Fauna
Invertebrate

Horned Nudibranch

Hermissenda crassicornis

Also known as Opalescent Nudibranch, Thick-horned Nudibranch, Opalescent Sea Slug

Horned nudibranchs are striking Pacific sea slugs recognized by their iridescent blue body lines and translucent, orange-tipped cerata. These aggressive carnivores actively hunt hydroids and anemones, storing their prey's stinging nematocysts inside their own bodies for self-defense. When two individuals meet on the seafloor, they frequently clash in fast-paced combat, biting off each other's cerata.

MarineColorfulCarnivorous
Horned Nudibranch reference1 / 7
"Opalescent Nudibranch" by Dan Hershman is licensed under CC BY 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/2.0/. · cc-by
—observations
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Not EvaluatedConservation statusIUCN Red List categoriesLCNTVUENCREWEXNot Evaluated — not placed on the risk ladderRed List categories, explained →
Key facts
Diet & preyHydroids, Sea anemones, Bryozoans
Predators
  • Sea stars
  • Kelp crabs
  • Navanax
ThreatsCoastal runoff, ocean warming, habitat loss
HabitatsRocky intertidal zones, tide pools, shallow reefs, marina pilings
Life span1–2 years
Classification
  1. FamilyMyrrhinidae
  2. SpeciesHermissenda crassicornis
Range & activity
Range data is not available yet.
Safe observation
Interaction riskLow
  1. Low
  2. Moderate
  3. High
  4. Very high
Why interaction risk is low
Safe to observe in tide pools; avoid touching to protect delicate tissues.
Responsible observation
  • Tide Pool Etiquette: Observe underwater without lifting the animal out of damp substrate.
  • Spotting Strategy: Scan submerged hydroid colonies where they frequently hunt and feed.
  • Lighting Tip: Use a diffused dive torch to illuminate their opalescent blue stripes.
Related species
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