Felimare bilineata, commonly known as ‘bicolored sea slug‘ or ‘bicolored nudibranch‘, is an opisthobranch mollusc of the family Chromodorididae and belongs to the order Nudibranchia. The genus name ‘Felimare‘ comes from Latin and refers to the beauty of the animal, while ‘bilineata‘ is derived from the Latin words ‘bi‘ (two) and ‘lineata‘ (with lines), in reference to the two colored lines that adorn its body. This species can be found in the waters of the eastern Atlantic Ocean, particularly along the coasts of the Canary Islands, Madeira and the Azores. It is also present in the southern Mediterranean Sea. Felimare bilineata is found in coastal marine habitats, preferably in rocky areas and coral reefs, where there is an abundance of sponges, its main food source.
Felimare bilineata is a small to medium-sized Nudibranch, which can reach a length of 2 to 3 centimeters. Its body is elongated and narrow, with a soft mantle that partially covers the foot of the animal, and whose edge has a yellow line. The most distinctive feature of this species is its coloration: the bottom of the body is dark blue, with two bright yellow longitudinal lines running along the back of the head to the tail. The rhinophores (sensory organs on the head) are deep blue. A distinguishing feature from other very similar species is that they have a thin yellowish line on the back of the rhinophores. The gills, located on the back of the body, are blue and have two thin yellow triangular-shaped lines on each of the gill blades. It is very frequent the confusion between several species of the genus Felimare. As a summary, we highlight the main characteristics of each one of them, which will allow us to differentiate them easily.
- Felimare tricolor
- Back: Blue with a yellow border and a yellow or white longitudinal stripe ending between the rhinophores, plus some yellow dots.
- Foot: Blue with some white or yellow dots on the sides.
- Rhinophores: Blue with white tips.
- Gills: Blue with a thin yellowish line on the outer side of each gill blade.
- Felimare fontandraui
- Back: Blue with a yellow border and a longitudinal white line ending in front of the rhinophores with an anchor (or “T”) design, plus some white dots on the sides of this stripe.
- Foot: Blue with an irregular white stripe design on each side.
- Rhinophores: Uniform blue color.
- Gills: May be uniform blue or have two yellow triangular stripes on the outer side and a yellow stripe on the inner side of each gill leaf.
- Felimare villafranca
- Back: Blue with a yellow border and a pattern of irregular longitudinal yellow and/or white stripes forming a circle around the gills.
- Foot: Blue with a pattern of irregular longitudinal yellow and/or white stripes.
- Rhinophores: Blue with a white stripe on the back.
- Gills: Blue with a white line on the inside and outside of each gill leaf.
- Felimare orsinii
- Size: Small.
- Back: Blue surrounded by a yellow border and a very thin white longitudinal stripe. The mantle is covered with small blue tubercles.
- Foot: Blue with a white or yellow stripe on each side.
- Rhinophores: Blue.
- Gills: Blue.
- Felimare bilineata
- Body: Black or dark blue with yellow lines and blue dots.
- Back: Two yellow longitudinal stripes surrounding the rhinophores and gills. Northern specimens usually have these stripes merged into one, while in African specimens the stripes are quite far apart and the space between them has a yellowish stripe pattern.
- Mantle border: Yellow, preceded by a black line. The area between the yellow border and the longitudinal midline is covered with blue dots.
- Foot: Yellow longitudinal lines on each side.
- Rhinophores: Black or dark blue with a yellow longitudinal line on the back.
- Gills: Black or dark blue with two yellow triangular lines on the outer side and a yellow line on the inner side of each gill blade.
- Felimare cantabrica
- Back: Blue surrounded by a yellow line, with a yellow longitudinal stripe and some yellow dots.
- Foot: Blue with numerous yellow dots on each side.
- Rhinophores: Blue with a yellow longitudinal line on the back.
- Gills: Blue with two yellow triangular lines on the outer side and a yellow stripe on the inner side of each gill leaf.
- Distribution: This species is not found in the Mediterranean.
As for its diet, Felimare bilineata is carnivorous and feeds mainly on marine sponges, from which it obtains the nutrients necessary for its growth and development. This nudibranch specializes in certain species of sponges of the genus Dysidea and Haliclona, among others. The feeding of Felimare bilineata is fundamental for its defense, since it acquires toxic compounds from the sponges it consumes, which it stores in its own body as a strategy to deter potential predators.
Reproduction of Felimare bilineata is sexual and hermaphroditic, meaning that each individual possesses both male and female reproductive organs. During the breeding season, which generally occurs in the warmer months, individuals mate and fertilize each other. After mating, both individuals lay whitish ribbons of eggs in coils or clusters on the substrate or sponges on which they feed. The eggs hatch into veliger larvae, which are planktonic and spend a period in the water column before settling on the seafloor and metamorphosing into juveniles.
A curiosity about Felimare bilineata is its ability to regenerate parts of its body, such as rhinophores or gills, if they are damaged or amputated. In addition, like other nudibranchs, this species is an excellent indicator of the health of marine ecosystems, as its presence and abundance are often correlated with water quality and prey availability. It is also of interest to photographers and divers because of its striking color and beauty, which make it easily recognizable in its natural environment.
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