A Rare Benthic Ctenophore, Lyrocteis cf. imperatoris, at Mesophotic Depths off Réunion Island

Abstract

Closed-circuit rebreather diving has opened new opportunities for direct observation of the rich biodiversity of unexplored mesophotic coral ecosystems. We recently encountered four specimens of a rare benthic ctenophore genus, Lyrocteis. The specimens were found on the east coast of Réunion Island off the harbor of Sainte Rose at ~95 m. This finding contributes to the limited number of records of the genus in the Indian Ocean. Continued exploration of mesophotic zones remains essential, as each dive has the potential to reveal remarkable and unexpected discoveries.

Some marine taxa remain remarkably elusive, encountered only by fortunate divers. Among these are the benthic ctenophores of the order Platyctenida, a little-known group comprising approximately 35 described species. On 13 November 2024, during a closed-circuit rebreather (CCR) dive off the east coast of Réunion Island, in the Bay of Sainte-Rose, we discovered and photographed a first specimen belonging to the rare genus Lyrocteis at approximately 95 m depth. Over the following weeks, three additional specimens were recorded at the same site—one on 22 December and two on 9 January (Figure 1). Only two species have been formally described within this genus, one from the Pacific (Japan), L. imperatoris Komai, 1941 [1], and one from Antarctica (McMurdo Sound), L. flavopallidus Robillard & Dayton, 1972 [2]. The Pacific species was originally described from seven specimens dredged off Japan at a depth of 70 m. It has also been reported from Korean waters at shallower depths (15–25 m, [3]) where two additional specimens were recorded. More recently, an aggregation of several specimens was observed in the Verde Island Passage, Philippines, at mesophotic depths ranging from 85 to 110 m [4]. In subsequent years, hundreds of specimens, displaying a variety of colors, have been observed at this site [4].

Our observations represent one of the few records of Lyrocteis in the Indian Ocean. A single specimen of Lyrocteis sp. was previously photographed at a depth of 250 m in Aldabra (Seychelles) [5], and several specimens have been recorded from the canyons off northern KwaZulu-Natal at ~70 m [6], using remotely operated vehicles (ROVs). Additionally, one specimen was recorded from Ashmore Reef (Western Australia, https://www.ncbi.nlm.nih.gov/biosample/SAMN44050843 accessed on 9 June 2025). Our sightings of several specimens over a two-month period suggest a potential local aggregation or seasonal recurrence, similar to what has been observed in the Philippines [4]. Like their congeners, the specimens were observed elevated above the substrate, attached to gorgonians, and reached lengths of up to 20 cm. Notably, some of the specimens hosted several associated organisms, including shrimps and gobies (Figure 2). The third and fourth specimens (Figure 1C,D) were sighted together on the same gorgonian. A few years earlier, another specimen was observed on the west coast of Réunion Island, in the Bay of Saint-Leu (P. Plantard, pers. obs.). Our specimens most likely belong to Lyrocteis imperatoris, but tissue samples are needed to perform DNA barcoding and confirm the identification. CCR diving continues to prove highly effective in advancing our incomplete knowledge of marine biodiversity, particularly within the largely unexplored mesophotic zone [7,8].