Nudibranchs: Mini marvels that hide beneath the surface of the Aotea coastline 

KYA DALY GRIFFITHS (Marine Biologist) 

Many wonderful creatures inhabit Aotea's coastline, from those that use camouflage for protection to those that use speed to their advantage, like our open ocean animals. Many of the tiny creatures go unnoticed. Sea slugs are the common term for the array of gastropods that belong to different taxonomic subgroups, such as nudibranchs, sapsucking slugs, sea hares, side-gilled slugs, and many more. Nudibranchs (order Nudibranchia) specifically are recognised for their small size but also their striking patterns and colours. Their vivid hues serve as a warning to predators due to their ability to release toxins. They make use of the toxic resources from species like sponges and anemones as a defense against their limitations in size and speed. The consumption of cnidarian species such as anemones, hydroids, and jellyfish allows these fabulous creatures to absorb those toxins (nematocysts) for their own chemical defense against unwanted predators(1).

Nudibranchs can be distinguished from other commonly described sea slugs through the placement of their external anatomy. The two most common types of nudibranch are Dorid and Aeolid. Dorids have their gills located near the dorsal of the mantle, usually positioned towards the rear. These external gills and their frilly-like display are a key feature not only to identify them but also what draws so much attention to these beautiful creatures. Aeolid have what is known as cerata, which essentially are tentacle-like gills formed in rows across the entirety of their mantle. Towards their head, there are two retractable rhinophores, appearing like two little horns; these rhinophores are their chemosensory organs(3). Nudibranchs use chemosensory organs to detect environmental stimuli, through taste and smell.

While commonly these sea slugs are noticed for their vibrant appearance, many species avoid such attention and use camouflage to merge with their environment, which generally corresponds with their food source. Species that don't possess the ability to obtain nematocysts (stinging-like toxins from anemones and sponges) have been shown to mimic species that do, giving rise to two relatively identical species, one with a toxic taste and the other without(5). Due to their ability to absorb nematocysts, predation on nudibranchs is minimised1. Known predators include humans, crabs, sea stars, species of fish, and other nudibranchs(6). The nudibranch, Roboastra luteolineata, has been commonly known to predate on other nudibranch species(7).

Why are these beautiful creatures so important? Besides their beauty and role in the ecosystem they are also good indicators of ocean health and environmental stress from processes associated with climate change(8). The average lifespan of nudibranchs is one year. Due to this short life cycle, their response to the quality of their environment is relatively quick. Depending on the species and their habitat, environmental impacts such as those associated with kina barrens can indicate how dire algae regrowth may be based on the presence or absence of nudibranch species. They are also a good indicator of the health of coral reefs. Solar-powered Nudibranchs (those that feed on coral) will be scarce in areas where corals may be stressed or undergoing bleaching events; a reflection of the lack of food sources. However, nudibranchs that feed on algae will thrive on corals that have died and have become coated in algae. High densities of algae feeding nudibranchs indicates that much of the coral is dead. As climate concerns confront us, it has become increasingly important to understand ecosystem indicators that provide insights into our environmental health.

Where do we find these cute little critters? They range in size from a few millimeters long to 700 mm(9). At Aotea nudibranchs are generally found in the intertidal zone to depths of around 40 metres(2). Their habitat can vary greatly from rocky shores and tidal pools, to sponges, soft substrates, and some species amongst plankton. Great variation occurs between species, including their diet. Food available will determine the species present. For instance, some species feed on algae, or sponges, or hydroids, and sometimes even other nudibranchs. Species have been found at depths of 1000 m, where their diet will be very different,  demonstrating how incredible these little guys are. Commonly around Aotea you will find species such as Jason miabilis, Ceratosoma amoenum (clown nudi) and Dendrodoris krusensterii(9). Jason miabilis, a species endemic to Aotearoa, is often found on a species of hydroid known as Solanderia ericopsis. Commonly, Jason miabilis can be found in groupings on the same hydroid as this is their main food source; eventually, they leave the hydroid completely bare and move onto the next.

