling dynamics and parasitic diseases. As decomposers, they enhance and , while parasitic forms like those in Tylenchida inform strategies for crop protection and understanding host immune responses. Their symbiotic associations, such as with in entomopathogenic nematodes, further illustrate complex interactions that affect and vector biology.

Morphology and Anatomy

Diagnostic Features

Secernentea are distinguished by their sensory organs, particularly the amphids and phasmids. The amphids feature pore-like or slit-like apertures located labially, serving as chemosensory structures that open directly into the . Phasmids, a defining trait, are paired posterior chemoreceptors typically positioned near the , consisting of glandular and sensory elements that aid in environmental sensing. Some taxa also possess deirids, which are small cervical sensory organs located near the nerve ring, providing additional tactile input. The in Secernentea is characteristically tubular, forming an H-shaped configuration with paired lateral s joining a prominent ventral that connects to a single excretory pore anterior to the nerve ring. This system includes a ventral gland cell responsible for and , often with cuticle-lined ducts for efficient fluid transport. Unlike simpler glandular systems in other nematodes, this tubular arrangement supports the diverse terrestrial and parasitic lifestyles prevalent in the class. The of Secernentea is multilayered, typically composed of two to four strata with fine transverse striations, and features a prominent lateral field that appears as longitudinal incisures or ridges along the body, enhancing structural integrity and flexibility. Internally, the exhibits variation, such as rhabditiform or tylenchiform regions, but consistently includes three esophageal glands that open anteriorly, a well-defined metacorpus (median bulb) for pumping, and a postcorpus for glandular . Reproductive and caudal features further define Secernentea morphology. Males generally possess a single reflexed testis, producing amoeboid , with the reproductive system opening into a equipped with paired equal spicules for copulation. Caudal alae, wing-like cuticular expansions, are common in males, often supported by numerous caudal papillae (typically more than 10), which function in sensory guidance during mating. These traits contrast with more variable arrangements in other groups, emphasizing the class's specialized reproductive adaptations.

Comparative Traits with Other Nematodes

Secernentea, in classical , were distinguished from the sister class Adenophorea (also known as or Aphasmidia) primarily through morphological traits that reflected adaptations to different environments and lifestyles, making the divide intuitively based on observable anatomy. These differences, particularly in sensory, excretory, and reproductive structures, underscored the historical separation into Phasmidia (Secernentea) and Aphasmidia (Adenophorea), with phasmids serving as a hallmark trait unique to Secernentea. A prominent contrast lies in the amphids, the principal anterior chemosensory organs. In Secernentea, amphids are typically labial in position, featuring simple pore-like or slit-like apertures that open directly at or near the lip region. In contrast, Adenophorea exhibit post-labial amphids, often more elaborate in shape, such as pocket-like, spiral, or convoluted forms that extend posteriorly from the head. This positional and structural variation likely relates to differing sensory demands in their respective habitats. Phasmids, paired cuticular sensory organs located posteriorly near the , are universally present in Secernentea, functioning potentially in chemoreception or . Their absence in Adenophorea contributed to the subclass name Aphasmidia and highlights a key evolutionary divergence in tail-end sensory capabilities. The excretory systems further differentiate the classes, with Secernentea possessing a more complex tubular arrangement featuring prominent lateral canals and tubules that facilitate active ion regulation and , essential for terrestrial existence. Adenophorea, conversely, have simpler glandular structures, often comprising renette cells or short canals without extensive lateral components, suiting their predominantly aquatic lifestyles. Caudal morphology in males also varies significantly: Secernentea typically display numerous caudal papillae and well-developed alae (wing-like cuticular expansions) that aid in copulation, with sensory papillae concentrated anteriorly but extending to the tail. Adenophorea males, by comparison, have fewer caudal papillae, rare alae, and more distributed somatic sensory papillae along the body. In females, Secernentea commonly feature paired (didelphic) reflexed gonads, supporting prolific egg production adapted to variable terrestrial conditions. Adenophorea show greater variability, often with single or paired outstretched gonads, reflecting their diverse aquatic niches. These traits correlate strongly with habitat preferences, as Secernentea are almost exclusively terrestrial, benefiting from enhanced osmoregulatory mechanisms to cope with and ionic fluctuations in environments. Adenophorea, dominating marine and freshwater ecosystems, retain simpler systems aligned with stable aquatic .

