Many shrimp species exhibit a distinct behavioral pattern of moving toward a light source, especially during certain phases of their life cycle. This attraction, often observed in marine life at night, is a fundamental biological response influencing their daily movements, feeding habits, and survival. Understanding this attraction requires looking into the specific biological mechanism that governs how these crustaceans perceive and react to light in their aquatic environment.
Understanding Positive Phototaxis in Shrimp
The movement of an organism in response to light is known as phototaxis, and when the movement is directed toward the light source, it is termed positive phototaxis. This is a deeply rooted behavioral mechanism, particularly noticeable in the larval and juvenile stages of many shrimp species. The sensory perception that drives this movement is mediated by the shrimp’s visual system, which includes specialized light-sensitive pigments called opsins.
For many young shrimp, this attraction serves an important evolutionary purpose, often tied to feeding and migration. Moving upward toward the surface light allows them to stay within the water column where their primary food sources, such as phytoplankton and algae, thrive. Some species, like brine shrimp, follow the light because it indicates the location of the algal blooms they consume.
This light-seeking behavior is a crucial component of diel vertical migration, a daily pattern where organisms move to the surface layers at night and descend to deeper, darker waters during the day. By moving into the illuminated zone, shrimp larvae can optimize their position for feeding and growth. The ability to detect and move toward light helps synchronize their internal biological clock, known as the circadian rhythm, with the natural light-dark cycles of their environment.
Factors Affecting the Response to Light
The attraction to light is not a constant, uniform behavior across all shrimp; it is highly dependent on a variety of internal and external factors. One of the most significant variables is the shrimp’s developmental stage, as the intensity of the light response often changes as the animal matures. Larval shrimp typically display the strongest positive phototaxis, but as they grow into adults, this response may weaken, become neutral, or even reverse into a negative phototaxis, where they move away from light.
The intensity of the light source also plays a large role in how the shrimp react to it. Extremely bright or intense light can overwhelm the sensory system and may lead to a reduced or erratic response, or even cause the animal to flee. Studies suggest that a shift in light intensity can alter which of the visual pigments are activated, affecting the overall spectral response of the shrimp.
The color of the light, determined by its wavelength, is another important factor that modulates the attraction. Since different light colors penetrate water to varying depths, many shrimp species have evolved to respond preferentially to specific wavelengths. Blue and green light penetrate deepest into the ocean, and some species, like the whiteleg shrimp (Litopenaeus vannamei), show a positive impact on growth when exposed to green light. However, the optimal color differs significantly between species and life stages, as juvenile northern shrimp actively swim away from green light.
How Light Attraction is Used in Commercial Practices
The predictable light-seeking behavior in shrimp is actively exploited in commercial settings, both in wild fisheries and in aquaculture. One of the most direct applications is in a technique known as light fishing, where powerful submerged lights are used to aggregate shrimp and other crustaceans. The lights draw the animals toward the surface or a specific area, making them easier to harvest with nets or traps, thereby increasing the overall catch rate.
In modern commercial fishing, this knowledge is also used to reduce the bycatch of non-target species. By strategically placing lights of specific colors, such as green LEDs, on fishing gear like trawl nets, fishers can sometimes deter smaller, non-marketable juvenile shrimp from entering the net. This application leverages the subtle differences in the light response between different life stages and species to improve the selectivity and sustainability of the fishing method.
In aquaculture, or shrimp farming, light is a carefully managed environmental parameter to optimize the health and yield of the stock. Manipulating the photoperiod—the duration of light exposure within a 24-hour cycle—is routinely used to influence the shrimp’s growth, metabolism, and reproductive development. Adjusting the light intensity and color spectrum in tanks can improve production performance and survival rates, demonstrating that controlled light exposure is a fundamental tool for managing farmed shrimp populations.