A Cannibal’s Tale
On an ordinary day at the Sunflower Sea Star Laboratory, you can find a particularly fuzzy and active juvenile sea star named Hannibal. Hannibal is our most famous sunflower sea star, who was quarantined early in his* life cycle for eating his brothers and sisters. Hannibal is named after the character from the 1991 crime thriller film Silence of the Lambs, in which Anthony Hopkins portrays Dr. Lecter, famously known as 'Hannibal the Cannibal.' Now, Hannibal the sunflower star lives alone, feasting on juvenile sea urchins, the forbidden delicacies of his youth hopefully forgotten. Most days, he can be found hiding underneath his rock, adverse to the limelight.
Introducing Hannibal the Cannibal weighing in here at an extremely active and fuzzy 1.8cm. Photo courtesy of Joe Paquin.
Hannibal’s famous habits asserted themselves when he transitioned from a larva to a post-settlement juvenile. So small, we had to observe him through a microscope. He dropped out of the water column to roam what would have been the surface of the reef, looking for similarly tiny prey. The post-settlement stage is when a sea star no longer eats microalgae and enters the juvenile portion of its life cycle. Hannibal is not unique in his thirst for the flesh of his brethren (perhaps paired with fava beans and a nice chianti). Cannibalism is a common trait in sea stars and other species who broadcast spawn. Broadcast spawners are marine organisms that release their eggs and sperm into the water column, where fertilization occurs externally. This method of reproduction is common in many marine species, including corals, many fish species, and some invertebrates like sea urchins and sunflower sea stars, though not all broadcast spawners exhibit cannibalistic behaviors.
Let’s take a deep dive.
Hannibal hunts down brine shrimp while a lab volunteer tries to measure him.
Why Eat Your Brethren?
Evolutionarily, it’s a bit counterintuitive to eat your kin; there are enough predators and environmental hurdles to clear without the additional threat of being eaten by your buddy when your back is turned. However, there are a few reasons that sunflower stars (pycnopodia helianthoides) eat each other anyway:
It’s easy: Broadcast spawning results in a large number of eggs and sperm released into the open water. The resulting larvae are extremely vulnerable to predation. Larger members of the same species (conspecifics) will eat other settled juveniles, as they are abundant and easy to capture.
Population control: If too many sunflower stars reach maturity, the environment's capacity to provide food is at risk, creating a boom-and-bust cycle.
Energy efficiency: There is a net savings when you don’t have to roam around expending energy to hunt for food and sea stars are rich in nutrients.
Cannibalism may exert selective pressure on the development of settled juvenile traits that help them avoid being eaten by other sunflower stars. For example, settled juveniles might develop behavioral adaptations, such as timing their development to avoid peak predation periods or dispersing to areas where cannibalism pressure is lower. Additionally, the timing and location of spawning events may evolve in response to cannibalism risks, with species adapting to release their gametes under conditions that minimize the chances of conspecific predation.
Hannibal next to his name tag and a juvenile purple sea urchin, soon to be prey. Photo courtesy of Jenny Adler.
What Role Does Cannibalism Play in Raising Sunflower Stars in the Lab?
In April 2024, the Sunflower Star Laboratory (SSL) received 400 settled juveniles from a Valentine’s Day spawning event at the Birch Aquarium at Scripps in San Diego, California. This was a big deal because it was the first successful spawn in California! The team at Birch Aquarium successfully fertilized millions of eggs, which were then distributed to three teams that partnered with the aquarium, one of them being the Sunflower Star Lab. Initially, there were tens of thousands of larvae, but soon after moving into the post-settlement phase, their numbers decreased dramatically.
Andrew Kim, current Laboratory Manager at the SSL and lead aquaculture tech at Moss Landing Marine Labs at the time, watched the counts go from tens of thousands to hundreds in a few weeks, eventually arriving at 400 settled juveniles to transfer to the SSL. Andrew has raised thousands of broadcast spawners so he knows that vast numbers at the beginning, narrowing down to fewer survivors is just part of the broadcast spawner life cycle in the ocean and therefore part of the rhythm of the lab. According to Andrew, the nature of an aquarium does not replicate the open ocean, where these hungry juveniles would disperse, floating for 2-10+ weeks as plankton and not have as many opportunities to feed on their full siblings.
Thanks to Hannibal’s antics, we’ve learned our lesson and have now isolated all post-settlement sunflower stars as a protocol to preserve enough juveniles to grow to adulthood, weaning them off other sea stars and onto sea urchins, shrimp, and mussels. They’re currently in separate habitats, consisting of small plastic tubs with a few rocks for substrate. Ultimately, we hope that sea urchins become their favorite food, encouraging this critically endangered species to reduce sea urchin populations and thus play their keystone predator role to control the grazers and restore California’s precious kelp forests.
Hannibal (in his separate enclosure) learns to pry open mussels.
The Man, the Myth, the Legend
Hannibal the Cannibal, now secluded and in isolation, has come to embrace the juvenile sea urchin as his a food source, along with brine shrimp and mussels as he grows. Gone are his days of terrifying and feeding on his kin. He is no longer the largest or the one with the most insatiable appetite in the group. As he ages, he will eventually become more gregarious and will learn to live in harmony with other sunflower stars. In a few years, once he reaches the size of a dinner plate, Hannibal will be an adult capable of participating in annual broadcast spawning and able to contribute to the first generations of California sunflower stars helping to restore their species to the kelp forest. Such is the life cycle of an infamous Pycnopodia helianthoides echinoderm invertebrate post-settlement sunflower sea star.
Hannibal gets measured as part of a feeding experiment on 10-17-24.
*We don’t know if Hannibal is male or female (observation of the gametes released during spawning is the best non-invasive way to tell), but after already anthropomorphizing him with the mantle of the most famous male cannibal in modern fiction, might as well run with it.
