ABCS101: Introductory Animal Biology - Phylum Cnidaria Study Notes

Phylum Cnidaria: General Characteristics and Evolutionary Status

Phylum Cnidaria, also known as coelenterates, represents the second least developed phylum within the kingdom Animalia. In the hierarchy of biological advancement, they are the first group to possess true tissues. The name "Cnidaria" is derived from the Greek word "cnidos", which translates to "stinging nettle," referencing their specialized stinging cells. These animals are uncephalized, meaning they lack a centralized head region, and possess only one body opening: the mouth. Their body plans typically exhibit radial or biradial symmetry. Examples of such symmetry include the radial symmetry seen in coral polyps and the biradial symmetry found in sea anemones equipped with a siphonoglyph. Some groups, such as specific echinoderm-like relatives, may exhibit pentamerous radial symmetry, indicated as $D5$. In contrast to these forms, sponges exhibit no symmetry, while beetles represent bilateral symmetry. Anatomical planes relevant to the study of these organisms include the sagittal plane, coronal plane, and transverse plane.

The Cnidocyte and Nematocyst Mechanism

The defining characteristic of this phylum is the cnidocyte, also referred to as a "nettle cell." Each cnidocyte contains a specialized capsule called a nematocyst, which is a specific type of cnida. The nematocyst functions as a harpoon-like structure used for both defense and prey capture. It consists of a coiled thread stored within a capsule. When the "trigger" (cnidocil) is stimulated, the capsule discharges the thread, which can eject with poisonous stinging structures to paralyze prey. These cnidocytes are primarily concentrated within the tentacles of the animal. This mechanism is critical for their survival as carnivorous predators.

Morphological Forms: Polyp and Medusa

Cnidarians are diploblastic, meaning their bodies are composed of two primary cell layers: the outer epidermis (ectoderm) and the inner gastrodermis (endoderm). Between these layers lies the mesoglea, a jelly-like middle matter. The mesoglea can vary significantly in thickness, serving as a simple glue in organisms like Hydra or acting as the primary mass of the animal in jellyfishes.

There are two basic body forms in this phylum: the polyp and the medusa. The polyp form is generally tubular with the mouth surrounded by tentacles at the top; it is sessile and attached to a substrate at the aboral end. In polyps, the mesoglea is typically thin. Examples include sea anemones and corals. In colonial polyps, individual members are connected by tube-like structures called hydrocauli (singular: hydrocaulus), and the colony is anchored by a root-like stolon. Specialized polyps within a colony, known as zooids, perform different functions: gastrozooids are for feeding, gonozooids are for reproduction (specifically budding off medusae), and dactylozooids are studded with nematocysts for defense.

The medusa form, commonly associated with jellyfishes, is shaped like an inverted bowl with tentacles hanging from the brim. In this form, the mouth faces downward. Medusae are free-floating and swim through pulsating contractions. The mesoglea in medusae forms the bulk of the organism, providing its characteristic jelly-like nature.

Some complex colonial forms, known as Siphonophores, consist of both polyps and medusae. An example is the Portuguese man-of-war ($Physalia\,physalis$). These colonies can be between $9$ to $30\,cm$ long, but their dactylozooid tentacles can reach lengths of $40$ to $50\,meters$, though they are typically about $10\,meters$. These colonies function as a single unit, with medusae serving as floats for propulsion and polyps acting as gastrozooids, gonozooids, and dactylozooids.

Physiology: Nutrition, Digestion, and Metabolism

Cnidarians are almost exclusively carnivorous predators. They capture food using tentacles and mucus secreted from the mouth. They are generally considered passive predators, as polyps are immobile and not all medusae possess advanced sensory structures, meaning they rely on organisms accidentally straying into their tentacles. The captured food enters the gastrovascular cavity, also known as the coelenteron. This blind-ending cavity serves multiple functions: digestion, gas exchange, metabolic waste removal, and the discharge of gametes.

Digestion occurs in two stages. First, gastrodermal gland cells produce proteolytic enzymes for extracellular digestion within the cavity. Second, specialized gastrodermal cells called nutritive-muscular cells take up partially digested food via phagocytosis to form food vacuoles where digestion is completed. These cells also facilitate the movement of materials into and out of the cavity through peristalsis. Some cnidarians maintain a symbiotic relationship with zooxanthellae (dinoflagellate symbionts). In this arrangement, the cnidarian absorbs sugars (dissolved food) produced by the symbionts, and in return, provides the symbionts with metabolic wastes like $CO_2$.

Respiration occurs via the diffusion of oxygen ($O_2$) across the body surface or through acquisition from symbionts. Excretion of digestive waste occurs through the mouth, while metabolic wastes diffuse across the body surface.

Nervous System and Sensory Structures

The cnidarian nervous system consists of a decentralized nerve net made of sensory neurons. Despite lacking a brain, they possess specific sensory structures. These include ocelli for light perception and statocysts for balance. Statocysts contain sacs with statoliths, which allow the animal to recognize up and down displacement and determine its orientation in the water column.

