Agnatha – Characteristics, Classification, Morphology, Reproduction

• The Agnatha, meaning “without jaws,” is a superclass of jawless fish within the phylum Chordata and subphylum Vertebrata. It encompasses both living species, known as cyclostomes, and extinct species like conodonts and ostracoderms. These jawless fish are closely related to vertebrates with jaws, known as gnathostomes.
• Extensive molecular and embryological evidence strongly supports the idea that the cyclostomes, or living agnathans, form a monophyletic group. Studies involving rRNA, mtDNA, and embryonic development have provided substantial confirmation for this hypothesis.
• The earliest known fossil evidence of agnathans dates back to the Cambrian period. Today, there are two families of agnathans that still exist: lampreys and hagfish. Together, these families comprise around 120 species.
• Hagfish, despite being jawless, are classified as members of the subphylum Vertebrata because they lost their vertebrae secondarily. Prior to the discovery of this characteristic through molecular and developmental studies, the classification system introduced by Linnaeus included the group Craniata, which referred to both hagfish and vertebrates. While some scientists argue that the living agnathans may only share superficial similarities and that these characteristics are likely ancestral traits of ancient vertebrates, recent taxonomic investigations have clearly established the close relationship between hagfish (Myxini or Hyperotreti) and lampreys (Hyperoartii), with both being more closely related to each other than to jawed fishes.
• Understanding the skeletal structure of agnatha provides valuable insights into the evolutionary history and relationships of these fascinating jawless fish.

General Characteristics of Class Agnatha

The Class Agnatha encompasses the earliest known vertebrates and is distinguished by several key characteristics:

1. Absence of jaws: Agnatha lack true jaws, making them distinct from other vertebrate classes.
2. Lack of teeth, paired appendages, and exoskeleton: Agnatha do not possess teeth or paired appendages like fins. Furthermore, they lack an exoskeleton.
3. Membranous roof in the skull: The skull of Agnatha has a membranous roof, which sets them apart from other vertebrates with bony skulls.
4. Single median nasal opening: Agnatha have a single nasal opening located in the midline of the skull.
5. Notochord and fibrous neural tube: The vertebral column of Agnatha consists of a persistent notochord, a flexible rod-like structure, surrounded by a fibrous neural tube.
6. Multiple gill slits: Agnatha typically possess a significant number of gill slits, ranging from 7 to 14 pairs.
7. Absence of a conus in the heart: Unlike other vertebrates, Agnatha lack a conus, a specialized region in the heart.
8. Persistent hypophysial sac: Agnatha have a persistent hypophysial sac, a structure related to the pituitary gland.
9. Semicircular ducts in the ear: Agnatha possess one or two semicircular ducts within their ear, aiding in balance and orientation.
10. Elongated kidneys and archinephric ducts: Agnatha have long kidneys and archinephric ducts, which are involved in excretory functions.
11. Absence of genital ducts: Agnatha lack specific genital ducts for reproductive purposes.
12. Well-developed pineal apparatus: Agnatha exhibit a fairly well-developed pineal apparatus, which is involved in regulating biological rhythms and light detection.
13. Larval stage and endostyle: Agnatha typically undergo a larval stage that is microphagus, meaning they consume small particles of food. They possess an endostyle similar to that found in protochordates.

Additional general characteristics of Class Agnatha include:

• Classification within the phylum Chordata and subphylum Vertebrata.
• Lack of a recognizable stomach in the digestive system.
• Complete absence of jaws, distinguishing them from jawed vertebrates.
• Paired fins are often absent or reduced in size.
• Thick bony scales and plates that were present in early species have been lost in modern Agnatha.
• The skeleton is primarily cartilaginous rather than bony.
• The notochord, which is a characteristic of embryonic development, persists into adulthood.
• Presence of seven or more paired gill pouches for respiration and water filtration.

