Coral reefs of La Paz

pocillopora coral speciesAlthough La Paz is not known for its coral reefs there are at least 39 species of stony coral and 21 species of soft corals found here. Living in among these corals are numerous species of invertebrates and fish all contributing the reef system as a whole. One of the dominant coral species here in La Paz is a stony coral found in the genus Pocillopora. Mound shaped with thick, short branches it creates large colonies which act as a home for a number of different species.There are species of coral on all of the dive sites in La Paz but two main reefs, the island of San Rafaelito and Suwanee Reef. These two dive sites have impressive coral gardens and plenty of macro species. These two reefs could be classified as patch or barrier reefs but our type of reef is a little different than your typical tropical reef. Although the dominant species is a stony coral it does not really contribute to the reef's physical substrate. One of the main components of the substrate, or rock, is mollusc shells. Corals tend to thrive here in micro-environments where the conditions for coral growth are present. Up-welling plays a major part, bringing cold, nutrient and oxygen rich water from the deep in contact with these reefs. Currents bring in a constant supply of food and most of the coral growth is concentrated on small islands and points of land. While our coral reefs are not extensive they are robust, with the reef at San Rafaelito displaying almost 100% cover of pocillopora and large black coral species (Antipathes galapagensis) present in the deeper reefs and shipwrecks of La Paz.

Creature Spotting and Macro Life

Most invertebrates are very small or highly camouflaged to escape predation. Finding them usually requires a little study beforehand, learning their habits and their preferred habitat. The most beneficial thing a diver can do to improve their creature spotting is to slow down. The slower you go the more chance you will have to spot these creatures. It's also a good idea to learn about their habitats. A large number of species are symbiotic with one species of coral, sponge or other invertebrate. They are easy to find because you just look for their host which is typically much bigger and easier to find.

Unfortunately the Sea of Cortez does not have any good invertebrate identification books, there is "Sea of Cortez Marine Animals: A guide to common fishes and invertebrates" but it only has a few invertebrate species. Another place to try is mexfish.com; they have a number of invertebrate species described as well as some birds and mammals. If you're scientifically inclined you can use The Gulf of California Invertebrate Database, it allows you to search using taxonomy and is quite extensive.

There are countless other examples of symbiosis among the marine species here in La Paz and there are far too many invertebrate species to describe here, over 3000 just in Baja Sur, represented by 16 different phyla (taxonomy groups). In short, if you are into macro you won't be disappointed.

Coral Reefs     

Besides man, corals are the only species with the ability to permanently alter the surface of the earth, creating large chains of islands and shaping the coastline. In many areas coral reefs protect the coastline from the open ocean, providing a habitat for mangroves and sea grass beds to thrive. Countless people directly rely on them for their livelihood, both in tourism and fisheries. Globally we rely on them for their photosynthetic activities, carried out by their symbiotic algae. The by-products of this photosynthesis serve as a vital input of food into the tropical/sub-tropical marine food-chain, and assist in recycling the nutrients. Coral reefs provide home and shelter to over 25% of fish in the ocean and up to two million marine species. They are also a nursery for the juvenile forms of many marine creatures. Corals are found throughout the world, in temperate, tropic and arctic waters, able to thrive in all zones of a coastline, from the inter-tidal area down to over 3000m/9800 ft.

What is coral?

A "coral head" is a colony made up of individual organisms. All of the individuals are in the sessile (not-mobile) polyp stage of their life cycle and are typically clones of one individual, the ancestral polyp. This individual is the result of coral spawning, a sexual means of reproduction in corals which happens just once a year. The first stage of the life cycle is a free swimming medusa very similar to a jellyfish. They drift until they come in contact with an available surface and than attach themselves and transform into their polyp stage. This ancestral polyp will than clone itself by either division or budding and form a large colony of individuals. This colony is what is termed a "coral head". There are over 2,500 different species of coral worldwide and all have adapted special ways to survive in a nutrient poor environment where space is highly limited.

A typical individual coral polyp is made up of three layers of cells, a thin mesoglea bordered by the exo and gastroderm. The body is cup shaped with a central opening (mouth) which leads into the gut cavity. This mouth is ringed with tentacles that extend into the water column and using nematocysts the polyp can catch prey that is floating by, mostly in the form of zooplankton, microscopic animals.

Corals can be further divided into two major groupings, Hermatypic (hard) corals, which produce stony skeletons and have zooxanthellae (algae) in their tissues and Ahermatypic (soft) corals, which have soft bodies and no zooxanthellae.

Symbiosis

zooxanthellaeTo understand how corals live you need to understand symbiosis, defined as the close physical interrelationship between two different species. There are three types of symbiosis; parasitism, commensalism and mutualism. All types of symbiosis are present in the marine environment and in the case of corals their survival depends on it. If we look at a colony of Pocillopora we can see examples of all three types of symbiosis. The coral exists because of a relationship between the coral and a specific species of algae which lives in its tissues, zooxanthellae. This microscopic plant acts like any other plant, absorbing CO2 and releasing oxygen as a byproduct. The coral, like any other animal, needs oxygen to breath and utilizes the oxygen produced by the algae in turn providing it with CO2. The waste products of photosynthesis, glucose, glycerol, and amino acids are utilized by the coral for food and to create its skeleton which is Calcium Carbonate. Up to 90 percent of the organic material photo-synthetically produced by the zooxanthellae is transferred to the host coral tissue. This efficient relationship, known as mutualism has allowed corals to become so successful.

coral hawkfishThe small fish and invertebrates that live outside the coral colony use its branches and fissures to hide from predators, either darting out to feed and returning for protection or simply feeding off the detritus that builds up in the cracks and crevices of the coral host. These species use the host as a home, taking advantage of the host but not creating any harmful effects. This relationship, known as commensalism, is where one species benefits from the relationship while there is no impact to the other. While it can be argued that the small fish and invertebrates keep their host colony clean and therefore benefit the coral host it is still a good example.

guard crabBurrowed into the coral branches you can find worm snails, marine molluscs belonging to the family Vermetidae. Unlike most marine creatures these worm snails do not start off their life as pelagic larvae. Born from the female they simply crawl until they come in contact with a coral host, attach themselves and secrete a tubular shell which eventually gets enclosed by the growing coral. The end of the shell remains open and the worm snail uses a modified part of its foot called the operculum to block the opening when threatened. From this shell the worm snail secretes mucus nets to trap floating plankton and reels them in once they are full. In this relationship the host coral is affected in two ways, firstly the worm snail takes up valuable space. Corals photosynthesize and their success partly depends on the amount of surface area exposed to sunlight, space which is taken up by the worm snail. While at first glance they do seem small and insignificant but when they exist in great numbers they have been shown to limit coral growth. Secondly the worm snail's mucus contains toxins which deter coral growth and are feeding on the same prey that the corals are. This direct competition for food sources and twormsnailhe harmful effects of the mucus impact the coral host in a negative way, an example of parasitism.  

There are countless other examples of symbiosis among the marine species here in La Paz and there are far too many invertebrate species to describe here, over 3000 just in Baja Sur, represented by 16 different phyla (taxonomy groups). In short, if you are into macro you won't be disappointed.

 

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