Cyclostomes represent a unique and ancient lineage among vertebrates. These fascinating creatures offer a glimpse into the early evolutionary history of backboned animals. Their unusual biology sets them apart from jawed fishes, making them subjects of scientific interest.
Distinguishing Features of Cyclostomes
Cyclostomes are defined by their lack of true jaws, earning them the common name “jawless fish.” Instead of a bony skeleton, their internal support structure consists entirely of cartilage, providing flexibility without the rigidity of bone. They also lack paired fins, such as pectoral or pelvic fins, moving through water using undulatory body movements.
A persistent notochord, a flexible rod-like structure, runs along their entire body length and remains present throughout their lives, unlike in many vertebrates where it is replaced by a vertebral column. Their respiratory system features internal gill pouches that open directly to the outside through separate pores, unlike the external gill slits of jawed fish. Cyclostomes also possess a single, median nostril on the top of their head, unlike the paired nostrils of other fish. The two living groups within cyclostomes are the lampreys and hagfish, each exhibiting these shared characteristics.
Habitats and Lifestyles
Cyclostomes inhabit a variety of aquatic environments. Lampreys are found in both freshwater and marine habitats, with many species being anadromous, migrating from saltwater to freshwater to spawn. Hagfish, conversely, are exclusively marine, dwelling in deep ocean waters where they burrow into soft sediments.
Their feeding strategies are specialized and reflect their jawless nature. Adult lampreys are often parasitic, attaching to other fish with their suction-cup like mouths to rasp away flesh or consume blood. Hagfish are primarily scavengers, locating dead or dying marine animals and burrowing into them to consume internal tissues. Some hagfish species also prey on small invertebrates, using their feeding apparatus to pull apart prey.
Evolutionary Significance
Cyclostomes are important in the study of vertebrate evolution, representing one of the oldest living lineages of vertebrates. Their anatomical simplicity provides insights into the primitive conditions from which complex vertebrate features, such as jaws and paired appendages, likely evolved. Scientists study their genetics and development to understand the origins of these innovations that define jawed vertebrates.
These animals are sometimes referred to as “living fossils” because their body plans have remained relatively unchanged over hundreds of millions of years. Their ancient lineage, stretching back to the early Paleozoic era, makes them direct descendants of the earliest vertebrates. The persistence of features like the notochord and absence of jaws offers a window into the evolutionary transitions that led to today’s vertebrate diversity.
Reproduction and Development
The reproductive strategies of cyclostomes vary between lampreys and hagfish, yet both involve external fertilization. Lamprey species are anadromous, migrating upstream to freshwater rivers and streams where they construct nests in gravel beds for spawning. Females release numerous small eggs, fertilized by males, and both adults typically die shortly after reproduction.
Lampreys undergo a larval stage called the ammocoete, which can last for several years. Ammocoetes are blind, filter-feeding organisms that burrow in soft sediments, consuming microscopic particles. After this prolonged larval phase, they undergo metamorphosis into their adult form, developing eyes and changing their feeding apparatus to become parasitic or non-feeding. Hagfish reproduction is less understood but involves the laying of large, yolky eggs, often encased in a tough, proteinaceous shell with hook-like anchors.