Reproductive Biology and Life Cycles

Reproduction in cnidarians is both asexual and sexual. Asexual reproduction occurs through regeneration, budding, or binary fission (splitting along the lateral axis). Most cnidarians are gonochoristic (dioecious), meaning sexes are separate. Gametes are released into the gastrovascular cavity; fertilization may occur there or in the open water.

Development begins with the formation of a blastula embryo, which elongates into a ciliated, free-swimming larva called a planula. The planula eventually attaches to a substrate to develop into a polyp. Adult polyps can produce both more polyps and medusae by budding. In some species, buds break away as individuals, while in others, they remain to form a colony.

Many cnidarians exhibit metagenesis, which is the alternation of generations between sexual (medusa) and asexual (polyp) phases. In Scyphozoans and Cubozoans, the larva becomes a polyp called a scyphistoma. Through a process called strobilation, the scyphistoma absorbs its tentacles and splits horizontally into a series of disks called ephyra (young medusae). These ephyra swim off to become adults, while the polyp regrows tentacles to continue the cycle.

Taxonomic Classification and Biodiversity

There are over $10,000$ known species of cnidarians, all of which are aquatic and mostly marine. They are classified into four main groups:

1. Class Hydrozoa: This class includes approximately $3,600$ species. Most have both polypoid and medusoid stages, though the polypoid stage usually dominates. Some, like $Hydra$, lack a medusoid stage entirely. In $Gonionemus$, the medusa stage is dominant. A characteristic of hydrozoan medusae is the velum, a shelf-like lip on the bell margin. This is the only class with freshwater species. Examples include $Hydra$ and the Portuguese man-of-war ($Physalia$).

2. Class Scyphozoa: These are the "true" jellyfishes, with about $228$ species. The medusoid stage is large and dominant, while the polyp (scyphistoma) is small and inconspicuous. Unlike hydrozoans, scyphozoan medusae lack a velum. Their thick mesoglea contains amoeboid cells, and their gastrodermal cells have cilia to circulate seawater. Movement is achieved through rhythmic muscular contractions of the bell and elastic recoil. An example is $Aurelia\,aurita$.

3. Class Anthozoa: Known as "flower animals," this class contains over $6,100$ species, making it the most abundant group. They exist only as sedentary polyps; there is no medusoid stage. The mouth leads to a tubular pharynx (stomodaeum), and the gastrovascular cavity is divided by radial membranes called mesenteries. They can be solitary (sea anemones) or colonial (corals). Some are oviparous (egg-bearing) while others are viviparous (brooding young polyps). Examples include $Metridium$ (sea anemones), $Corallium$ (stony corals), $Gorgonia$ (sea fans), $Pennatula$ (sea pens), and $Renilla$ (sea pansies).

4. Class Cubozoa: These are the box jellyfishes, with about $42$ species. They have a cuboidal swimming bell with four tentacles (or groups) hanging from the corners. They are active, strong swimmers and are highly toxic, possessing dangerous nematocysts. Examples include Sea wasps and Box jellyfish.

Ecological and Practical Significance

Cnidarians and sponges have significant roles in ecosystems and human industry. Sponges provided the basis for anti-viral and anti-cancer drugs, such as the HIV drug azidothymidine ($AZT$) and anti-leukemia treatments. Historically, sponges like $Spongia\,officinalis$ were used for padding, painting, and bathing.

In ecosystems, cnidarians provide habitat and protection for crabs, shrimps, and brittle stars. Corals are central to the aquarium trade and are used medicinally for antimicrobial substances. In East Asia, jellyfishes have been a food source for centuries, with commercial fisheries existing in Asia and the USA. Additionally, corals and other cnidarians are used for decorative purposes.

Questions & Discussion

• Compare the methods of digestion in protozoans and sponges: This requires looking at intracellular vs. extracellular mechanisms.
• Structure and function of Sponge cells: Explain Archaeocytes (totipotent cells involved in digestion/regeneration) and Pinacocytes (plate-like cells for the outer covering).
• Define or describe the following:     - Endocytosis: The process of taking in matter by a living cell by invagination of its membrane.     - Phagocytosis: A type of endocytosis where a cell engulfs large particles or whole cells.     - Schizogony: Asexual reproduction by multiple fission, found in some protozoa.     - Gastrovascular cavity: The primary organ of digestion and circulation in Cnidaria and Platyhelminthes.     - Amoebocytes: Mobile cells in the body of invertebrates like sponges or cnidarians.
• Identification of Classes:     - Hydrozoa: $Hydra$, $Obelia$.     - Scyphozoa: $Aurelia$, $Desmonema\,glaciale$.     - Zoomastigophorea: (Requires reference to protozoan material).
• Contrast the life cycles of Plasmodium, Paramecium, and Obelia: Focus on the movement between hosts for Plasmodium, conjugation in Paramecium, and metagenesis (polyp/medusa) in Obelia.
• Determine the cause of green color in Hydra: This is due to a symbiotic relationship with green algae (Chlorella).