Sea Butterflies - The "Flying" Snails of the Ocean.
- bioworld090
- Nov 19, 2023
- 6 min read
Updated: Jun 6, 2024
INTRODUCTION
Sea butterfly is a suborder of small holoplanktonic (live as plankton for their whole life) sea snails. Its scientific name is Thecosomata. It occupies the pelagic zone of the oceans.
MORPHOLOGY OF SEA BUTTERFLY
Sea butterflies are small swimming sea snails, with sizes ranging from a few millimeters to around 2-3 cm. Sea butterflies usually have a calcified shell that is transparent, fragile and tiny. For instance, the sea butterfly Limacina rangii has a shell that is 2-9 μm thick and 0.5-6 mm wide.
The bilaterally symmetrical shells of sea butterflies come in various shapes such as coiled, needle-like, triangular and globular.
They have a pair of wing-like appendages called parapodia that are used for swimming. These parapodia flap slowly, allowing sea butterflies to move through the water column. Despite their small size, sea butterflies are known for their graceful and delicate movements.
HABITAT & DISTRIBUTION
Sea butterflies are unique in that they spend their entire lives as plankton. Unlike other snails that have a benthic lifestyle, sea butterflies remain in the water column, drifting with the currents. This adaptation allows them to occupy the pelagic zone of the oceans.
Sea butterflies are known to exhibit diel vertical migration, meaning they migrate vertically in the water column on a daily basis. During the day, they descend to deeper depths to avoid predators, and at night, they ascend to shallower depths to feed on phytoplankton. This behavior helps them optimize their survival and feeding strategies in the changing light and food availability.
SWIMMING MECHANISM
Sea butterflies have a pair of highly flexible structures known as parapodia. It consists of two wing-like structures that extend horizontally from the base of the body. They develop during the growing phase of sea butterflies. Parapodia are thin, translucent, flexible structures that sea butterflies use to swim by creating rhythmic “clap and fling” movement of their parapodia.
The “clap and fling” movement of parapodia, in simple terms, is the closing and opening of parapodia. The “clap” involves bringing the two “wings” together in a clapping motion. Then, comes the “fling” which involves rapidly opening the “wings” of parapodia and creating a sudden burst that generates thrust and propels the organism forward in any desired direction.
This is sometimes also explained in terms of power and recovery stroke.
Power Stroke:
The powers stroke is basically the “fling” motion of parapodia. During the power stroke, muscular contractions in parapodia lead to the downward or “fling” movement. This creates thrust and propels the animal through the water.
Recovery Stroke:
The recovery stroke is the “clap” motion of parapodia. It occurs right after the power stroke and during the recovery stroke, muscles in the parapodia relax and the wing-like extensions move upwards to prepare for the next power stroke.
FEEDING & PREDATION
FEEDING
Sea butterflies thrive on a planktonic diet that obviously consists of plankton. Plankton are tiny organisms that are incapable of moving independently against the water current. So, they flow along with the current of water. Planktons can be divided into two types: phytoplankton (plankton that are capable of photosynthesis) and zooplankton (plankton that are incapable of photosynthesis and they feed on phytoplankton or other zooplankton). Sea butterflies also belong to the category of zooplankton.
Sea butterflies are very small animals but they can catch prey that is many times larger than them. They create a mucous web with the help of mucous they secrete. The mucous web can be as big as 5 cm. Then, they use this mucous web to trap and entangle their prey. As the plankton they feed on are incapable of swimming against water currents, they are carried towards the mucous web of sea butterflies and get trapped there.
ROLE IN MARINE FOOD WEB
Sea butterflies are very tiny animals but their role in the marine food web is not that tiny. Sea butterflies, in fact, play many crucial roles in the marine ecosystem. Some of their roles include energy transfer through different trophic levels, nutrient cycling and acting as an indicator species to potentially indicate various environmental changes.
Energy transfer:
Sea butterflies are primary consumers. They feed on the primary producers, aka, phytoplankton. Then, they are eaten by other animals or predators who are secondary consumers. The secondary consumers are fed upon by tertiary consumers and so on. This means that energy is being transferred through the food chain from the first trophic level to the second trophic level and so on.
