Obelia is a genus of small, colonial marine animals that belong to the phylum Cnidaria, a diverse group that includes jellyfish and sea anemones. These organisms are commonly studied in biology as a representative example of a complex life cycle. Obelia species are often called “sea fur” due to their delicate, whitish or light brown, plant-like appearance when found attached to surfaces in marine environments. They are diploblastic, meaning their body wall has two distinct tissue layers: an outer epidermis and an inner gastrodermis, separated by a jelly-like layer called the mesoglea.
The Stationary Polyp Colony
The Obelia colony typically begins as a small, sessile polyp, resembling a miniature sea anemone, which then grows to form a branched, tree-like structure. This colony is anchored to a substratum, such as seaweed, rocks, or shells, by a horizontal, root-like structure called the hydrorhiza. From the hydrorhiza, vertical branching stems, known as hydrocauli, extend upwards, reaching heights of around 2.5 centimeters. Both the hydrorhiza and hydrocauli are hollow tubes, encased in a transparent, protective chitinous exoskeleton called the perisarc.
Within the colony, individual Obelia polyps, also called zooids, are specialized for different functions. The feeding polyps, known as gastrozooids or hydranths, are vase-shaped and equipped with a mouth surrounded by numerous tentacles. These tentacles bear stinging cells called cnidocytes, which are used to capture prey. The gastrozooids are responsible for ingesting food and distributing partially digested nutrients throughout the entire colony via a continuous gastrovascular cavity.
Reproductive polyps, termed gonozooids or blastostyles, are elongated, lack tentacles and a mouth, and thus do not feed. They are enclosed within a protective perisarc covering called the gonotheca. The primary function of gonozooids is asexual reproduction through budding, producing small, bell-shaped structures known as medusae. These medusa buds develop within the gonotheca before being released into the surrounding water.
The Free-Swimming Medusa Stage
The medusa stage of Obelia is a small, free-swimming organism, typically measuring 1 to 2 millimeters in diameter, and appears as a transparent, bell-shaped form resembling a miniature jellyfish. The medusa has a convex upper surface, called the exumbrella, and a concave lower surface, the subumbrella. Hanging from the center of the subumbrella is a short, hollow structure known as the manubrium, which contains the quadrangular mouth at its distal end.
Numerous tentacles extend from the margin of the bell, which are equipped with stinging cells for defense and prey capture. Internally, the medusa possesses a well-developed gastrovascular cavity system, including four radial canals extending from a central stomach to a circular canal located at the bell’s edge. The primary purpose of the medusa is sexual reproduction and dispersal of the species across wider marine areas. While male and female medusae are physically indistinguishable externally, their sex is determined by examining the gonads, which are typically four knob-like structures located on the subumbrella beneath the radial canals.
The Alternation of Generations Life Cycle
Obelia exhibits a complex life cycle characterized by an “alternation of generations,” involving distinct asexual and sexual stages. This cycle begins with the stationary polyp colony, which reproduces asexually. Specialized reproductive polyps, the gonozooids, form buds that develop into numerous small medusae. These medusae are then released from the gonozooids and swim freely in the water.
The free-swimming medusae represent the sexual stage. Male and female medusae release sperm and eggs, respectively, into the surrounding seawater, where external fertilization occurs. The fertilized egg, or zygote, undergoes a series of cell divisions, forming a solid mass of cells that develops into a ciliated, free-swimming larva called a planula. This oval-shaped planula larva swims for a period, allowing for dispersal.
Eventually, the planula larva settles onto a suitable substratum, loses its cilia, and undergoes metamorphosis. It then develops into a new, single polyp, which subsequently grows and begins to bud off additional polyps, establishing a new colonial hydroid. This new polyp colony then continues the asexual phase, producing more gonozooids that in turn release new medusae, thus completing the intricate life cycle.
Habitat and Feeding
Obelia species are widely distributed throughout marine environments globally, with the exception of the high-Arctic and Antarctic seas. The sessile polyp colonies are typically found in shallow coastal waters, often at depths up to 80 meters, where they attach to various underwater surfaces. These surfaces include seaweeds, rocks, mollusc shells, and wooden structures like pilings and wharves, often forming a delicate, fur-like growth. The free-swimming medusa stage, however, is commonly found in both coastal and offshore plankton, allowing for broader dispersal.
Obelia colonies are carnivorous, primarily feeding on small zooplankton, which they capture using their tentacles. The gastrozooids, or feeding polyps, extend their tentacles into the water column. When small organisms like copepods, larval crustaceans, or other invertebrate larvae come into contact with the tentacles, specialized stinging cells called nematocysts are deployed to immobilize the prey. The tentacles then direct the captured prey towards the mouth, located at the center of the hypostome, for ingestion. Digestion begins in the gastrovascular cavity and is then completed intracellularly, with nutrients distributed throughout the interconnected polyps of the colony. Some Obelia species, like Obelia dichotoma, have also been observed to consume fecal pellets and microalgae, demonstrating some dietary flexibility.