Thuridilla hopei, commonly known as Hope’s Elysia, is a sacoglossan sea slug native to the Mediterranean Sea. Although predominantly found in the Mediterranean, it has occasionally been spotted in the Red Sea. This shell-less marine opisthobranch gastropod mollusk belongs to the family Plakobranchidae.
Identifying Thuridilla hopei is straightforward due to its distinctive appearance. These sea slugs have a long, smooth, and thin body, measuring between 10 to 25 mm. They possess parapodia, lateral extensions that fold and join on their back, forming an elongated sheath from which the head emerges. The most common colors are blue to violet, though they can also be green or dark gray. Their parapodia display several colored bands, with the outermost border often being yellow or orange. Additionally, they have two prominent rhinophores on their head, robust and widened at the top, with a distinctive white ‘V’ shaped mark extending from the parapodia to the rhinophores.
Thuridilla hopei exhibits various unique characteristics. The parapodia form an elongated sheath, and the body color ranges from blue to violet, with occasional green and dark gray variations. They have three colored bands on the upper parapodia: a yellow or orange outer line, a blue to turquoise intermediate line, and a white or yellow innermost line. The head features two long rhinophores and a distinctive white ‘V’ shaped mark with two small black eye spots at the base. These sea slugs are vegetarian, feeding on green algae by perforating the cells with their radula.
Thuridilla hopei is commonly found in shallow waters, usually up to 40 meters deep, near brown or green algae. They inhabit rocky, well-lit sea beds where these algae grow. This species is vegetarian and feeds on various green algae. Thuridilla hopei are hermaphrodites, and their reproductive process involves a head-to-head contact where both partners entwine.
One fascinating aspect of Thuridilla hopei is their bright red penis, a rare and notable characteristic among sacoglossan sea slugs. Additionally, these sea slugs are capable of absorbing chloroplasts from the algae they consume, allowing them to photosynthesize and produce energy, a process known as kleptoplasty.
Diving in Malta provides an excellent opportunity to observe Thuridilla hopei in their natural habitat. Some of the best sites to spot these sea slugs include:
Reqqa Point – Located on Gozo’s north coast, this site offers a diverse marine environment where Thuridilla hopei can be found at shallow depths.
Hondoq Bay – Situated on Gozo’s southeast coast, this area is known for its clear waters and abundance of green algae, attracting these sea slugs.
Blue Hole – Found near the Azure Window on Gozo, this popular dive site features rocky crevices and algae-rich environments ideal for spotting Thuridilla hopei.
Diving at these sites, particularly during daylight when the algae are more visible, increases the chances of encountering the vibrant and fascinating Thuridilla hopei.
