On coral reefs, there are many different habitats (biotopes), each with its own unique characteristics. Each plant and animal species has specific conditions that it needs for survival and reproduction. Some species are highly specialized and can only live in specific habitats, while others are adaptable and can be found in a variety of habitats. The distribution of certain species may seem mysterious, but it is actually influenced by factors that are not fully understood or are random, such as the settlement of larvae or the impact of a storm.
At the base of the coral reef's food chain are marine plants, including single-celled forms such as diatoms, planktonic algae, and symbiotic zooxanthellae, as well as multicellular benthic algae and seagrasses. These plants are followed by consumers, which are divided into three trophic levels: herbivores that eat plants, omnivores that feed on both plants and animals, and predators that consume other animals. Among the consumers, there are smaller groups based on their diet, such as planktivores, coral feeders, and detritivores that consume decaying organic matter. Corals and other animals containing zooxanthellae are unique, as they are herbivorous and cultivate their own plants within their bodies. The majority of major animal groups on the coral reef encompass both herbivorous and predatory forms, as well as omnivorous forms.
Among the animals of the coral reef, one can find a wide range of behaviors and lifestyles. Even seemingly "inanimate" animals like corals exhibit remarkably complex behaviors. They actively capture planktonic organisms and engage in chemical warfare with their neighbors by releasing toxins that prevent overgrowth. Of course, it is easier to observe behavior in active and mobile animals such as fish. Almost all behavioral patterns are linked to feeding, avoiding predators, and reproduction. Different species employ various strategies to tackle these fundamental tasks, resulting in diverse lifestyles: solitary or social, sedentary or migratory, territorial or peaceful, and active during the day or night.
Protective coloration and mimicry are effective strategies for approaching prey or concealing oneself from predators. Protective coloration enables an animal to blend seamlessly with its surroundings.
The skin of skinks and sculpins is adorned with growths, brushes, and warts that resemble leaf-like plates of algae. In some cases, these growths even become covered in algae. Numerous grouper species possess transverse stripes that disrupt and camouflage their outlines, rendering them virtually invisible (known as disruptive coloration). Juvenile gobies, such as Novaculichthys taeniuorus, have the appearance and swimming behavior of a detached clump of algae.
Mimicry refers to a phenomenon where one organism, known as the mimic, resembles another organism, known as the model, which is protected from predators due to its unpalatability, toxicity, or other means of defense. There are four forms of mimicry: Batesian mimicry, where an unprotected species mimics a protected species; Müllerian mimicry, where two or more protected species resemble each other; aggressive mimicry, where the mimic utilizes its resemblance to the model for predation or other hostile actions; and social mimicry, where two or more species resemble each other and interact, collectively defending themselves against predators due to their numerical abundance.
Among reef animals, Batesian mimicry is commonly observed. Both the model and the mimic possess distinctive warning coloration that predators tend to avoid. Typically, the model species is more widespread than the mimic. A characteristic example is the slow-swimming venomous black-saddled pufferfish (Canthigaster margaritata) and the mimicking spotted pufferfish (Paraluteres arqat). In the case of aggressive mimicry, the mimic utilizes its resemblance to a harmless or beneficial species to attack other animals. This type of mimicry is prevalent in the family Labridae. A classic example is the bluestreak cleaner wrasse (Aspidontus taeniatus), which mimics the appearance of the blue cleaner wrasse (Labroides dimidiatus). The cleaner wrasse provides cleaning services to other fish by removing parasites and dead skin. Predatory fish also benefit from the services of cleaner fish and do not prey on them. The mimic cleaner wrasse deceives the fish by making it assume a cleaning posture and then swiftly bites off a piece of fin or scale! Experienced fish can usually distinguish the mimic from the cleaner and avoid the imitator. Social mimicry is employed by schooling and group-living species to enhance collective defense by increasing the overall size of the group. It becomes more challenging for predators to focus on a single target when it is surrounded by a larger group of similar individuals compared to a smaller group. Different species can increase the size of their groups if they resemble one another and form mixed aggregations. An example of this is the females of the fairy basslets (Pseudanthias squammipinis and Pseudanthias taeniatus) and the bicolor blenny (Ecsenius midas), which exhibit similar coloration and mingle with one another.
Symbiosis is the close interaction between two different organisms. There are three main forms: mutualism, where both organisms depend on each other; commensalism, where one organism benefits from another without causing harm; and parasitism, where one organism benefits at the expense of another.
In the life of reef animals, there are numerous examples of all forms of symbiosis. The coral reef itself is built thanks to symbiotic relationships between corals and zooxanthellae. A classic example of mutualism is the relationship between cleaner fish and cleaner shrimp. Anemonefish in anemones provide the most famous example of commensalism. Almost every reef organism carries parasites. Most of them are internal and not visible to the naked eye, but there are also external ones: some copepods and isopods live attached to the skin of fish, and barnacles invade the bodies of echinoderms and sometimes feed on their internal organs.
The reproductive strategies of coral reef organisms exhibit a remarkable diversity. Many primitive invertebrates engage in both sexual and asexual reproduction. Flatworms simply divide in half, while sponges and stoloniferous organisms can produce new individuals through budding, resulting in the formation of large colonies of genetically identical organisms, known as clones. Colonial organisms often grow in a manner that resembles the shape or branching patterns of plants. Vertebrate animals exclusively rely on sexual reproduction, where an egg and sperm from different individuals fuse together, giving rise to the development of a new organism. Occasionally, this process takes on unique forms. Hermaphroditism is prevalent among marine animals, with certain species possessing both male and female reproductive organs simultaneously. These organisms can exchange gametes, fertilizing each other, or take turns assuming male or female roles. In some cases, animals are sequential hermaphrodites, starting their lives as one sex and then transitioning to the opposite sex at a certain age. Fertilization can occur either internally, with the male introducing sperm into the female's body using a penis or other copulatory organ, or externally, with gametes being released into the water, where fertilization takes place.
Most marine creatures produce offspring in large quantities. Mortality rates are high, but enough individuals survive to reproductive age to ensure the species' survival. Some larger creatures, like sharks, rays, and marine mammals, give birth to a small number of fully developed offspring that have a higher chance of survival. Marine creatures that produce a large number of eggs deposit them on the seafloor (nests, spawns) or release them into the water column. Most mollusks and some fish (damselfish, dottybacks, wrasses, gobies, parrotfish, and triggerfish) lay their eggs on the seafloor. Mouthbrooders, such as cardinalfish and bigmouths, carry the eggs in their mouths until they hatch into larvae. Other fish and many larger invertebrates release their eggs into the water column.
Early development of marine animals occurs through either direct development or a larval stage. Direct development is uncommon among reef organisms and is observed in certain mollusks, all sharks and rays, and only three species of bony fish (not from our region). Generally, species with direct development have relatively few offspring, and their distribution is limited.
Most marine creatures go through a long pelagic larval stage in their development, which helps them spread out over a wide area. During this stage, the larvae look like planktonic organisms and often have a yolk sac on the outside that gives them nutrients until their digestive system develops. The larvae float along with the currents and initially feed on phytoplankton. As they grow, they switch to eating zooplankton. The open water is much safer for these larvae than the reef environment, which is full of different filter-feeding and plankton-eating animals. Some larvae actively swim and use various environmental cues to find suitable places to settle down.
Reproduction among organisms in coral reefs typically follows a cyclic pattern. Spawning often takes place during the full moon or new moon, when tides are at their highest and currents are strongest. This enables eggs and larvae to disperse over a wide area. Many animals spawn at dusk near the reef edge to move their eggs away from the reef, away from most planktivorous fish that sleep at night. Reproduction for many animals occurs in the spring, coinciding with the period of settlement and initial growth of juveniles when water temperatures are at their maximum. Once a year, some large migratory fish, such as groupers and snappers, gather in large numbers at their preferred spawning sites. These sites are usually found in large channels or promontories, where strong currents quickly carry the eggs into the open ocean. Many corals only spawn once or twice a year, typically shortly after sunset during the full moon in the middle of summer.
Growth rates and lifespan can vary significantly among reef organisms. Some smaller organisms reach reproductive maturity within a few months and have a lifespan of less than a year. Many commonly found reef fish, including groupers, snappers, wrasses, butterflyfish, angelfish, damselfish, and surgeonfish, have surprisingly long lifespans, ranging from 10 to 20 years. Larger species take several years to reach reproductive maturity and can live for up to 80 years or even longer. Colonial organisms like corals have the potential to live for thousands of years.
Reproductive strategy and maximum lifespan are important traits of a species that influence its ability to withstand human exploitation, such as fishing. Species that have short lifespans, grow quickly, and reach reproductive maturity early, while producing a large number of offspring, show significant resilience to overfishing. These species tend to recover quickly once protective measures are implemented. On the other hand, species that grow and mature slowly, and produce few offspring, are more vulnerable. They are often the first to disappear and are at a higher risk of extinction.