Cypris morphology in the barnacles Ibla and Paralepas (Crustacea: Cirripedia Thoracica) implications for cirripede evolution

Abstract

We used scanning electron microscopy (SEM) to describe cypris morphology in species of the barnacles Ibla and Paralepas, both of which are pivotal in understanding cirripede evolution. In Ibla, we also studied late naupliar stages with video and SEM. Special emphasis was put on the lattice organs, the antennules and the thorax and telson. In Paralepas we had settled specimens only and could therefore only investigate the carapace with the lattice organs. Cyprids of Ibla quadrivalvis and Paralepas dannevigi have five sets of lattice organs, grouped as two anterior and three posterior pairs. The organs are of the pore-field type and the terminal pore is situated anteriorly in the first pair, just as in the Rhizocephala and the Thoracica. In Ibla the armament of antennular sensilla resembles that found in the Thoracica but differs from the Rhizocephala. The absence of setules on the A and B setae sited terminally on the fourth antennular segment is a similarity with the Acrothoracica. The attachment disc is angled rather than facing distally and is encircled by a low cuticular velum. The thoracopods have two-segmented endopods and exopods as in the Thoracica, but the number, shape, and position of thoracopodal setae differ somewhat from other species of that superorder. Both Ibla and Paralepas cyprids have a deeply cleaved telson, but no independent abdominal part. In cypris morphology, Ibla and Paralepas show several synapomorphies with the clade comprising Rhizocephala and Thoracica and there are no specific apomorphies with either the Acrothoracica, the Rhizocephala or any particular subgroup within the Thoracica. This is in agreement with recent molecular evidence that Ibla (Ibliformes) is the sister taxon to all other Thoracica and the ibliforms therefore become the outgroup of choice for studying character evolution within the superorder. Paralepas, and other pedunculated barnacles without shell plates, are apparently not primitive but are secondarily evolved and nested within the Thoracica.

Scanning electron microscopy of the cypris larvae ofCapitulum mitella(Cirripedia: Thoracica: Scalpellomorpha)

“…The attachment disc is encircled by a velum in C. mitella and I. quadrivalvis and by a s kirt in O. angulata . In C. mitella a series of slender cuticular flaps forms the distinct velum, whereas the velum of I. quadrivalvis certainly does consist of small cuticular flaps (Hoeg et al , 2009). And the flaps in I. quadrivalvis do not run far down the side of the segment, therefore they differ from the long, narrow flaps in C. mitella and balanomorphans.…”

“…Within the Cirripedia, only the Acrothoracica have a separate abdomen, Rhizocephalan and thoracian cyprids have no free abdomen, and the cleaved telson (or abdomen–telson) connects directly with the thorax (Kolbasov et al , 1999; Høeg et al , 2009). Although there is no trace of an independent, abdominal body part in cyprids of I. quadrivalvis (Høeg et al , 2009), cypris larvae of the family Poecilasmatidae have a rudimentary abdomen which seems to consist of two small segments (Kolbasov et al , 2013). Cyprids of C. mitella also have a rudimentary abdomen according to our observation.…”

“…For example, the telsonic cleft is shallow in Trypetesa , of intermediate depth in Armatoglyptes , while it almost cleaves the telson in cyprids of Weltneria and Kochlorine (Kolbasov & Høeg, 2007). Rhizocephalan cyprids have a shallow telsonic cleft (Rybakov et al , 2002), and the telson is almost completely cleaved in thoracican cyprids (Kolbasov et al , 1999; Høeg et al , 2009). Because the telson divided into two equal parts looks like basal segments of the furcal rami in B. amphitrite cyprids, Glenner & Høeg thought they had a two-segmented furcal rami (1995).…”

Fossil Calibrations for the Arthropod Tree of Life

“…Generally, the cyprid larval morphotype (with an elongated head shield, six swimming thoracopods, and robust modified antennules) is considered a synapomorphy of Thecostraca (Høeg et al, 2004). R. reduncus differs from cirripede cyprids as it lacks attachment discs, and its abdomen extends past the head shield; a differentiated abdomen is a condition of Ascothoracida (Briggs et al, 2005;Høeg et al, 2009a). Based on the presence of five shell plates in a juvenile specimen, Høeg et al (2009b) suggested that R. reduncus may be placed on the cirripede stem lineage.…”

The sensory dorsal organs of crustaceans

“…Lastly, it has been shown that precursors of the LOs of cyprids (or cyprid-like larvae) exist in nauplii in the form of pore-bearing setae (Walossek, Høeg & Shirley, 1996;Rybakov et al, 2003;Høeg et al, 2009). …”