Metabolism of Agnatha

• The metabolism of Agnatha, the jawless fish, exhibits certain characteristics due to their ectothermic nature. Ectothermic organisms do not possess the ability to regulate their own body temperature, and as a result, their metabolic rates are influenced by the temperature of their environment. In cold water, Agnathans have a slower metabolic rate compared to warm-blooded animals. Consequently, they require less food to sustain their energy needs.
• One notable feature of Agnathan digestion is the absence of a distinct stomach. Instead, they have a long gut that is relatively uniform throughout its length. This simplistic digestive system reflects their primitive evolutionary status. The absence of a stomach suggests that Agnathans have a more straightforward approach to nutrient absorption and digestion.
• Lampreys, a group of Agnathans, have a feeding strategy that involves consuming other fish and sometimes even mammals. They attach themselves to their host using their suction-like mouth and rasping tongue. During feeding, lampreys inject anticoagulant fluids into the host’s body, which prevents blood clotting. This action enables them to obtain a continuous flow of blood-rich nutrients from the host. Lampreys rely heavily on this feeding method to meet their nutritional requirements.
• On the other hand, hagfish, another group of Agnathans, have a different feeding behavior. They are scavengers, primarily feeding on dead animals. With their row of sharp teeth, hagfish tear apart the carcasses of their food sources. This feeding strategy allows them to access the nutrient-rich tissues of deceased organisms. However, the structure of Agnathan teeth limits the types of food they can consume. Unlike vertebrates with movable jaws, the teeth of Agnatha are unable to move up and down, which restricts their ability to consume certain types of prey.
• Overall, the metabolism and feeding habits of Agnatha demonstrate adaptations that enable these jawless fish to survive and obtain necessary nutrients within their respective ecological niches. Whether as predators like lampreys or scavengers like hagfish, Agnathans have developed distinct strategies to acquire the energy they need for their survival and reproduction.

Morphology of Agnatha

• The morphology of Agnatha, the jawless fish, is characterized by several distinctive features. Firstly, modern agnathans lack jaws, which sets them apart from their jawed counterparts. In addition to the absence of jaws, they also lack paired fins, further differentiating them from other fish groups.
• Another defining characteristic of Agnatha is the presence of a notochord, a flexible rod-like structure that provides support along the length of the body. The notochord is present in both larvae and adult stages of agnathans. Additionally, these organisms possess seven or more paired gill pouches, which are used for respiration.
• Lampreys, a type of agnathan, have a light-sensitive structure called the pineal eye. This structure is homologous to the pineal gland found in mammals and is involved in detecting light and regulating biological rhythms.
• Most agnathans, both living and extinct, do not possess a recognizable stomach or any appendages. The absence of appendages further distinguishes them from other fish groups. Reproduction in agnathans involves external fertilization and development, and there is no parental care exhibited within the Agnatha class.
• Agnathans are ectothermic, meaning they are cold-blooded. They lack the ability to regulate their own body temperature and rely on the environment for heat regulation. Their skeletal system is cartilaginous, providing support and flexibility. The heart of agnathans typically consists of two chambers, which is a simpler cardiac structure compared to the multi-chambered hearts found in some other vertebrates.
• Regarding body covering, modern agnathans have skin without dermal or epidermal scales. However, hagfish, a group of agnathans, possess copious slime glands in their skin. This slime serves as their defense mechanism and can clog up the gills of potential predators, causing them harm or death. In contrast, many extinct agnathans exhibited extensive exoskeletons composed of either heavy dermal armor or small mineralized scales.
• While most agnathans lack paired appendages, some fossil species, such as osteostracans and pituriaspids, did possess paired fins. This trait was inherited by their jawed descendants and played a significant role in the evolution of vertebrates with jaws.
• Overall, the morphology of agnathans is characterized by their jawless nature, absence of paired fins (excluding some fossils), presence of a notochord, gill pouches, cartilaginous skeletons, and unique body coverings. These features highlight the distinctiveness of agnathans within the broader classification of vertebrates.

