The Solar-Powered Sea-Slug

P. ocellatus found during our dive. Credit: 

HIMB/L. Noonan

While snorkeling off the sandbars of Moku O Loe I took a pause to defog my mask, giving a fellow intern (Rachel), the opportunity to take a moment and smell the seagrass. While waiting, she discovered a small patch of sand moving across the algae as if it were late to its walking tour. She then noticed two little stalk-like projections from the moving cluster of coral remains and discovered it was not sand at all.

These little projections are called rhinophores which are typically used for chemoreception and rheoreception. Chemosensing is when organisms can determine the presence of certain chemicals so it’s similar to our sense of smell. Rheoreception is the ability to sense the direction of the currents. The organism to which these rhinophores are attached: Plakobranchus ocellatus, or more commonly known as the ringed sap-sucking slug. They have several rings distributed amongst their body which range from blue to green to yellow, hence the term “ring” in the name. There are two black dots located between the two rhinophores which contain ocelli. These ocelli are simple photoreceptors, meaning the slugs primarily use them to detect movement in the environment around them.

A. Arrow pointing to parapodia. B. Arrow pointing to

rhinophores C. Arrow pointing to ocelli. D. Parapodia

spread, revealing the green lamellae. Red arrow

points to the stomach. E. The underside.

Credit: Zookeys/M. A. Munoz

Well what about the rest of the name? What is sap-sucking? The raddest function of this slug is that it has the ability to extract and use chloroplasts from algae that it has ingested (functional kleptoplasts). This potential kleptoplasty classifies this gastropod as a mixotroph, which is an organism that is capable of phototrophy and heterotrophy. According to an article published in 2012 by Maeda & Hirose et. al., photosynthesis is a mechanism that commonly only occurs in starved sap-sucking slugs. Their primary source of nutrition stems from the consummation and digestion of algae, which concurrently provides them with fresh kleptoplasts. These kleptoplasts are usually visible within the lamellae (gills) when the parapodia of the slug has been spread.

                Kleptoplasty. An eyeful upon approach, but quite easy to remember when you break it down. Klepto is a term that has been derived from the term kleptomaniac. A person who is a kleptomaniac is someone who constantly has an urge to steal. When you add klepto- to the term -plasts (chloroplasts) you get an organism who steals the chloroplasts from others (in this case algae). Similarly, cnidae (stinging cells contained by cnidarians) that are ingested by a nudibranch would be termed kleptocnidae.

With all the new knowledge you have you’re probably wondering where you can find them. These little gastropods of the phylum mollusca are typically found in the indo-pacific and are comfortable in sandy and silty reef areas. However, these slugs can still reach depths of down to 50 ft (Hoover 2007).

These gastropods are hermaphroditic and have a spiral cream-colored eggs sac that holds the young. These sacs have reportedly been found on algae and on a species of sea cucumber that is commonly found here in Kaneohe Bay. This commensal association with the sea cucumber species Holothuria atra which is the black sea cucumber you usually find in the touch tanks. An experiment done by Mercier and Hammel in 2005 discovered that P. ocellatus favor H. atra because of the sea cucumbers ability to produce a toxin that deters possible predators of P. ocellatus. This allows the gastropods spawn under protection.

Ring details of P. ocellatus. Credit: The University

of Queensland Australia/W. L. Hong, 2011.

On the topic of protection, P. ocellatus also produces a mucus that duals as a camouflage cover and as a means of locomotion. The mucus is secreted over the parapodia which adheres sediment to their dorsal side, allowing them to blend in with their environment. The mucus is similarly secreted ventrally, where they are able to smoothly glide over it and move safely without harming their epidermis.

 Now that you’ve been SUCKED in by these little sap-sucking slugs I encourage you to come out to Moku O Loe and explore the reefs. We have a variety of other unique invertebrates that are pleasant to observe... and who knows, maybe you’ll find one we’ve never encountered!

-LiAn

FOR MORE FACTS:
https://www.gbri.org.au/Species/Plakobranchusocellatus.aspx?PageContentID=2560

REFERENCES:
Hoover J. P. 2007. Hawaii’s Sea Creatures: A Guide to Hawaii’s 

Marine Invertebrates, Revised Edition.

Maeda T., Hirose E., Chikaraishi Y., Kawato M., Takishita K., 

Yoshida T., Verbruggen H., Tanaka J., Shimamura S., Takaki 

Y., Tsuchiya M., Iwai K., Maruyama T. 2012. Algivore or 

Phototroph? Plakobranchus ocellatus (Gastropoda) 

Continuously Acquires Kleptoplasts and Nutrition from 

Multiple Algal Species in Nature. Vol. 7 Iss. 7. Accessed July 

10, 2019.

Mercier A., Hamel J. 2005. Note on the association between 

Plakobranchus ocellatus (Mollusca: Gastropoda: 

Opisthobranchia) and Holothuria atra (Echinodermata: 

Holothuroidea). Cah. Biol. 46 : 399-402. Accessed July 11, 

2019.

Munoz M. A., Velde G., Meij S., Stoffels B., Alen T., Tuti Y., 

Hoeksema B. 2016. The phylogenetic position of a new 

species of plakobranchus from West Papua, Indonesia 

(Mollusca, Opisthobranchia, Sacoglossa). ZooKeys 594: 73-

98. Accessed July 12, 2019.