Classical Taxonomy

Subclasses and Orders

The classical taxonomy of Secernentea, established by Chitwood in , divides the class into subclasses such as Rhabditia, Spiruria, Diplogastria, and Tylenchia (though groupings vary among authors), encompassing numerous orders based primarily on morphological traits such as phasmid structure and esophageal features. Subclass Diplogastria includes the order Diplogastrida, which comprises predominantly free-living soil nematodes adapted to bacterial-feeding habits through specialized stomal structures. Subclass Rhabditia is a paraphyletic assemblage featuring the orders (such as free-living forms in the family Rhabditidae) and Strongylida (including parasitic hookworms), unified by the presence of rhabditiform larvae in their developmental stages. Subclass Spiruria encompasses the orders Ascaridida (e.g., intestinal parasites like ), Oxyurida (pinworms), and Spirurida (filarial worms), reflecting a range of parasitic lifestyles in animal hosts. Subclass Tylenchia, whose is disputed, contains the orders Tylenchida (plant parasites, e.g., root-knot nematodes) and Aphelenchida (feeders on wood and plants), often characterized by a hollow stylet for host penetration. Note that some classifications place Tylenchida under Diplogastria. This morphological framework, while foundational, has been superseded by molecular phylogenies that reveal inconsistencies in these groupings.

Key Families and Examples

Within the classical taxonomy of Secernentea, the family Rhabditidae (order ) represents free-living bacterivores that are abundant in and freshwater sediments, serving as primary consumers of microorganisms. Genera such as Rhabditis include species like Rhabditis sp., which thrive in decaying and contribute to nutrient cycling in terrestrial ecosystems. A key example is , a transparent approximately 1 mm in length that feeds on and has become a foundational for studying and due to its simple anatomy and short life cycle. The order Strongylida encompasses several families of parasitic nematodes, including Strongylidae, with species that infect vertebrates and feature specialized caudal structures in males. Within this order, hookworms such as (family Ancylostomatidae) are notable intestinal parasites of humans, attaching to the gut wall and causing significant blood loss that leads to . The family Ascarididae (order Ascaridida) includes large intestinal parasites primarily affecting vertebrates, characterized by their robust bodies and lack of a stylet. Ascaris lumbricoides is a prominent species, infecting an estimated 772–892 million people worldwide, particularly in tropical and subtropical regions with poor sanitation. In the order Oxyurida, the family Oxyuridae comprises small nematodes with pin-like tails, often parasitizing the intestines of mammals. Enterobius vermicularis, the human pinworm, is a common example that predominantly affects children through fecal-oral transmission, leading to infections in school-aged populations. The family Heteroderidae (order Tylenchida) consists of plant-parasitic cyst nematodes equipped with a hollow stylet for feeding on host tissues. Heterodera glycines, the cyst nematode, is a major agricultural pest that forms persistent cysts on roots, ranking as the most damaging to crops in the United States and causing substantial yield reductions. The family Onchocercidae (order Spirurida, superfamily ) includes filarial worms transmitted by arthropod vectors, featuring microfilariae in their life stages. is a critical species that causes , or river blindness, primarily in through bites from blackflies.

Modern Phylogeny

Molecular Evidence and Reclassification

The transition to molecular phylogeny in nematode taxonomy began in the with the application of small subunit (SSU rDNA) sequencing, which provided a robust to test the classical morphological classifications established by Chitwood in 1958. These early molecular approaches demonstrated that the traditional dichotomy between Adenophorea and Secernentea did not reflect monophyletic groups, as genetic data revealed deep evolutionary divergences, with Adenophorea being paraphyletic and Secernentea nested within it as a monophyletic . Phasmids—sensory organs diagnostic of Secernentea—were confirmed as a synapomorphy reflecting shared ancestry rather than convergence. A seminal study by Blaxter et al. (1998) analyzed 53 SSU rDNA sequences from diverse nematodes and proposed a new framework dividing the into five major clades (I–V) based on maximum likelihood and parsimony methods. This analysis highlighted the of classical classes, with Secernentea corresponding to clades III, IV, and V, supporting the of the group. Building on this, De Ley and Blaxter (2002) expanded the dataset and confirmed Secernentea's through integrated morphological and molecular comparisons, advocating for a clade-based system where traditional classes were restructured, with Secernentea demoted to subclass rank within the class . These studies emphasized the 18S rRNA gene as a reliable marker for deep phylogeny due to its conserved regions suitable for alignment and variable domains for resolution, enabling tree-building that aligned genetic and morphological data for Secernentea. The timeline of reclassification accelerated through the as additional SSU rDNA and emerging multi-gene datasets reinforced the molecular consensus, with Secernentea retained as a monophyletic subclass under in modern taxonomy. For example, comprehensive phylogenomic analyses as of 2021 and classifications as of 2022 continue to recognize Secernentea, integrating genetic data to resolve its position within . This shift prioritized evolutionary relationships over superficial traits in some contexts, but Secernentea remains useful for grouping based on shared synapomorphies like phasmids.

