Master of Disguise

Pygmy seahorses (Hippocampus bargibanti) are so good at camouflage that, until at least 45 years ago, they were completely unknown. 

While knowledge of their biology remains fragmentary, mainly because it is hard to find them or keep them in aquaria, a new study analyzed the genome of these seahorses and found why they are so elusive. 

Pygmy seahorses, one of the smallest vertebrates, measuring about the size of a thumbnail, live in symbiosis with gorgonian corals in the western Pacific Ocean, clinging to the branches with their tails, day and night, waiting for food to pass by. 

Through evolution, pygmy seahorses evolved small knobs on their skin, which mimic the shape and color of the coral’s polyps and allow them to blend seamlessly into the reef. Their snouts are also shortened to the length of polyps, making them even more difficult to spot. 

“Seahorses usually have an elongated snout that resembles a horse. However, this would set the pygmy seahorse apart from the coral’s shape,” lead study author Axel Meyer said in a statement. “We were highly interested in finding out at what stage of development this deviating appearance of the pygmy seahorse is caused to provide good camouflage and which genes are responsible.” 

To carry out the research, the team looked at the expression pattern of the pygmy seahorse’s genes at different stages of development of the snout region. 

In the early stages, all seahorse species have short heads and squat facial proportions that align with the “baby schema,” cute features that trigger caregiving responses, Smithsonian Magazine explained. 

“Normally, a combination of different genetic components causes the snout of a seahorse to grow proportionally faster than other parts of the body from a certain age and thus become elongated,” said Meyer. “In the pygmy seahorse, however, we have now discovered that these different growth rates are suppressed because the hoxa2b gene has been lost.” 

As a result, the pygmy seahorse’s head never outgrows the “childlike” developmental stage and can visually merge with the corals. 

Researchers also analyzed the genetic basis of the skin color, the formation of skin knobs, and the immune system of these seahorses. Among the many things they found, they noticed that the pygmy seahorse has shed an unusually high number of immune genes throughout its evolution, having diverged from full-sized seahorses about 18 million years ago. In terms of the immune system, they also have the smallest known set of genes of all vertebrates. 

Meyer explained that this is probably related to the seahorses’ tolerance to coral toxins, which protect them from microbes, so their immune systems no longer need the genes that would normally provide this protection. 

Moreover, the sex roles of seahorses have been switched, and males incubate the eggs in their brood pouch. Since the eggs are not genetically identical to the cells of the male’s body, shedding immune-system genes was essential to prevent them from being attacked as foreign tissue. 

Pygmy seahorses are prime examples of evolution, researchers said, demonstrating that what is advantageous for survival prevails, while what is unnecessary disappears. 

At the same time, these seahorses have genetically evolved to live alongside gorgonian corals so perfectly that they cannot live without them. 

“The research reveals a cruel irony,” Richard Smith, a marine biologist not involved in the study, told the New York Times. “The very traits that made these sea horses successful, such as their perfect mimicry, tiny size, and specialized biology, are now their greatest vulnerability.”