Ctenophores are one of the first diverging metazoan clades and most likely the sister group for all the remaining animal lineages (Schultz et al. 2023). Ctenophores (a.k.a. comb jellies) include 205 + accepted marine species that are primarily planktonic and found throughout the world's oceans (Mills 1998). Ctenophores possess a wide variety of morphologies but typically can be described by the presence of swimming structures composed of fused macrocilia arranged in eight ctene rows, a rotationally symmetrical body plan (Dunn et al. 2015), an apical sensory structure (statocyst), and either colloblasts (multicellular secretory organs) or macrocilial teeth (Horridge 1965) used to capture prey.

Ctenophores' remarkable ability for regeneration has placed them as a promising model system for studying regeneration processes (Edgar et al. 2021). However, very little is still known of the regenerative abilities of the entire phylum, with most studies focusing on two species: Mnemiopsis leidyi (Edgar et al. 2021) and Vallicula multiformis (Freeman 1967).

During a dive in May 2023 in the Velikaya Salma strait, Kandalaksha Bay of the White Sea, Dr. Alexander Semenov observed a Beroe sp., showing a potential regeneration after an injury of unknown origin. The Beroe specimen (Fig. 1), which resembled Beroe cucumis, was observed and photographed at a depth of 3 m with a remarkable feature—two developed mouths (Fig. 1). More precisely, the animal was observed retaining a large central mouth (Fig. 1a: white arrow) typical of the genus but possessed a secondary mouth protruding halfway down the body on one side (Fig. 1a: teal arrow). Upon closer inspection, both mouths appeared to be directly connected to the main gastric chamber of the animal (Fig. 1c, d) and possess simple bands of macrocilial teeth (Fig. 1b). The eight typical ctene rows are still present in the observed animal but appear to be warped on the inflicted side before merging with the secondary mouth's radial canal. The ctene row then reemerges from the secondary ring canal at a separate location and continues up to the ring canal surrounding the primary mouth.

Fig. 1
figure 1

Beroe sp. from the Kandalaksha Bay in the White Sea, column: a Frontal plane, white arrow: primary mouth, teal arrow: secondary mouth; b closeup of the secondary mouth featuring macrocilial teeth in green; c and d various views/angles of the sagittal plane. Photographs by Alexander Semenov

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Beroids have been assumed to be unable to regenerate from an injury due to the anatomical absence of tentacle bulbs, which house stem cells (Martindale 2016; Ramon-Mateu et al. 2022). However, this observation constitutes one of the first evidence of morphological abnormalities in Beroe and, to our knowledge, the first documentation of a Beroe sp. with multiple mouths. Ultimately, to better understand the possible causes for this abnormality, further research is needed to explore the influence of regeneration and to rule out other causes, such as the influence of developmental stages and naturally occurring abnormalities such as the development of multiple statocysts during development in M. leidyi (Martindale 2016). This brief observation opens new avenues for research on abnormal morphologies and regeneration capacities in Ctenophores and further emphasizes the need to study additional non-model species.