Correspondence to Chromadorea

The classical class Secernentea largely corresponds to the modern class , which represents a major monophyletic in nematode phylogenetic trees, encompassing predominantly terrestrial and parasitic characterized by the presence of phasmids (lateral tail organs). This equivalence highlights shared morphological traits, such as tubular excretory systems and diverse feeding strategies, but is broader, incorporating additional free-living marine forms previously classified under Adenophorea. In terms of subgroup mappings, the classical subclasses Rhabditia and Diplogasteria align with the order within , including free-living and microbivorous nematodes like those in the family Rhabditidae. The subclass Spiruria maps to multiple orders in , such as Ascaridida and Spirurida, which contain many animal parasites like species. Similarly, Tylenchia corresponds to Tylenchida, featuring plant-parasitic forms with stylet-bearing mouthparts, such as root-knot nematodes in Meloidogyne. Discrepancies arise because some traditional Secernentea groups, particularly certain , have been reclassified into early-diverging lineages, reflecting their basal position in molecular trees. Additionally, marine nematodes once tentatively included in Secernentea are now excluded from core Secernentea groups and placed in other clades or . The modern structure of comprises approximately 12 orders, with Secernentea-derived groups dominating the diversity, including (e.g., the Caenorhabditis elegans), Spirurid orders, and Tylenchida. These orders emphasize adaptations to terrestrial and host-associated habitats, accounting for over 80% of described species. While the Secernentea framework retains utility for morphological identification in diagnostic contexts, it aligns with evolutionary studies as a monophyletic subclass in phylogenomic analyses.

Ecology and Biology

Habitats and Distribution

Secernentea nematodes, now often classified within the , are predominantly terrestrial, inhabiting a wide array of environments worldwide. They are most commonly found in aerated, moist soils with a pH range of 5–7, where they thrive in microhabitats associated with , such as rhizospheres, heaps, and forest litter. This terrestrial bias is largely due to their sensitivity to , which limits their persistence in drier or more extreme conditions, although some exhibit adaptations like anhydrobiosis to survive temporary stress. While rare in marine or freshwater ecosystems compared to Adenophorea, certain Secernentea groups, such as some , occasionally occur in marine sediments or freshwater bodies, but these represent exceptions to their overwhelmingly soil-based distribution. Their cosmopolitan range spans all continents, with highest diversity reported in tropical soils, where warm, humid conditions support abundant free-living and plant-parasitic forms. For instance, plant-parasitic Secernentea, like those in the Tylenchida order, are ubiquitous in agricultural fields globally, infecting roots and contributing to widespread distribution through human-mediated soil movement. In fertile soils, Secernentea densities can reach millions to tens of millions of individuals per square meter, playing a crucial role in cycling through bacterivory and facilitation of breakdown. Parasitic species further occupy specific microhabitats, such as animal intestines or zones, enhancing their in both natural and managed ecosystems. Global estimates indicate billions of Secernentea contribute to , underscoring their importance in terrestrial food webs.

Life History and Reproduction

Secernentea nematodes exhibit a conserved life cycle consisting of an egg stage, four successive juvenile molts (J1 to J4), and the adult stage, with development typically occurring through where the is shed between stages. This pattern is characteristic across the class, including free-living and parasitic forms, and allows adaptation to diverse environments. In many , particularly within the , a facultative dauer stage replaces the normal J3 under stressful conditions like nutrient limitation or overcrowding, enabling long-term survival and dispersal; for instance, in , dauer larvae resist and pathogens via a thickened and reduced . Reproduction in Secernentea is primarily sexual and dioecious, with males possessing spicules for copulation and females producing eggs after mating, though hermaphroditism and occur in select lineages. In free-living rhabditids like C. elegans, self-fertilizing hermaphrodites produce both and oocytes, yielding up to 300 progeny total, with peak egg-laying of 10–20 eggs per day during the 3–5 day reproductive period. Parasitic tylenchids often employ facultative or obligatory , such as mitotic parthenogenesis in root-knot nematodes (Meloidogyne spp.), where unreduced eggs develop without males, facilitating rapid in host tissues. Fecundity and developmental timing vary by lifestyle and conditions, with free-living species completing generations in 3–10 days at 20–25°C and producing hundreds of eggs per individual, while plant parasites like Pratylenchus spp. lay 1–2 eggs per day initially, increasing to support indirect cycles involving free juvenile stages for host invasion. Longevity spans days to weeks in active free-living adults but extends to years in dormant forms, such as stages in some tylenchids (Heterodera spp.), which protect eggs and juveniles against environmental stressors. These strategies underscore the class's versatility, from rapid turnover in bacterivores to prolonged dormancy in parasites.

