Plankton are not defenseless drifters; they possess a remarkable array of shells, covers and carapaces, each intricately designed for survival in the vast marine environment.
The Slimy Defense of Marine Bacteria
While some plankton rely on hard shells or exoskeletons, marine bacteria have a different trick up their sleeve. Under certain conditions, like abundant nutrients or calm waters, these tiny organisms (as well as some microalgae) can produce a slimy substance called mucilage. This gelatinous soup, a complex mix of carbohydrates and proteins, offers a protective barrier against predators and harsh environmental conditions. Mucilage can also trap nutrients and organic matter, creating a readily available food source for the bacteria themselves. However, excessive mucilage production can disrupt the delicate balance of the marine ecosystem, so it’s a strategy to be used cautiously.
Protists: Masters of Architecture
The sunlit surface layers of the ocean are dominated by phytoplankton, the plant-like members of the plankton community. Diatoms, single-celled algae, reign supreme with their exquisitely crafted silica shells called frustules. These intricate structures, meticulously patterned with ridges, spines, and pores, offer not only structural support but also play a crucial role in buoyancy control. By adjusting their position within the water column, diatoms optimize light exposure for photosynthesis.
Diatom of the genus Chaetoceros. Albert Calbet
Another group of phytoplankton, the coccolithophores, utilizes a different approach to shell construction. These tiny algae fashion their protective coverings from intricately arranged plates of calcium carbonate called coccoliths. Each coccolith is a marvel of geometric precision, meticulously crafted by the coccolithophore cell and arrayed around its surface like the scales of a microscopic suit of armor. These coccoliths not only shield the cell from predation but also contribute to the global carbon cycle as the calcium carbonate eventually settles on the seafloor as sediment.
Unlike their counterparts with well-defined silica or calcium carbonate armor, dinoflagellates exhibit a fascinating diversity in their protective coverings, known as thecae. Some dinoflagellates, like Ceratium and Protoperidinium species, sport armored plates composed of cellulose or a combination of cellulose and silica. Others boast more complex thecae, possibly incorporating additional carbohydrates and proteins alongside cellulose. These proteins might even enhance strength or provide functionalities like buoyancy or improved movement. While some thecae resemble simple plates offering protection, others in the genus Ceratocorys are intricate shells adorned with spines, ridges, and ornamentation, serving both defensive and buoyancy purposes.
Dinoflagellates. A) Ceratium sp. B) Ceratocorys sp. Albert Calbet
Tintinnids, a group of ciliate protozoans, construct delicate “houses” or shells called loricae from secreted organic materials. These shells vary in shape and composition among different species, ranging from simple cylindrical tubes to elaborate spiraled structures. Tintinnid loricae protects the organism’s soft body and serve as a means of attachment to floating debris or marine surfaces. Some species even incorporate mineral particles for added strength and rigidity.
Tintinnind ciliate. Albert Calbet
Radiolarians are renowned for their intricate mineral skeletons, named tests, primarily composed of silica. These tests come in a dazzling array of shapes and designs, from spherical to filamentous, and often exhibit elaborate ornamentation, including spines, ribs, and lattice-like structures. Radiolarian tests provide structural support, protection against predators, and aid in buoyancy regulation, allowing these delicate organisms to thrive in diverse ocean habitats, from surface waters to the deep sea.
Crustacean Zooplankton: Chitinous Champions
Shifting our focus to zooplankton, the animal-like denizens of the oceans, we encounter a diverse array of carapaces and exoskeletons. Copepods, some of the most abundant zooplankton, use a chitinous exoskeleton for both protection and buoyancy. This rigid shell, segmented for flexibility, encases the copepod’s body, allowing for efficient movement through the water column. Krill, another crucial group of zooplankton, rely on a carapace composed of tough, transparent chitin. This sturdy exoskeleton protects them from the crushing pressures of the deep sea and predation. Additionally, the carapace serves as a site for muscle attachment, facilitating efficient swimming.
Copepod of the genus Acartia molting. Albert Calbet
Shells for Swimming and Survival
Pteropods, also known as sea butterflies and sea angels, are a group of pelagic gastropod mollusks. These delicate creatures possess intricately shaped shells composed of calcium carbonate, which may be transparent or translucent, allowing for camouflage and protection against predators. Pteropod shells come in a variety of forms, including coiled, straight, and wing-like structures, each adapted to the species’ unique mode of swimming and feeding. Despite their fragile appearance, pteropod shells provide essential protection for the organism’s soft body and contribute to its buoyancy control in the water column.
Other mollusks, including clams’ larvae, various species of heteropods and pelagic gastropods also boast remarkable shells adapted for life in the open ocean. Heteropods, a group of predatory gastropods, possess coiled shells equipped with a keel or spines, which aid in swimming and maneuverability. Pelagic gastropods, such as species of the genera Carinaria and Atlanta, produce delicate, transparent shells that offer protection while allowing for efficient locomotion through the water.
Gasteropod larvae. Albert Calbet
Beyond Hard Shells: The Jellyfish Solution
Jellyfish, members of the phylum Cnidaria, offer a unique adaptation. They possess a gelatinous bell that serves as both a protective covering and a means of propulsion. Composed primarily of water and a jelly-like substance, this bell allows jellyfish to move with remarkable agility despite lacking the hard shells of other plankton. However, they compensate for this with stinging tentacles armed with specialized cells for capturing prey and deterring predators. Within jellyfish, Velella possesses a rigid, sail-like structure called a pneumatophore. This gas-filled float, often triangular or rectangular, is crafted from chitin and protrudes from the colony like a miniature sail. By tilting the pneumatophore and catching the wind, Velella can propel itself across the ocean’s surface, steering its course with remarkable precision. This unique adaptation allows Velella to navigate currents, escape predators, and optimize its position for feeding on plankton near the surface.
A masterpiece of complexity
Among the most ingenious adaptations in the plankton realm are the houses of appendicularians. These tiny marine filter feeders construct remarkable, elaborate structures from mucus and cellulose. Dwarfing the appendicularia itself, the house functions as a super-efficient net. As the appendicularia swims, it pumps water through the house, sieving out minute food particles like algae and bacteria. This clever design concentrates food in the surrounding water, making meals a breeze. These intricate houses are disposable; if a predator attacks, the appendicularia simply discards the compromised house, escapes unharmed, and builds a fresh one, leaving behind a trail of mucous mansions in the ocean’s depths.
Appendicularia. Albert Calbet
A Testament to Evolution
The world of planktonic shells and carapaces is a marvel of biological diversity and adaptation, with each group exhibiting unique structures finely tuned to their respective ecological niches. From the silica frustules of diatoms to the delicate loricae of tintinnids and the intricate tests of radiolarians, these protective coverings play a crucial role in the survival and success of planktonic organisms in the dynamic and challenging environment of the world’s oceans. As scientists continue to explore and unravel the mysteries of planktonic life, the study of these remarkable structures offers valuable insights into the evolution and ecology of marine ecosystems.
Plankton Ocean 