Reproduction of Agnatha

• Agnatha, a primitive group of jawless fish that includes lampreys and hagfishes, exhibits unique reproductive strategies. While lampreys and hagfishes share some similarities in their reproductive processes, there are notable differences between the two.
• In lampreys, fertilization occurs externally. During the mating season, male lampreys release sperm into the water, and females release their eggs. The sperm and eggs meet in the water, where fertilization takes place. The exact mode of fertilization in hagfishes is not well understood, as their reproductive behavior remains largely unknown.
• Development in both lampreys and hagfishes is likely to be external. However, the details of the developmental process are better understood in lampreys. After fertilization, lamprey eggs develop externally in freshwater riverbeds. Working in pairs, lampreys construct nests and bury their eggs about an inch beneath the sediment, providing some protection from predators and water currents. The eggs develop and hatch into larvae, commonly known as ammocoetes, which have a distinctive appearance with a round, jawless mouth and a well-developed notochord.
• Interestingly, lampreys exhibit a unique reproductive trait. They are only able to reproduce once in their lifetime. After the external fertilization process, the lamprey’s cloacas, the common opening for excretion and reproduction, remain open. This allows a fungus to enter their intestines, leading to their eventual demise. This phenomenon is known as semelparity, where an organism reproduces only once and then dies.
• In contrast, hagfish reproduction is not well documented, and there is limited knowledge about their reproductive processes. It is believed that hagfishes have a low reproductive output, with females producing a small number of eggs, estimated to be around 30 eggs throughout their lifetime. The specific details of hagfish larval development are unclear, and further research is needed to understand their reproductive behavior fully.
• Overall, lampreys and hagfishes represent intriguing examples of reproduction within the Agnatha group. Lampreys display a complex life cycle with distinct larval stages and semelparity, while hagfishes have a relatively limited understanding of their reproductive biology. Further scientific investigations are required to uncover the mysteries surrounding the reproductive strategies and behaviors of these fascinating jawless fish.

Evolution of Agnatha

• The evolution of Agnatha, the jawless fish, provides us with fascinating insights into the early development of vertebrates and the emergence of adaptive immune systems. While Agnatha is a minor component of modern marine fauna, it played a prominent role among early fish during the early Paleozoic era.
• In the Early Cambrian period, two types of animals with fins, vertebrate musculature, and gills were discovered in the Maotianshan shales of China: Haikouichthys and Myllokunmingia. These ancient organisms, tentatively assigned to Agnatha by Janvier, offer evidence of early jawless fish. Another possible agnathid, Haikouella, also hails from the same region. Additionally, a potential agnathid from the Middle Cambrian Burgess Shale of British Columbia was reported but remains undescribed. Conodonts, a class of agnathans, emerged in the early Cambrian and persisted until their extinction in the Triassic. Their fossilized teeth are frequently used as index fossils from the late Cambrian to the Triassic.
• During the Ordovician, Silurian, and Devonian periods, agnathans developed heavy bony-spiky plates as armor. The first armored agnathans, known as Ostracoderms, appeared in the middle Ordovician. They served as precursors to bony fish and ultimately to tetrapods, including humans. By the Late Silurian, agnathans had reached their evolutionary zenith. However, most of the ostracoderms, such as thelodonts, osteostracans, and galeaspids, were more closely related to the jawed vertebrates (gnathostomes) than to the surviving agnathans, known as cyclostomes. Cyclostomes split from other agnathans before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts. Agnathans experienced a decline in the Devonian period and never regained their former prominence.
• Approximately 500 million years ago, during the evolution of jawed vertebrates, two types of recombinatorial adaptive immune systems (AISs) emerged. The jawed vertebrates, including humans, developed diverse immunoglobulin domain-based T and B cell antigen receptors through the rearrangement of V(D)J gene segments and somatic hypermutation. However, none of the fundamental AIS recognition elements found in jawed vertebrates have been identified in jawless vertebrates. Instead, the AIS of jawless vertebrates, including lampreys and hagfish, relies on variable lymphocyte receptors (VLRs). VLRs are generated through the recombinatorial usage of highly diverse leucine-rich-repeat (LRR) sequences. Lampreys and hagfish possess three VLR genes (VLRA, VLRB, and VLRC), each expressed on distinct lymphocyte lineages. Cells expressing VLRA and VLRC resemble T-cells and develop in a thymus-like structure called thymoids. On the other hand, VLRB-expressing cells resemble B-cells, differentiate in hematopoietic organs, and give rise to plasma cells that secrete “VLRB antibodies.”
• The evolution of Agnatha provides a glimpse into the early stages of vertebrate development and the emergence of adaptive immune systems. The diverse forms and characteristics exhibited by Agnatha throughout history contribute to our understanding of the complexity and diversity of life on Earth.

