Briozoos: Morfología, alimentación, reproducción y ecología

Bryozoa

The phylum Bryozoa includes small marine organisms, commonly called moss animals or bryozoans. These animals form colonies with up to millions of zooids, which resemble tiny polyps with oral and aboral cavities. Each colony may occupy a single surface area, while others can grow into large masses. Let's delve deeper into their morphology, feeding mechanisms, reproduction, ecology, and evolution.

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Morphology

Structure

Morphologically, bryozoans consist of two main parts: a protective outer shell known as the calcareous envelope and an inner part composed of a gelatinous material. The individual members of the colony are referred to as zooids, which are connected by tissue called zooecia. A typical zooid consists of five regions: the mouth, stomach, intestine, anus, and buccal cavity. Some species of bryozoans have specialized structures such as spicules, glands, and filaments, adding more complexity to their overall structure.

Size and Size Range

Most bryozoans range from 1–15 mm in diameter, although some species can reach up to several centimeters in size. The colony size also varies significantly among different species, ranging from small clusters of zooids to massive aggregations covering vast surfaces.

Coloration and Appearance

Bryozoans come in various colors, primarily due to the presence of pigments in their cells. Commonly observed colors include red, orange, yellow, green, brown, and purple. Their coloration is influenced by both the growth environment and the location within polypide or the corallite.

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Feeding Mechanisms

Food Consumed

Bryozoans consume microscopic plankton, including bacteria, diatoms, dinoflagellates, foraminiferas, copepods, and radiolarians. They rely on water currents generated by the beating of tiny hair-like appendages, called cilia, to obtain food particles from their surroundings. Many of these food sources are tiny enough for unfiltered ingestion through the same ciliated opening used for expulsion of waste products.

Filtration and Transport System

In general, water transport occurs via the extensive network of channels inside the bryozoan skeleton. In bryozoans with a calcareous envelope, the water moves through the interstices between the calcified plates and the underlying organic matrix. This system allows them to filter feed continuously without actively moving around in search of food.

Digestive System

Bryozoans possess a simple digestive system consisting of an oral cavity, a short esophagus, a stomach, and a narrow intestine. Digested food is absorbed directly into the bloodstream, nourishing the surrounding tissues and helping to maintain the structural integrity of the colony.

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Reproduction

Modes of Reproduction

Bryozoans reproduce both sexually and asexually. Sexual reproduction involves the release of gametes that are exchanged during copulation between two colonial members, ultimately resulting in the development of new colonies. On the other hand, asexual reproduction occurs through budding, whereby new zooids develop from the parental ones, forming new colonies without the involvement of sexual reproduction.

Sexual Reproduction

In sexual reproduction, bryozoans release gametes into the surrounding water, where they are mixed and fertilized. The resulting zygote then develops into a larval form, called a veliger larva. The veliger larva undergoes metamorphosis to form a free-living adult, which then settles and forms a new colony.

Asexual Reproduction

In asexual reproduction, new zooids develop from the parental zooids, growing outwards from the existing colonies. This process, known as budding, can occur both terminally and laterally, leading to the formation of new colonies.

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Ecology

Habitat

Bryozoans can be found in a variety of habitats, including intertidal and subtidal zones, in the deep sea, and in freshwater environments. Their distribution is influenced by environmental factors such as temperature, salinity, and nutrient availability.

Ecological Role

Bryozoans play a crucial role in marine ecosystems. As filter feeders, they help to maintain water quality by removing excess nutrients and organic matter. Their structures also provide important habitats for various marine organisms, serving as substrate for the growth of other species.

Ecological Threats

Bryozoans are threatened by habitat degradation, overfishing, and pollution. Additionally, some species are considered pests due to their ability to form dense colonies, which can outcompete native species and create ecological imbalances.

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Evolution

Evolutionary History

Bryozoans are believed to have evolved from a common ancestor shared with other ectoprocts (such as tunicates) around 540 million years ago. They diverged into two main groups, the Cheilostomata and the Stenolaemata, around 450 million years ago.

Evolutionary Adaptations

Bryozoans have evolved a variety of adaptations to cope with their environment. For example, some species have developed unique structures such as spicules and filaments to enhance their stability and feeding capabilities. Additionally, their ability to reproduce both sexually and asexually has allowed them to adapt and colonize a wide range of habitats.

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In conclusion, bryozoans are fascinating marine organisms that exhibit a variety of morphological, ecological, and evolutionary adaptations. Their role as filter feeders and habitat providers in marine ecosystems highlights their importance and underscores the need for their protection and conservation.