The blobfish (Psychrolutes marcidus) has gained worldwide attention due to its unusual appearance when displayed outside of its natural habitat. This strange look often leads to confusion regarding its fundamental biology and challenges common assumptions about fish structure. The primary question surrounding this deep-sea resident is whether it is classified as an invertebrate or a vertebrate.
Defining Vertebrates and Invertebrates
The classification of animals into vertebrates and invertebrates is based on a single anatomical feature. Vertebrates are members of the subphylum Vertebrata within the phylum Chordata. These animals are characterized by the presence of a vertebral column, or backbone, which protects the spinal cord. They also possess an internal skeleton, or endoskeleton, that provides structural support and a framework for muscle attachment.
In contrast, invertebrates are animals that lack a vertebral column entirely. This group is far more diverse and numerous than vertebrates, encompassing approximately 97% of all animal species. Invertebrates include organisms like insects, mollusks, worms, and jellyfish. They may have an external skeleton, a hydrostatic skeleton, or no rigid skeleton at all.
The Classification of the Blobfish
Based on these biological standards, the blobfish (Psychrolutes marcidus) is definitively classified as a vertebrate. As a species of ray-finned fish, it is placed in the Phylum Chordata and the Class Actinopterygii. The blobfish possesses a rudimentary, though highly modified, spinal column and a skeleton, meeting the core requirement for this classification.
The skeleton of the blobfish is largely cartilaginous and less dense than the bones of shallow-water fish. This modification allows the body to cope with the extreme hydrostatic pressure of its deep-sea environment. While its bones are soft and minimal, the presence of a segmented vertebral column places it firmly within the subphylum Vertebrata.
Physical Adaptations and Deep-Sea Appearance
The confusion about the blobfish’s classification often stems from its famous “blob” appearance, which is not its natural state. Psychrolutes marcidus lives at depths between 600 and 1,200 meters, where the water pressure is up to 120 times greater than at the surface. To survive, the fish evolved a body made of gelatinous tissue that is less dense than the surrounding seawater.
This low-density flesh allows the blobfish to achieve neutral buoyancy, enabling it to float just above the seafloor without expending energy. The animal lacks a functional swim bladder, a gas-filled organ that helps most fish regulate buoyancy, because the high pressure would cause it to collapse. The gelatinous mass provides the necessary support and structure under the intense pressure of the abyssal zone.
When the blobfish is rapidly brought to the surface, the sudden decompression causes its less-dense tissues to expand and lose structural integrity. This results in the shapeless form that the public recognizes as the “blobfish.” In its natural deep-sea habitat, the pressure compresses its features, giving it a more streamlined and typical fish-like silhouette.