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DescriptionAscidia mentula is a large solitary sea squirt that can be over 18 cm in length. Its elongate body is usually smooth, often with low rounded swellings. The test is thick, cartilaginous and translucent red - pink, or greenish - olive green, although it can be grey in low light deeper waters. This species is usually attached to the substratum on its left side, unlike most ascidians which attach at the base. Its siphons are inconspicuous with small lobes often bearing white bands or spots. The inhalant (oral) siphon is terminal and the exhalant (atrial) siphon is half to three quarters down the body and can be obscured by detritus and encrusting growths.Ascidia mentula is fertilized externally and produces a tadpole larvae up to 0.8 mm in length, with a short planktonic phase. Breeding in this species occurs throughout the year but predominantly during the summer. The branchial sac of Ascidia mentula is commonly inhabited by the pea crab Pinnotheres pinnotheres or the copepod Notopterophorus papilio. The small bivalve Modiolarca tumida can also live commensally in the test. Ascidia mentula may be mistaken for Ascidia virginea which has a smooth, retangular, transparent rose-pink test with red veins on the mantle.