Classification of Agnatha

The Class Agnatha can be further classified into several subgroups, each with its own unique characteristics and representative examples:

1. Cyclostomes:
   • Example: Myxini (hagfish)
   • Myxini, commonly known as hagfish, are eel-shaped marine animals that produce slime. They possess a skull but lack a vertebral column, making them unique among living animals. Hagfish are considered living fossils and are basal to vertebrates. The classification of hagfish has been a topic of debate, with some considering them degenerate vertebrates closely related to lampreys, while others suggest they represent a stage preceding the evolution of the vertebral column. The original classification groups hagfish and lampreys together as cyclostomes (or Agnatha), the oldest surviving clade of vertebrates alongside jawed-vertebrates (gnathostomes). Recent DNA evidence supports the original classification.
2. Hyperoartia:
   • Example: Lamprey
   • Hyperoartia is a disputed group that includes modern lampreys and their fossil relatives. Lampreys have a long fossil record, with early forms like Endeiolepis and Euphanerops existing during the Late Devonian period. The placement of Hyperoartia among jawless vertebrates is still a matter of debate. Traditionally, they were grouped together with hagfishes as cyclostomes. However, recent proposals suggest that lampreys are more closely related to certain “ostracoderms” (jawless armored “fishes”) or other extinct jawless fish. Resolving their exact relationships with other vertebrate groups remains an ongoing scientific inquiry.
3. Myllokunmingiida:
   • Example: Haikouichthys
   • Myllokunmingiida is a primitive order of agnathans endemic to the Cambrian-aged Maotianshan Shales lagerstätte in China. These creatures are the earliest known craniates, which are chordates with a skull made of hard bone or cartilage. The group includes genera such as Haikouichthys, Myllokunmingia, and Zhongjianichthys.
4. Conodonta:
   • Example: Conodonts (extinct)
   • Conodonts were eel-like agnathans that existed from the Cambrian to the beginning of the Jurassic period. They exhibited a wide range of lifestyles, with some species being filter feeders and others being macropredators. Initially, these animals were known only through microscopic tooth structures called “Conodont elements.” Later, body fossils of conodonts were discovered, providing insight into their true appearance. Conodonts serve as excellent index fossils due to their rapid evolution and relatively short lifespan. They experienced peaks in diversity during the middle of the Ordovician and mid-late Devonian but declined during the Carboniferous. They became relatively rare in the Permian but experienced a resurgence in the early Triassic before going extinct at the end of the period.
5. Ostracoderms:
   • Examples: Pteraspidomorphi (extinct), Thelodonti (extinct), Anaspida (extinct), Cephalaspido-morphi (extinct)
   • Ostracoderms are an extinct group of early jawless fish. They exhibited extensive shielding of the head and possessed armoured bodies covered in dermal bone. Some ostracoderms had hypocercal tails, which aided in movement through water. The group includes subgroups such as Heterostraci, Astraspida, Arandaspida (within Pteraspidomorphi), Thelodonti, Anaspida, and Cephalaspido-morphi. These fish occupied various ecological niches, with some preferring reef ecosystems. Ostracoderms lived in both freshwater and marine environments, appearing during the Ordovician period and going extinct during the Late Devonian extinction event.

Each subgroup within the Agnatha classification represents a distinct evolutionary lineage of jawless fish, contributing to our understanding of vertebrate evolution and the diversity of ancient aquatic ecosystems.

FAQ

References

• https://nhpbs.org/wild/Agnatha.asp
• http://fsc.fernbank.edu/stt/vertbio/agnatha.htm
• https://www.notesonzoology.com/superclass-agnatha/classification-of-superclass-agnatha-zoology/6327′
• https://faculty.weber.edu/rokazaki/Zoology1120/Chapter%2024%20Fishes.pdf
• https://biolearnspot.blogspot.com/2016/03/agnatha.html