Members of the Subclass Cirripedia (barnacles) are crustaceans that display the basic maxillopod tagmata with five cephalic, six thoracic and usually four abdominal somites and a telson. However, in most species the adult body is greatly modified for an attached or parasitic way of life. The first thoracomere is often fused with the cephalon and bears the maxillipeds. The six thoracal segments bear paired and biramous appendages; the adult abdomen is greatly reduced in most species and is without limbs. The telson is absent in most species, though caudal rami may be present on the abdomen in some. The compound eye is lost in adults, median eyes are reduced or absent in adults.
The carapace is "bivalved (in fact, folded) as in cypris (i.e. the cirriped larval stage) and in the parasitic Order Ascothoracica, or lost as in the parasitic Order Rhizocephala, or the carapace consists of a fleshy or membranous sac-like mantle that produces calcareous plates enclosing the body as in the Order Thoracica and Order Acrothoracica. Note that this calcification is greatly reduced in the latter order, of which members burrow in calcareous substrates including mollusc shells, and in some commensally Thoracica.
The non-parasitic (yet sometimes commensally) Order Thoracica and Order Acrothoracica have adapted their feathery thoracopods (called cirri) for suspension feeding.
Adult cirripeds are sessile animals, either (endo)parasitic or attached to a usually firm substrate. A few thoracicans are adapted to a pelagic way of life, being attached to floating objects as seaweeds, logs and other flotsam, or nektonic marine animals as sea turtles, sharks, dolphins and whales. In particular this type of barnacles from the North Sea are here of interest, notwithstanding the fact that they are a bit of an exception to the usual zooplankton.
Larval development is metamorphic and generally includes a nauplius and cypris. Go to Barnacles: key to larval stages for a text key to naupliar stages (but not to species).
The Subclass Cirripedia comprises the following orders:
¥ Order Acrothoracica. Usually non-parasitic, burrowing cirripeds in living and dead coral, limestone and (mollusc) shells; calcarous plates strongly reduced or even lacking. Present in our area with a single species, but not included in the key.
¥ Order Thoracica. Free-living (i.e. non-parasitic, but often commensal) cirripeds attached permanently as adults; usually calcarous plates present, sometimes reduced. This order consist of three suborders: Suborder Lepadimorpha or stalked barnacles, Suborder Balanamorpha and Suborder Verrucomorpha. The key involves lepadimorph species.
¥ Order Rhizocephala. Mostly endo- rarely ectoparasitic on particularly decapod Crustacea. Present in the area on (hyper/pelago) benthic Decapoda, but not included in the keys and not described here.
¥ Order Ascothoracica. Parasitic; piercing and sucking as ecto- and endoparasites of coelenterates and echinoderms. Not present in our area.
Distribution in the North Sea
— Stalked barnacles. Regarding the pelagic stalked barnacles (Suborder Lepadimorpha) from the North Sea, two Conchoderma and five Lepas species are keyed out (Broch, 1959). These species are not epizoic, although Conchoderma auritum has been reported frequently from toothed whales also. The key starts at Page 149.
— Whale barnacles. The four strictly epizoic species attached to sharks and whales, which can be encountered in the area, are not keyed out here since their hosts are beyond the limits of the present system. These species are (see Nilsson-Cantell, 1978 for descriptions):
¥ Anelasma squalicola (Lovén, 1845). Suborder Lepadomorpha. Body without scales, stalk embedded in skin of host (shark).
¥ Xenobalanus globicipitis Steenstrup, 1852. Suborder Lepadomorpha. Body resembling a naked capitulum; shell rudimentary and star-shaped, six scales; embedded in skin of host (dolphin or whale). Xenobalanus globicipites
¥ Coronula diadema (Linnaeus, 1767). Suborder Balanomorpha. Body contained in a shell with six scales; shell crown-like; attached to skin of whales. Shell elevated with convex rugose ribs. Coronula diadema. The stalked barnacle Conchoderma is often found attached on Coronula.
¥ Coronula reginae Darwin, 1854. Suborder Balanomorpha. Body contained in a shell with six scales; shell crown-like; attached to skin of whales. Shell depressed; parietal ribs flattened. Coronula reginae. The stalked barnacle Conchoderma is often found attached on Coronula.
[Descriptions after Brusca and Brusca, 1990]
The following species of Cirripedia are included in the present volume: