At a shrimp pond in Mai Po Nature Reserve in Hong Kong, researchers have found a new species of Tripedalia jellyfish. The animal is only about half an inch long, which makes it hard to spot in murky pond water.

This discovery adds a fourth described member to the family Tripedaliidae, a small group of closely related box jellyfish. These strange creatures as a whole comprise a small group of cnidarians, with only 49 species known worldwide so far.

Surprising jellyfish discovery

The work was led by Professor Qiu Jianwen at Hong Kong Baptist University (HKBU), a marine biologist who tracks how coastal ecosystems respond to change. His research focuses on marine invertebrates, animals without backbones that live in the sea.

His team spent the summers from 2020 to 2022 taking nighttime samples from shrimp ponds that sit between mangroves and the open estuary. An estuary, a coastal area where river and sea meet, brings in tides that refresh these ponds.

The water there is brackish, slightly salty water that blends fresh and seawater. In that brown green mix, a transparent jellyfish can glide past nets unless someone looks very carefully.

Meet Tripedalia maipoensis

Tripedalia maipoensis has a clear, nearly colorless bell with a rounded cube shape. The bell reaches about six tenths of an inch from top to bottom, which keeps the animal small enough to hide among plant stems.

At each corner of this jellyfish, three flattened pedalia, muscular pads that anchor the tentacles, extend outward and help push water as the animal swims. From the tip of every pad hangs a single tentacle that can reach about four inches in length.

As the bell squeezes and relaxes, a thin velarium, a muscular sheet partially closing the opening, helps focus the jet of water behind the jellyfish.

Tripedalia maipoensis uses this focused thrust to swim more quickly than many other jellyfish that drift with weaker pulses.

How they know it is a new species

Under the microscope, the team saw that this jellyfish shared some features with its Caribbean relative Tripedalia cystophora but differed in several key traits.

The new species has three pedalia at each bell corner, only one tentacle on each pad, and forked canals in its velarium.

To go beyond looks, the scientists built a phylogeny, a family tree showing how species are related, using DNA from several genes.

Those comparisons placed Tripedalia maipoensis next to Tripedalia cystophora, confirming that the two are close cousins rather than one species.

One key dataset came from rRNA, a type of genetic material often used to compare species. In the 16S rRNA gene, Tripedalia maipoensis differed from Tripedalia cystophora by about 17.4 percent, a gap large enough to support naming a new species.

What 24 eyes can actually do

Like its close relatives, Tripedalia maipoensis carries 24 eyes arranged in four clusters on structures called rhopalia. A rhopalium, a sensory club that holds each cluster of eyes, hangs slightly below the edge of the bell.

In each cluster, two larger lens eyes, image forming eyes with small lenses, handle most of the detailed vision. The remaining four pit and slit eyes are simpler organs that mainly track light and dark rather than sharp shapes.

In experiments with Tripedalia cystophora, scientists found that some upward facing eyes help the jellyfish stay under the mangrove canopy. 

Those eyes constantly point toward the world above the water, giving the animal enough visual guidance to steer around roots and other obstacles.

Tripedalia maipoensis shares this layout of eye types, so it may also use particular eyes for tasks such as hunting and navigation.

Researchers still need to test exactly how this new species responds to light and shadows in its own muddy pond habitat.

Learning and behavior with no brain

Box jellyfish look simple, yet their behavior hints at surprising mental abilities for such small, soft animals. They can swim quickly, avoid obstacles, and hold position in sunlit patches where tiny crustaceans gather.

Recent learning experiments with Tripedalia cystophora show that these jellyfish can change how they swim after bumping into obstacles. 

That behavior fits associative learning, a process where animals link experiences with later actions. Tripedalia cystophora has no single brain, yet it does have a central nervous system, a network that coordinates signals from its eyes and muscles. 

Because Tripedalia maipoensis jellyfish is so closely related, scientists suspect it may share some of these learning abilities once they are tested directly.

Lessons from Tripedalia maipoensi

Tripedalia maipoensis is the first box jellyfish ever formally reported from Chinese coastal waters, so it changes how scientists map this group’s global range. 

Its home in a managed shrimp pond beside a busy city also shows that human shaped landscapes can still hide unnamed species.

For conservationists, each new species adds to local biodiversity, the full variety of living things in an ecosystem.

Finding extra species in a well studied reserve such as Mai Po makes scientists suspect that quieter wetlands may hide even more life.

The study is published in Zoological Studies.

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