Nature in SB: California Killifish

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Illustrated by Bridget Rios

Minjae Ryu

Contributing Editor

The campus lagoon is home to a variety of organisms, ranging from distinct species of birds dipping their beaks in the water, to wild flowers and grasses growing near the shore. However, hidden beneath these waters is a whole ecosystem of unseen fish species. One of these fishes is the California Killifish. 

The California Killifish are a non-migratory fish that are most often found in shallow waters, taking residence on beds of aquatic vegetation in estuaries and lagoons. They are small, with an average size of 11 cm, yet they are thick. Their gray-green and yellow-brown coloration along their dorsal and ventral sides camouflages their appearance in the dark lagoon water from our eyes. With an average of a one-year life cycle, the end of their lives are usually marked (or “paralleled”) by the end of their breeding season. Their sexes can be identified by unique patterns along their mid-flank — females have a faint stripe across it, while males have up to twenty black bars along theirs. 

One of the most fascinating facts about the California Killifish is their adaptability to waters of varying salinity levels. They are able to thrive in freshwater to saltwater that is four times saltier than the ocean. This level of adaptability can be observed by their residence in the lagoon, where both freshwater and saltwater flow freely. With the influx of seawater gushing into the lagoon from the nearby ocean, the Pacific in our case, the Killifish are most commonly found in the salt marshes along the shore. 

As for their diet, the Killifish have mouths that are angled upwards, allowing them to easily swipe small insects and larvae floating on the surface of the water. They have their most diverse feeding options near high tidal zones, which is rich with a variety of organisms and aquatic vegetation for the fish to choose from. Conversely, Killifish are predated on by mostly aquatic birds that coexist with them in the lagoon. As a species that is far from going extinct due to their tolerance to pollution, changing temperatures, and varying oxygen levels, the Killifish act as a stable food source for magnificent water bird species like the Great Egrets or the Great Blue Herons

Recent research has found that a certain parasite species of larval trematodes, a class of parasitic flatworms, take residence in some Killifish. These parasites modify the Killifish’s motor function, making them highly susceptible to predators. Examples of modified behavior include contortions and jerking movements. Though ruinous to the Killifish, this parasitical relationship makes the aquatic organism an interesting topic of study. 

Despite being hard to spot from the shore, the Killifishes’ presence can be felt on campus with their contribution to both the lagoon ecosystem, by providing balance within the food chain, and the pursuit of science, as a prime example of a common euryhaline species that is easily observable and susceptible to research.