How do you tag a jellyfish? It may sound like a metaphor for frustration, but it's a question that's occupying a team of scientists at the Woods Hole Oceanographic Institution (WHOI). The team has developed a new technology called Integrated Tracking of Aquatic orGanisms (ITAG), which is designed to place instruments on squid, jellyfish, and other small invertebrates as a way to provide detailed information about the animals and their habitat.

Invertebrates are valuable to marine scientists. Soft-bodied creatures like jellyfish and squid are a very important resource as both food and a source of food for fisheries, but they are also the "canaries" of the oceans. According to WHOI, invertebrates have an almost open circulatory system, which makes them important indicators of environmental change. The problem is, tracking them is not an easy task.

One obvious answer is to tag them, but squids and jellyfish don't take kindly to tags. In recent decades, biologists have attached tracking devices to everything from otters to blue whales using harpoons, adhesives, suction cups, collars, and even screws, depending on the animal, but it's more of a challenge when it comes to the squishier sealife.

What the WHOI team wanted was a tag that could attach to something as fragile as a moonjelly without harming or hindering the host. In addition, they wanted telemetry equipment with a faster sampling rate to return more detailed information. This meant taking the various requirements of the scientist and the engineers and balancing one against the other so that the integrated sensors, batteries, flotation devices, attachment and release mechanisms, and recovery beacons not only fit into a single package, but were satisfactorily functional enough to answer the scientists' questions.

"When squid go down to a couple hundred meters, are they foraging at night at that depth, or are they resting and getting away from top predators? What are their respiration rates?" says Aran Mooney, a biologist at WHOI. "These are the types of behavior questions we wanted to answer."

The result was the ITAG, which is based on the Digital acoustic recording TAG (DTAG) developed by WHOI in the late 1990s. The DTAG was built to track large marine mammals and record their pitch, speed, and depth. But it was too large and heavy for invertebrates, so Mooney and his collaborators at Stanford University, the University of Michigan (UM), and WHOI developed a smaller, more advanced device in the form of a high resolution data-logging tag made specifically for small and delicate invertebrates that not only monitors their behavior, but the ocean conditions around them.

The ITAG measures 108 x 64 x 28 mm (4.25 x 2.5 x 1 in) and looks almost like a toy. Built in two parts, it consists of the body, which is watertight and streamlined with a hydrodynamic shape that's designed to impede the particular species of invertebrate as little as possible. Inside is the sensor package, data recorder, and a VHF radio beacon. This is set on a bespoke base that attaches to a variety of animals. For a jellyfish, the tag base attaches to the bell using suction cups and veterinary-grade adhesive, while the more vigorous squid require biodegradable sutures to attach it and keep the tag in line with the body for minimum drag.

The team says that the ITAG records an animal's movement, orientation, dive profiles, and environmental data, such as light and temperature. When it completes its job, it detaches from the base, floats to the surface, and the beacon activates for recovery and transmits the information it had recorded via VHF radio.

To find the best design for the ITAGs, the research team attached prototypes to five jellyfish (Aurelia aurita) and eight squid (Loligo forbesi) for up to 24 hours. The jellyfish were tested at the Marine Biological Laboratory and Environmental Systems Laboratory from August to November 2013, while the squid were trialed in early 2014 at the Porto Pim Aquarium in the Azores with the help of the University of the Azores and the Marine and Environmental Sciences Centre in Portugal.

These were then allowed to swim about while the scientists recorded their behavior on video and compared it to that of untagged animals. At the same time, the output of the accelerometers on the tags was compared in real time to the video, so the team could correlate the readings to the behavior and how the animals responded to changes in their environment. According to WHOI, this information will be of help in interpreting later field data when sea trials begin.

The team's findings were published in Animal Biotelemetry.

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