Harmful Algal Blooms, Echinoderms, Mollusks, and Eutrophication
Harmful Algal Blooms
Harmful Algal Blooms (HABs) are the result of the rapid multiplication of a single species of microorganisms known as photosynthetic microalgae. This proliferation reaches millions of cells per liter and can be colored by pigments on the cell surface, hence the popular name “Red Tide.” Those blooms produced by microalgae that are present in different oceans across the globe and are capable of producing potent toxins are termed Harmful Algal Blooms, a term coined by the Intergovernmental Oceanographic Commission of UNESCO (IOC). HABs are caused by the proliferation of several species of microalgae. Among the main causes of toxin production and mass poisonings associated with human consumption of contaminated shellfish are dinoflagellates and diatoms.
Factors that Enhance the Formation of HABs
Most dinoflagellates, including toxic species, can reproduce by simple division. This allows a few cells present in the environment to reach concentrations of cells/L. This is achieved by two mechanisms:
- Biological growth stimulated by appropriate regimes of low salinity and high nutrients.
- Physical mechanisms favoring the concentration of cells in well-defined areas of water, such as winds and sunstroke.
The occurrence of the highest concentrations of toxic organisms, decreased salinity, and increased water temperature precede toxic event occurrence and abundance. These maximum concentrations were observed in low salinity and relatively high temperatures at the beginning of the bloom. High-pressure situations of relative calm, marked by light winds (generally not exceeding 5 m/s) and high sunstroke, help maintain the stability of the water column, thus favoring the development of harmful algal blooms.
Echinoderms
Echinoderms are exclusively marine animals that are usually part of the benthos. They are common on rocky shores whipped by waves. Their bodies have radial symmetry and always consist of five or more repeated radii around the animal’s body axis.
Echinoderm Classes: Morphology
There are several differences between the various classes. Thus:
- Class Asteroidea: The typical starfish, with a central disc from which five arms arise that encompass the entire disc. The mouth is in a central position. Movement is mainly due to the ambulacral system. If we lift a star and look at the bottom, we observe some small extensions called ambulacral feet that the star uses to get around.
- Class Ophiuroidea: Like other echinoderms, brittle stars have five branches that arise from a central disc and become 10 to 15 times longer than it. These arms are used for rhythmic movement, hence the name “sea dancers.” In some species, their arms detach at the slightest touch as a defensive measure. This phenomenon in echinoderms shows that from the grappling arms and swimmers of the crinoids, it passes to the reptating arms of brittle stars.
Mollusks
- Bivalves or Lamellibranchs: Their bodies are covered with a shell formed by two pieces or valves. They present various forms, and the leaflets are attached to each other by the dorsal region and articulate by an area called the hinge, which allows opening and closing, preventing lateral displacement of the leaflets. Their growth is continuous, and the shells present ribs or striations. They also have strong muscles called adductors.
- Gastropods: Mollusks characterized by having their bodies covered by a shell in one piece, more or less coiled. Their name means “stomach foot.” Their bodies are distinguished by a head, foot, and visceral mass. The number of shell turns varies, with some common gastropods having one turn, like the limpet.
- Scaphopods: A small group of marine mollusks bearing a conical, tubular shell that is open at both ends. The most common, Dentalium, lives in the benthos and receives its name because it resembles an elephant’s tusk.
- Cephalopods: This class is the most evolved of the group. Their most prominent feature is having a crown of tentacles as arms or head extensions. The shell is very reduced in the majority of these species, even disappearing in some of them. There are rare species with shells, such as those in the Pacific Ocean. They are curious swimmers and are mainly part of the nekton. During the day, they hide and only come out at night to forage for small crustaceans.
Eutrophication
A lake or a river suffers eutrophication when its waters become rich in nutrients. If there is an excess of nutrients, plants and other organisms grow. Later, when they die and decay, they fill the water with bad smells and give it a sickening appearance, drastically decreasing its quality. The putrefaction process consumes a large amount of dissolved oxygen, and the water is no longer suitable for most living things. The result is an almost destroyed ecosystem.
Eutrophic and Oligotrophic Waters
When a lake or reservoir is nutrient-poor (oligotrophic), the water is clear, light penetrates well, algal growth is small, and it keeps a few animals that are characteristic of well-oxygenated waters, such as trout. As the lake becomes loaded with nutrients, it becomes eutrophic. Algae grow in large quantities, making the water turbid. When algae and other organisms die, they are decomposed by bacterial activity, which consumes oxygen. Fish that need oxygen-rich waters cannot live. In some lakes, anaerobic decomposition produces foul-smelling odors. The water is murky and of low quality for human consumption and sports use. The bottom of the lake fills with sediment, and its depth decreases.