introduction to Coral biology

Introduction

Seeing as coral is the backbone of the reefs, it is only logical that we analyze its physiology and biology more in depth.

The individual animals that make up coral colonies are called Coral Polyps, which are part of a phylum of invertebrates called Cnidarians.

Within this taxon we can also find other organisms such as Jellyfish, Anemones, and Hydras.

Body Structure

All cnidarians share a similar body structure, both in terms of appearance and physiology. Most adult Cnidarians have either a free-swimming form called Medusa, or a sessile form called Polyp, while some Hydrozoans alternate between the two forms.

Opposite ends of the body are called the Oral end (closest to the mouth) and the Aboral end (furthest). In polypoid cnidarians the aboral end is connected to the substrate, while in medusoid ones it is not attached to anything and it constitutes the head of the organism.

One or more rings of tentacles can usually be found around the oral cavity; their function is to capture prey and to bring it towards the mouth to be ingested, as well as protection against predators.

To achieve this end, they employ a particular type of stinging cells called Nematocysts.

As seen in the image below, a nematocyst is a capsule containing a coiled up, hollow, barbed tubule. When triggered, the nematocyst expels the tubule like a harpoon, exerting a tremendous amount of force and puncturing the target. When expelled, the harpoon also injects a small amount of venom, which can have different effects on the target depending on various factors. The effects can range from a mild irritation to paralysis or even death. If the target is a prey, the tentacles can drag it towards the mouth, where it will be ingested and then digested in the gastric cavity.

Zooxanthellae

The vast majority of warm water corals live in conjunction with one of many photosynthetic algae called Zooxanthellae (Symbiodinium spp.); a relationship called mutualistic symbiosis, where each organism provides something for the other, so that they may both thrive: coral polyps offer a safe environment for the algae to live in, while the zooxanthellae produce nutrients through photosynthesis that are then passed on to the polyp.

In addition to providing corals with essential nutrients, zooxanthellae are also partially responsible for the unique and beautiful colors of many corals.

Skeleton

While nematocysts offer a suitable degree of protection against predators for most cnidarians, many coral polyp species have evolved a hard skeleton as a secondary defense mechanism (Hard Corals). This structure is composed of Calcium Carbonate, and it offers a shelter inside which polyps can retract and hide to evade predation, as well as mechanical resistance to physical stress agents such as waves and impact. Corals that do not secrete a Calcium Carbonate skeleton structure are known as Soft Corals.

Reproduction

Throughout their lifetime coral polyps go through a number of different phases which can be very different from one another; they can grow and reproduce in a number of different ways depending on both genetic factors and environmental pressures.

Corals begin their life as small free-swimming larvae called Planulae, suspended in the water column until they settle on the ground; depending on the species, this phase can last from a few days up to several weeks.

If they settle on suitable substrate, the newly formed coral enters its benthic phase, during which it undergoes metamorphosis and begins to reproduce asexually, a process called budding.

Budding occurs when a portion of the parent polyp pinches off to form a new individual; it is very similar to cellular mitosis, where the mother cell grows in size and duplicates all its parts in order to then split and form two identical daughter cells.

This process allows the colony to grow at relatively fast speed compared to sexual reproduction, but due to the fact that all polyps of one colony are clones, their genetic variability is minimal, making them all equally susceptible to stress factors and disease.

A second advantage of all polyps of one colony being clones is that they are not specialized. Every polyp performs the same functions as any other, which in turn means that each polyp is completely autonomous and does not rely on functions performed by other polyps to survive. This also implies that if for any reason a part of the colony gets broken off, provided the environmental conditions remain favorable, that piece of colony can continue to thrive and reproduce asexually, creating a new autonomous colony. This process takes the name of fragmentation, and it is the basis on which the coral propagation technique was developed.

Coral colonies can eventually also undergo sexual reproduction, during which they produce gametes (sperm and eggs), which are released in the water column to eventually fuse and form new planulae, restarting the life cycle.