Prey for Higher Trophic Level:
As already mentioned above in the “Energy transfer” section, sea butterflies are fed upon by other predators who are above them at the trophic level. So, sea butterflies play a crucial role in providing food and energy, directly or indirectly, to the animals at higher trophic levels and thus, play a role in their survival. They are also the sole food source of their relatives, the Gymnosomata or the sea angels.
Nutrient Cycling:
Nutrient cycling is the cyclic process of the flow of nutrients (organic and inorganic matter) from the physical environment to living organisms and back to the physical environment. In the case of sea butterflies, they consume plankton and are also consumed by animals at the higher trophic level. This means that sea butterflies take nutrients from plankton and animals at higher trophic levels, directly or indirectly, take the nutrients from sea butterflies. The acquired nutrients are eventually released back into the environment through excretion and decomposition. From, there it gets into the physical environment (soil, ocean bed, etc.) and then again into living organisms (primary producers).
Indicator Species:
Sea butterflies are sensitive to environmental changes and may be impacted by them. This can directly affect their abundance or distribution in a particular region or ecosystem. The different ecological changes can be shifts in water temperature, nutrient availability and ocean acidification.
POTENTIAL THREATS
Invasive species, Loss of biodiversity (factors that affect plankton negatively), pollution (water pollutants), habitat loss (human activities such as coastal activities), overfishing (imbalanced predator-prey dynamics), climate change, and ocean acidification.
Invasive species:
Invasive species may compete with plankton that sea butterflies primarily feed on. If invasive species are primary consumers like sea butterflies, they may outcompete sea butterflies and thus sea butterflies may face a reduction in the availability of their primary food source, plankton.
There may even be a chance of an invasive species feeding directly upon sea butterflies which may result in the decline of the sea butterflies population. This may directly or indirectly affect the animals at the higher trophic levels.
Ocean Acidification:
Ocean acidification is the reduction in the pH of the ocean over an extended period of time primarily due to the excess absorption of CO2 from the atmosphere. This makes the ocean water to become more acidic.
As most sea butterflies have a thin, transparent shell made up of aragonite, a form of calcium carbonate, the decreased pH of the ocean water results in dissolution and reduced thickness of the shells of sea butterflies. This can potentially compromise their chances of survival.
Sea butterflies (pteropods), just like other many other gastropods, possess shells. The shell has an outermost protective layer known as periostracum. The periostracum is slowly dissolved under low pH but things get worse for these animals when the periostracum is already damaged either due to some mechanical or chemical factors. The damaged periostracum makes the shell wall susceptible to damage by the acidified water. If the acidified water breaches or passes through the shell wall, the soft body of the animal becomes vulnerable to infection and predation. This also compromises their internal chemistry and the ability to regulate buoyancy.
Sea butterflies face a real threat from ocean acidification but they possibly have a way to counteract its adverse effects up to a certain extent. Many, if not all, can internally repair and thicken their shell by secreting more carbonate on the inner surface of their shell. This is evident from a study conducted on a sea butterfly species, Limacina helicina.
CONCLUSION:
In the vast ocean, thecosomata pteropods, or sea butterflies, are like tiny guardians of the sea's delicate balance. With their see-through shells and gentle wing movements, they may seem small, but their role in the ocean is enormous.
Sea butterflies are essential players in the ocean's story. They help keep the water healthy by eating tiny plants, playing a crucial role in the food web. Their dance-like movements contribute to the beauty of the underwater world, reminding us of the intricate connections that sustain life beneath the waves.
Yet, despite their importance, sea butterflies face threats. Changes in the ocean, like rising temperatures and ocean acidification, can affect them. These changes make it harder for sea butterflies to build their shells, putting their delicate existence at risk. As we marvel at their beauty, it's crucial to recognize the challenges they face and work together to protect their ocean home. By safeguarding thecosomata pteropods, we're ensuring the health of the entire ocean ecosystem and preserving the magic of these tiny, essential dancers of the sea.
REFERENCES:
Comments