Hydra or planaria in a shrimp tank: identify and assess

Hydra and planaria are different animals. Establish the shape and movement first. Their presence does not prove that they killed a dead shrimp or ate missing young. Record the observation before choosing a control product.

Look at shape, attachment and movement

• A hydra attaches by a foot and has tentacles around its mouth. These can retract strongly, making the same animal look different in two photographs.
• Many free-living flatworms glide and have a flattened body. Some show eyespots and lateral head lobes. Not every small aquarium worm has a triangular head.
• Take a magnified photograph or short video with a size reference. Record whether the animal is on glass, in the substrate or actually attached to a shrimp. For the latter, use the articles about worm-like growth and white fuzz.

What does their biology tell us about risk?

Hydra capture small aquatic animals with stinging cells. Free-living flatworms include predators, scavengers and other feeding types. This supports assessing contact with very small animals, but provides no general prediction for every shrimp species or life stage. A worm on a carcass may also be scavenging; it does not establish an attack on a living shrimp.

Manage specific findings and follow the result

Remove visible leftover food and check uptake of the food offered. Do not starve shrimp to eliminate every other organism; fan shrimp need suitable suspended food. Visible animals can be siphoned out selectively with equipment dedicated to this aquarium. Check the collection vessel for accidentally removed shrimp and do not return its contents.

If using a trap, assess the entrance, possible bycatch and timely bait removal. This can catch only part of a population and cannot establish that eggs or hidden animals are absent. Count again at the same locations and record any observed contact with shrimp.

If demonstrable harm persists or identification remains uncertain, seek an assessment. A wormer for another animal is not automatically a suitable aquarium product. Check the exact active ingredient and implications for every inhabitant using the medication checklist. Do not add a fish solely to control an unidentified organism.

Sources and scope

• Missouri Department of Conservation: Hydras
• Missouri Department of Conservation: Turbellarians
• UF/IFAS: Fish Health Management Considerations in Recirculating Aquaculture Systems
• Seachem: Cupramine directions and invertebrate warning

The field guides describe these groups’ structure and feeding. They provide neither a shrimp-tank mortality estimate nor a treatment dose. Observation and removal steps are our practical judgement, not an eradication guarantee.