General Coral Biology Part I: Basic Physiology
Chris Maupin (“galleon”)
Corals belong to an evolutionarily unique phylum known as the Cnidaria. The cnidaria are probably an evolutionary dead end, arising from the original ancestral metazoan: an aquatic conglomeration of choanoflagellates. Choanoflagellates are much like free living examples of collar cells found in sponges today: a flagellated cell with a collar of fingers called microvilli surrounding the flagellum. Within the phylum Cnidaria, we have four classes: Scyphozoa (true jellyfish), Cubozoa (box jellies, sea wasps), Hydrozoa (fire coral, Portuguese man-o-war, hydroids), and Anthozoa, which contain all species of corals and anemones.
Stony corals belong to the subclass Hexacorallia and
soft corals belong to the subclass Octocorallia.
While many cnidarians go through a polyp and a medusa stage in their life cycle, making them dimorphic, corals and other Anthozoans only exist as polyps and planktonic larvae, called planula. An entire coral colony consists of only two cell layers; the epidermis or ectoderm and the gastrodermis or endoderm. Thus, it is said that corals, like all cnidarians, are “diploblastic.” Between the ectoderm and gastroderm lies a very thin layer of acellular matrix known as the mesoglea. In this article, gastrodermis and endoderm are used interchangeably, as are epidermis and ectoderm. Epidermal cells are highly polymorphic, meaning they develop many different kinds of special structures and functions. The functions will be covered in part II.
One of the most identifying structures of the cnidaria are cnidocytes. These are specialized epidermal cells that project threads perform various prey catching and defense actions. While there are about 30 different types of threads, called cnidae, by far the most studied are the nematocysts (“thread-bag”). Nematocysts are used primarily as prey capture tools. Prey touches cnidocil, which are tiny triggers that line the bag covering (called the operculum). This trigger somehow allows osmotic pressure to violently build within the nematocyst, causing the operculum to open and the thread to lash out and secure itself via barbs. The thread is stored in an inside-out position within the bag and inverts itself as it is fired. After the thread “harpoon” is secure, proteinaceous toxin is pumped through it and into the prey.
Corals and other cnidaria are also believed to be some of the earliest organisms to have evolved some type of nervous system. In cnidaria, while the nervous system is not central or very advanced, it is efficient and effective. Modified epidermal cells called neurons contain long, thin strand-like processes, called neurites, that synapse into each other. This creates a neural network, of which corals and other cnidarians have two separate ones: one fast conducting and one slow conducting.
The slow conducting network generally has single, unfused neurites coming off of many sides of the cell (multipolar). Synapses can fire in a single direction or back and forth across the neural network, depending on how the neurites come together. The fast conducting network lies underneath the slow network and is formed by fused neurites coming from only two sides of the neuron cell (dipolar). This reduced branching of neurites and their fusing to a larger size allows for a faster and more direct movement of signal to the musculature. When neurons that are exposed to the water, “neurosensory cells,” are stimulated (by touch, etc.), waves of synapses fire, spreading across the animal and ending at a neuromuscular junction, which we’ll get to next.
The main portion of epidermal cells are epitheliomuscular cells. Whereas you and I have muscles formed from our third, inner cell layer, the mesoderm, corals and other cnidarians must make do with their two cell layers. In the ectoderm, most cells have long, thin, cylindrical bases, called myoneme, that reach into the mesoglea and are anchored there. These muscular processes run longitudinally along the entire epidermis. They contain contractile proteins, much as our muscles do, that enables them to relax and lengthen, or contract and shorten based on signals coming to their neuromuscular junction. If you imagine a cube (the mass of the cell) with a cylinder (the contractile base) up against one side (the side facing towards the mesoglea), that’s pretty much what and epitheliomuscular cell looks like. In corals, the epidermal cells give way to gastrodermal cells in the actinopharynx, which will be explained later. Typically, these last epidermal cells are responsible for mucus production.
Between the ectoderm and gastroderm is the thin mesoglea. While the mesoglea is nonliving and gelatinous, it contains interstitial cells called archaeocytes, which probably came from developing epidermal cells. The archaeocytes most likely function to transfer and store nutrients and act as a pathogenic defense system.
On the other side of the mesoglea lies the gastrodermis or endoderm. This cell layer mainly consists of nutritive muscular cells. These cells are identical in form and function to epitheliomuscular cells; the only difference is that nutritive muscular cell processes are oriented sideways, so the myoneme, when connected, run in a circle around the polyp. Imagine hula-hoops stacked perfectly on top of one another. This is what the complete set of nutritive muscular cells looks like. Don’t forget, since endodem and ectoderm make up the entire polyp, that means that the two different types of muscular cells are in tentacles as well, allow corals to transfer prey caught with nematocysts and other cnidae to the mouth.
Zooxanthellae, a unicellular, photosynthetic dinflagellate, are cultured within the gastrodermal cells of many Anthozoans. These algae are stimulated by the coral host to release from 20% to 95% of its photosynthetically-produced organic compounds; mostly glycerol, amino acids, fatty acids, glucose and lipids. Zooxanthellae density within corals is about 1-5 million cells per cm^2 (Pechenik, 2000). The zooxanthellae also assist skeletogenesis in the subclass hexacorallia in several ways, which will be discussed in upcoming installations.
In the center of the polyp is the gastrovascular cavity. This cavity is open to the water at the mouth. In corals, the upper gastrovascular cavity is modified to be a pharynx, called the actinopharynx. This pharynx is created by ectoderm running down inside the mouth, behind which is mesoglea and gastroderm support. On one or more sides of the actinopharynx, there are groves running vertically, down the length of the pharynx, and containing ciliated, water pumping cells. These grooves are known as the siphonoglyphs. The ectoderm terminates as the pharynx opens to the gastrovascular cavity, which is divided by radially symmetrical folds of gastrodermis (behind which is mesoglea) called mesenteries. Mesenteries are modified to be multidisciplinary: along with being crucial for digestion, coral embryos are formed in the gonads contained within the mesentaries.
There are two types of mesenteries, those that attach to the actinopharynx, which are called complete mesenteries, and those that are free folds in the gastrovascular cavity, called incomplete mesenteries. These incomplete mesenteries play very important roles. The edge of the mesentery is divided into three stubs that are loaded with cells that pump water by cilia, cells that secrete digestive enzymes, bacteria-consuming phagocytes (cells that engulf and digest foreign matter), and also nematocysts. Water pumping cells are crucial in the gastrovascular cavity, as cell respiration is accomplished solely through diffusion. Thus, oxygenated water must be brought to the cells within the gastrovascular cavity and coelenteron (open internal space, most often part of the gastrovascular cavity). Another quirk of mesenteries is the reorientation of what once were nutritive muscular cell myoneme. The myoneme have been turned vertically and are heavily clustered, creating thick powerful muscles used for improved longitudinal retraction ever what the epitheliomuscular cells alone can provide.
In the upcoming parts, skeletogenesis, feeding, behavior, and special adaptations of different Anthozoan subclasses (Hexacorallia, Octocorallia) will be discussed.
References Cited
Pechenik, J. A. (2000). The Cnidarians.
Biology of the invertebrates. New York: McGraw-Hill Companies.
