Sessilia

Lamarck, 1818

acorn barnacles, sessile barnacles

Family Guides

Balanidae(acorn barnacles)
Chthamalidae(Chthamalid Barnacles)
Coronulidae(Whale Barnacles)

Sessilia is an order of barnacles (Cirripedia) characterized by the absence of a stalk (peduncle), with the body attached directly to the substrate by a calcified base. The group comprises two orders: Verrucomorpha (two ) and Balanomorpha (14 families). Sessilia is considered monophyletic and likely derived from stalked ancestors. The Australasian region preserves the earliest stratigraphic records for several families including Pachylasmatidae, Tetraclitidae, Archaeobalanidae, and Verrucidae.

PSM V08 D693 Coronet barnacle by Unknown authorUnknown author. Used under a Public domain license.

Balanus nubilus (3484682809) by Minette Layne from Seattle, Washington. Used under a CC BY-SA 2.0 license.

Chthamalus dalli - Allison Gong by Allison Gong. Used under a CC BY 4.0 license.

Pronunciation

How to pronounce Sessilia: //sɛˈsɪliə//

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Distinguished from pedunculate barnacles (Lepadomorpha) by the absence of a fleshy stalk; the body is cemented directly to the substrate via a calcified base. The capitulum (body wall) is composed of interlocking calcareous plates. Traditional characteristics used in phylogenetic analysis have been shown to be homoplasious due to convergence.

Images

Habitat

Marine environments from intertidal zones to shallow subtidal waters. The Oligocene fossil record from New Zealand indicates association with intertidal to shallow subtidal , with some occupying upper subtidal zones.

Distribution

Global distribution with fossil evidence from the Oligocene of Southland, New Zealand; earliest records for multiple documented in the Australian and New Zealand region. Present-day distribution includes waters around Denmark, Norway, and the northeastern United States (Vermont).

Similar Taxa

LepadomorphaStalked or goose barnacles possess a distinct fleshy peduncle attaching the capitulum to the substrate, whereas Sessilia lack this stalk and attach directly via a calcified base.
ScalpellomorphaPedunculate barnacles with capitulum covered by calcareous plates; distinguished from Sessilia by the presence of a stalk.

More Details

Taxonomic History

Historically treated as an unranked clade within Cirripedia; currently recognized as order-rank . Pachydiadema, once considered the earliest balanomorph, has been re-evaluated as a pedunculate cirripede.

Phylogenetic Considerations

Cladistic analyses indicate that traditional morphological characters used to evaluate balanomorph are homoplasious due to convergence, complicating evolutionary reconstructions.