Leptostracans have a bivalved carapace (the two valves are fused dorsomedially but have no hinge) enclosing head and thorax, or enclosing head, thorax, and part of the abdomen, laterally compressed and smooth. The anterior end of the carapace dorsum continues as a movable articulated rostrum. The eyes are stalked, compound, with visual elements either present or absent (genus Dahlella). The antennules (antenna one) are usually biramous and the antennae (antenna two) are uni-ramous. The eight pairs of thoracic limbs (thoracopods) are leaflike turgor appendages, foliaceous (phyllopodous), and bi-ramous. The abdomen has six pairs of appendages (pleopods): the first four pairs are birramous and large, and the last two are uniramous and reduced. The seventh abdominal segment continues as a telson having a long caudal furca.
Most leptostracans are 0.19-0.59 in (5-15 mm) long, but one species (Nebaliopsis typica) is a giant at nearly 1.5 in (4 cm) in length.
In life, specimens appeared mostly transparent (colorless), except for the bright red eyes. When viewed with the naked eye the thoracic region appears almost white. All leptostracans, except Nebaliopsis typica, show pronounced sexual dimorphism. Males also occur much more rarely than females. The carapace of the male is less deep than that of the female and the antennae are much longer.
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