While there are over 33,000 known fish species, they have two things in common: they live in water and have spines.
Fish are vertebrates, which means they have backbones. While most fish have a backbone, some don’t. The spine is one of the most vital structural components fish have. It protects their soft organs and spinal cord, helps them swim, and attaches to their muscles. The backbone also supports the production of blood cells and platelets. There are two types of backbone structures: the trunk vertebrae and the caudal spine.
Similarly, fish are split into 3 species types, depending on their spinal anatomy. The species include jawless, bony, and cartilaginous fish. The backbone structure differs slightly between them, but the essential components are the same.
Do Fish Have Spines?
Most fish have backbones, though not all do. Bony fish possess a chordate body structure, which means they have a stiff rod called the vertebral column (or notochord) running through the length of their bodies.
The spinal cord, which is a hollow tube containing nervous tissue, is located just above it. The gastrointestinal tract lies underneath the backbone. Examples of bony fish include:
Specifically, the vertebral column is comprised of a segmental series of stiff bones (the vertebrae) that are separated by mobile joints called interverbal discs. The vertebral column consists of the:
- Centrum: the structural core of the vertebra. It’s usually concave at each end, limiting motion.
- Vertebral arches: they protrude from the top and bottom of the centrum
The Laboratory of Biological Structures Mechanics describes how the vertebral column of fish differentiates in two regions:
- Trunk vertebrae, which are connected to the ribs and feature a neural arch and spines
- Caudal spine, which has no ribs but a hemal arch (or chevron), which protects the caudal artery and vein
Lobed-finned fish – a clade of bony fish with fleshy, paired fins joined to the body with a single bone – have three bony elements in their vertebrae. These bones include the:
- Vertebral arch, which surrounds the spinal cord
- Pleurocentrum. This protects the upper surface of the notochord
- Intercentrum, an arch-shaped bone protecting the lower border
In ray-finned fish, the pleurocentrum and intercentrum are fused and embedded with a solid piece of bone that resembles a mammal’s vertebral body.
In comparison, cartilaginous fish (e.g., sharks and stingrays) have vertebrae consisting of two cartilaginous tubes. Vertebral arches form the upper tube, but additional structures comprised of cartilage fill in the vertebrae gaps. The lower tube surrounds the notochord and includes layers of calcification.
Are Fish Vertebrate or Invertebrate?
As mentioned, because fish have backbones, they’re vertebrates. They share this in common with mammals, amphibians, reptiles, and birds. There are approximately 66,000 vertebrate animal species in the world in total. While this sounds like a large number, this is only 3% of the world’s population.
Invertebrates don’t have backbones. They have soft bodies or a hard outer casing that protects their bodies.
Why Do Fish Have Backbones?
The spine is one of the most essential features of a fish’s body, as it supports all other bones, body parts, vital organs, and muscles. As a result, the vertebral column is the body’s main structure. As described by Science Direct, the vertebral column:
- Provides attachment to the muscles
- Supports the trunk
- Protects the spinal cord and nerve roots
- Supports the production of blood cells and platelets (hemopoiesis)
Specifically, fish backbones are vital for the following reasons:
Supports the Body
The backbone is a fundamental part of the skeleton, providing support to the soft and delicate organs. It also supports the fins, which extend from the spine and perform several crucial swimming functions.
The backbone is responsible for propping up the area between (and including) the head and trunk. One of these things is the ribs. Fish have two of them, which are attached directly to the upper and lower part of the spine. This means that after the skull, the backbone is the most vital part of the skeletal system.
Not only does the backbone support the organs, but it protects them. While the spinal cord is essential, it’s fragile.
Fish need their spines to be in excellent working order because they have many nerves that send information to the brain and other parts of the body, acting as a messaging pathway between the brain and body.
The spinal cord runs through the spine, so the bony structure protects it. Without healthy, efficient backbones, fish wouldn’t move properly and would struggle to survive.
The backbone helps fish swim. While the fins and tail play an essential role in how fish move through the water, the backbone enables it to happen in the first place.
Most body parts and organs that aid with movement are connected to the backbone. Fish engage their spine when they want to move, steer, and change direction. Fish can do this because their backbones are so flexible.
The backbone determines the shape of the fish. Like all chordates, the body takes after the spine. Without it, fish wouldn’t have structure, and their organs would be vulnerable to accident and injury.
What Fish Don’t Have Backbones?
You might be wondering, do all fish have backbones? While most do, there are a few fish species that lack spines. Fish without backbones include:
- Bony fish
- Jawless fish
- Cartilaginous fish
While almost all fish have some kind of bony spinal structure, some fish don’t. These species include:
Hagfish have unfairly developed a reputation for being the most disgusting creatures in the ocean. They’re shaped similarly to eels and have four pairs of thin sensory tentacles around their mouths that they use to find food. As soon as they find a meal, they bury into it with their faces and tunnel into its flesh.
Smithsonian Magazine describes how hagfish are considered a part of the jawless fish species, but no one is sure whether they belong to their own group between invertebrates and vertebrates.
Even though they have a partial cranial skull, hagfish lack a backbone. That’s why they can’t be classed as true vertebrates. Hagfish don’t have any bones at all – their skeletons are made up of cartilage.
That being said, they share many traits with vertebrates, including jaws, an enlarged head, and a well-developed sensory system. Out of all sea creatures, hagfish are most akin to lampreys.
However, as described by the Laboratory of Biological Structures Mechanics, a recent study suggests that a rudimentary spine that develops in the embryo with the same mechanisms found in other vertebrates exists. More studies are currently needed into the anatomy of hagfish to get a complete picture.
Though lancelets aren’t are true species of fish, they’re considered to be close to vertebrate ancestral lineage. They contain many organs and organ systems that closely resemble modern fish, but they’re more primitive. They have a hollow nerve cord running along the back, pharyngeal slits, and a tail that runs past the anus.
While they have a nerve cord, it’s not protected by bone but by a notochord. This differs from that of an invertebrate’s, as it’s far simpler in structure. That’s because it’s made up of a series of tightly packed cylinders of cells. Also, unlike invertebrates, a lancelet’s notochord extends into the head.
Lampreys closely resemble eels. They have long, scaleless bodies, a solitary nostril on the top of their head, and 7 gill pores on each side.
Like hagfish, lampreys have a cartilaginous skeleton. They also have several cartilaginous structures called arcualia. These are located above the notochord, meaning lampreys, like the other two species, aren’t true vertebrates.
Despite this, they’re some of the most energy-efficient swimmers in the ocean. Their movements generate low-pressure zones around the body, pulling their bodies through the water instead of pushing them.
Fish are closer in anatomy to us than you might think. Their backbones are one of their most important features, and without them, they wouldn’t survive.