Berthella martensi is a “side-gilled” pleurobranch species of variable presentation; however putting the colour differences and the sometimes large pustules aside the general arrangement, as described here, is common throughout.
This is a medium-sized sea slug usually observed in the 25 to 60 mm range, with adults averaging 40 to 60 mm, although extremely large specimens up to 120 mm have been rarely reported. The body shape is oval though slightly elongated, reasonably high sided but flat on the top with the mantle curving over the sides and completely covering the foot. When “entire”, the animal can have a bulky appearance. The oral veil is deployed anteriorly when on the move and is seen to be wide and trapezoidal, almost triangular in shape due to the acutely tapering sides. The lateral ends of the straight anterior edge are formed into blunt cylindrical points. Sitting conspicuously above and behind the oral veil are the enrolled rhinophores with their longitudinal slit. They are fused at their bases and together tend to form a vee shape jutting upright or angling forwards.
The mantle has a deep anterior indentation to accommodate the rhinophores and the eyespots that are situated on the posterior base of each rhinophore. Behind this is a distinctive flat oval patch delineated by lines, almost furrows, that indicate the location of three sheer zones for autotomizing three separate sections of the mantle – one on each side and one posteriorly, running transversely across the mantle. At times, specimens may be found with one, two or even all three of these mantle sections cast off as a defensive measure. Autotomized sections are known to regenerate. The bipinnate gill, lying down along the right side, between mantle and foot, is not usually visible but becomes exposed when the right mantle segment is autotomized. It is composed of a single smooth rachis from which arise a number of pinnae on the dorsal surface. It is attached anteriorly for part of its length by a branchial membrane to the body wall. The genital aperture is located anterior to the gill attachment and the anus just posterior to the posterior end of that attachment. The tail is mostly covered by the mantle but when the posterior mantle segment is autotomized it can be easily seen to finish in a curve posteriorly or to be bluntly tapering. The aforementioned central flat region is never autotomized. It contains the small elongated shell remnant lying over the internal viscera.
In addition to autotomizing mantle segments Berthella martensi exudes a milky white fluid when disturbed as a defensive reaction and is believed to be of an acidic nature, reportedly burning human skin if handled.
Locomotion is achieved by cilia on the foot assisted by mucus released from a transverse semicircular anterior pedal groove but it is a slow moving sea slug.
Berthella martensi exhibits quite a wide variety of primary body colours and patterning across the range of its distribution. The most familiar body colour is a translucent white with regular brown spotting however the white may be replaced by cream, yellow, brown or even black and the spotted pattern may even be absent. At times a reticulation of brown lines may be present instead. Often the rhinophore tips and oral veil extremities are darkly coloured. Sometimes the lobes that are able to be cast off are delineated by a darker outline as can be the oral veil and foot. Most usually the body surface is smooth but there have been finds of specimens whose spots are actually raised tubercles. Different colour forms may even be encountered at the same dive site.
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This piece was prepared online by Panuruji Kenta, Publisher, SEVENSEAS Media