Coryphellina albomarginata, a species present in Aotea waters, is only found growing on the hydroids, Tubularia sp. and Syncoryne sp. at depths of around 25 metres(9). Aeolidiella drusilla is a beautiful, slightly transparent nudibranch, that has specks of purple and brown amongst its cerata. It can commonly be found within sand as it feeds on anemones(8). Due to its diet, Aeolidiella drusilla can also be found in and amongst tidal pools, commonly between 0-10 metres(9).

These are just some of the many beautiful nudibranch species that can be found around the Aotea and Aotearoa coastlines. With a variation in size from the microscopic to a few centimeters in length, these creatures make for a thrilling find when you do come across one of these frilly slugs. From camouflage to flamboyant, and tidal pools to the deep depths, always keep your eyes peeled because you never know when one of these tiny, yet magnificent creatures, will appear.

Kya has a degree in marine biology and has been working as a marine biologist in coral restoration on the Great Barrier Reef. At present she is a whale shark and humpback whale swim guide. 

References

1. Goodheart, J. (2023). How sea slugs steal the defenses of their prey, Smithsonian Ocean. Available at: https://ocean.si.edu/ocean-life/invertebrates/how-sea-slugs-steal-defenses-their-prey (Accessed: 12 March 2025) 

2. Department of Conservation (2023) Flamboyant, fantastical and fascinating: Nudibranchs of the Hauraki Gulf |, Conservation blog. Available at: https://blog.doc.govt.nz/2021/01/12/flamboyant-fantastical-and-fascination-nudibranchs-of-the-hauraki-gulf/ (Accessed: 11 March 2025) 

3. Shipman, C. (2012) Sea slug senses part I, California Academy of Sciences. Available at: https://www.calacademy.org/blogs/project-lab/sea-slug-senses-part-i (Accessed: 12 March 2025) 

4. Turner, K. (2025) Dorid vs Aeolid Nudibranchs , The weird and wonderful worlds of Nudibranchs . Kitty Turner . Available at: https:// www.kittyturner.com/nudibranch-book (Accessed: 2025) 

5. Nudibranchs: How sea slugs steal Venom. Natural History Museum (NHM). Available at: https://www.nhm.ac.uk/discover/ nudibranchs-psychedelic-thieves-of-the-sea.html (Accessed: 12 March 2025) 

6. Celine (2023) Nudibranchs fun facts, Pacific Beach Coalition. Available at: https//www.pacificbeachcoalition.org/nudibranchs-fun-facts/ (Accessed: 11 March 2025) 

7. Wassilieff, M. (2012) Sea Slugs, Te Ara Encyclopedia of New Zealand – Te Ara Encyclopedia of New Zealand. Available at: https:// teara.govt.nz/en/shellfish/page-3#:~:text=However%2C%20the%20Roboastra%20luteolineata%20preys,(cerata)%20on%20 their%20backs (Accessed: 11 March 2025) 

8. Heathcote, A. (2018) Nudibranchs: Indicators of climate change, Nudibranchs: indicators of climate change. Available at: https:// www.australiangeographic.com.au/topics/wildlife/2017/09/nudibranchs-indicators-of-climate-change/ (Accessed: 12 March 2025) 

9. Willan, R.C. et al. (2020) Super Sea slugs a guide to the sea slugs of New Zealand . Available at: https://niwa.co.nz/sites/default/ files/Super%20Sea%20Slugs_Version%201_2020.pdf (Accessed: 10 March 2025) 

10. iNaturalist (no date) Jason mirabilis. Available at: https://uk.inaturalist.org/taxa/391127-Jason-mirabilis (Accessed: 09 June 2025)

11. Pinterest, wool creature lab (no date) Wool creature lab. Pinterest. Available at: https://nz.pinterest.com/pin/668010557200676937/ (Accessed: 09 June 2025) 

12. MolluscaNZ (no date) Dendrodoris krusensternii. Available at : https://www.mollusca.co.nz/speciesdetail.php?taxa=3888 (Accessed: 09 June 2025)

13. Wikipedia (December 2024) Coryphellina albomarginata. Available at https://en.wikipedia.org/wiki/Coryphellina_albomarginata (Accessed: 09 June 2025)