Importance and Applications

Parasitic and Pathogenic Roles

Secernentea encompasses numerous parasitic nematodes that infect humans, animals, and , contributing to significant and agricultural burdens through diseases such as , , and root galls. These parasites, primarily within the subclass Chromadoria, exploit host tissues for nutrition and reproduction, often leading to chronic infections in endemic areas. In humans, causes , a affecting approximately one billion people worldwide, primarily through intestinal obstruction and malnutrition in children. Enterobius vermicularis, responsible for enterobiasis (), is highly prevalent in school-aged children, with estimates up to 50% in affected populations and about 40 million cases alone, leading to perianal itching and secondary infections. induces (river blindness), with around 19.6 million infections globally in 2021, causing skin lesions and through microfilarial migration; as of 2025, the has verified interruption of transmission in , the first country in to achieve this milestone. Among animals, hookworms of the genus , such as A. caninum in dogs and A. tubaeforme in cats, attach to intestinal walls, leading to and loss, with A. caninum being particularly pathogenic due to its voracious feeding. Filarial worms like (heartworm) infect dogs via vectors, with a global prevalence of about 10.91% in canine populations, causing pulmonary vascular damage and . Plant-parasitic Secernentea include root-knot nematodes (Meloidogyne spp.), which induce on roots by stimulating , disrupting vascular function and nutrient uptake in crops like tomatoes and carrots. Cyst nematodes (Heterodera spp.) are sedentary endoparasites that form persistent syncytial feeding sites in roots, extracting nutrients over extended periods and leading to stunted growth in cereals and soybeans. Pathogenic mechanisms in these nematodes involve tissue penetration via stylets or mouthparts, nutrient theft from host cells, and secretion of effectors or toxins that manipulate host responses; for instance, plant parasites exhibit migratory feeding (moving through tissues while damaging cells) or sedentary strategies (establishing fixed sites like or cysts for prolonged exploitation). In animal and human hosts, similar processes include esophageal secretions that anticoagulate blood and enzymes that degrade tissues, facilitating larval migration and adult attachment. Transmission of Secernentea parasites varies: geohelminths like Ascaris and hookworms spread via the fecal-oral route or soil contamination, where embryonated eggs or larvae in feces-contaminated environments are ingested or penetrate skin. Filarial species such as Onchocerca and Dirofilaria rely on arthropod vectors like blackflies or mosquitoes to deliver infective larvae during blood meals. Soil serves as a key reservoir for many, amplifying transmission in warm, humid climates.

Agricultural and Research Relevance

Plant-parasitic nematodes within Secernentea, such as species in the genus Meloidogyne, inflict substantial economic damage on global , with annual losses estimated at $100–157 billion worldwide. For instance, the Meloidogyne incognita significantly reduces yields in key crops like tomatoes and ; in tomatoes, it can cause losses of 42–54% in field conditions, while in , susceptible varieties experience yield suppressions ranging from 18–50% depending on levels and environmental factors. These impacts arise from root galling and disrupted nutrient uptake, exacerbating vulnerability in systems. Management of Secernentea pests relies on integrated approaches to minimize reliance on synthetic chemicals. Chemical nematicides, including fumigants like methyl bromide, were historically effective but have been largely phased out since the early 2000s due to their ozone-depleting properties under the , prompting shifts to alternatives such as . Biological controls, such as the Paecilomyces lilacinus, offer sustainable options by parasitizing eggs and juveniles, achieving up to 70% reduction in populations when applied in combination with other agents. Cultural practices like with non-host plants (e.g., cereals following ) and planting resistant varieties further suppress densities, reducing reproduction cycles and soil buildup. In research, Secernentea species serve as vital models for advancing agricultural solutions. The free-living nematode Caenorhabditis elegans has been instrumental in screening anthelmintic compounds, enabling the identification of drug targets that inform nematicide development against parasitic relatives. Genomic sequencing of plant-parasitic species, such as Meloidogyne hapla, provides insights into effector proteins and host interactions, guiding the design of targeted pesticides and resistance mechanisms. Economically, integrated pest management (IPM) strategies incorporating these tools promote sustainable farming by curbing losses while preserving soil health; non-parasitic Secernentea decomposers enhance nutrient cycling and organic matter breakdown, supporting microbial communities essential for fertile soils. Looking ahead, emerging technologies like editing target plant susceptibility genes to confer durable resistance against Secernentea nematodes, as demonstrated in edited crops showing reduced infection without yield penalties. poses challenges by altering nematode distributions through warmer soils and shifting precipitation, potentially expanding ranges of pests like Meloidogyne into new agricultural zones and intensifying outbreaks. These trends underscore the need for adaptive IPM frameworks to safeguard productivity amid environmental shifts.

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