Chthamalus stellatus (Poli,1791) text I.F. Smith
CURRENT TAXONOMY: World Register of Marine Species www.marinespecies.org/aphia.php?p=taxdetails&id=10623

DESCRIPTION **Glossary below.

opercular aperture (top opening)
Approximately broadly oval [01 Chthamalus stellatus], but often with tendency to flat sections of perimeter; some may approach kite-shape [02 Chthamalus stellatus] of C. montagui

opercular valves (lid plates)
Four valves, two terga and two scuta, open along longitudinal midline with a tergum and scutum each side [03 Chthamalus stellatus]. Whether closed or open, valves usually just below, or level with, rim of aperture, and each tergum fixed to its scutum (sometimes even after treatment with bleach) [04 Chthamalus stellatus]). Terga fill a third or more of aperture. A few growth lines may be seen on the rostral end of the scuta [03 Chthamalus stellatus].

tergoscutal flap (membranous seal on lid plates).
Bluish; varying grey-blue, white faintly tinted blue, bright blue to entirely electric blue. Bright orange mark at micropyle [05 Chthamalus stellatus] Orange-brown to dark brown mark at rostral end. Bright orange longitudinal lobe at carinal end (fully exposed when valves open [06 Chthamalus stellatus]). Between micropyle and carina, away from the rim, the flap may be orange-brown.
wall plates
Six; delineating sutures often only visible on juveniles as usually fused [07 Chthamalus stellatus] and difficult to discern on older ones. Rostral plate narrow [08 Chthamalus stellatus], butts onto lateral plates. Plates solid, not porose, but some holes near aperture internally [09 Chthamalus stellatus]. After bleaching, white with some pink-brown, especially internally near aperture. Often discoloured dull yellow in life and heavily eroded and/or punctured by Cliona celata, boring sponge [10 Chthamalus stellatus].

profile
When isolated, low mound [08 Chthamalus stellatus] often very depressed. When crowded, low column about 6mm high [11 Chthamalus stellatus].

base
Black membrane. Bright orange-red [12 Chthamalus stellatus] flesh visible in barnacle when removed from substrate.

diameter & height
Usually under 10mm diameter, occasionally to 14mm. Crowded columns about 6mm high.

IDENTIFYING FEATURES
Aperture: Approximately broadly oval [01 Chthamalus stellatus], BUT can vary to rounded kite.
Tergoscutal flaps: White to blue, with bright orange at micropyle and carinal end. [06 Chthamalus stellatus]
Base plate: Black membrane. Bright orange flesh inside barnacle [12 Chthamalus stellatus]
Wall plates: Rostral plate narrow [08 Chthamalus stellatus] butts onto lateral plates.
Tide level: Commonest MTL to MLWS, some up to HW. Not sublittoral.

SIMILAR SPECIES
Chthamalus montagui
Aperture: Kite shape (not-equilateral diamond), BUT can vary to rounded kite.
Tergoscutal flaps: White to blue, with brown at micropyle and carinal end [01 Chthamalus montagui].
Wall plates: Rostral plate narrow Cm, butts onto lateral plates [02 Chthamalus montagui]
Base plate: Black membrane. Dull yellow flesh inside barnacle [12 Chthamalus stellatus]
Tide level: Commonest HWS to HWN, some down to LWN. Not sublittoral.

Semibalanus balanoides (some juveniles in wave exposed positions)
Aperture: Oval.
Tergoscutal flaps: Whitish to grey with white rim, brown/blackish at rostral end, brown at micropyle, but white at carinal end [ 10 Semibalanus balanoides].
Wall plates: Lateral plates butt onto wide rostral plate [04 Semibalanus balanoides].
Profile: Depressed.
Base plate: Membranous.

ECOLOGY
Stenohaline, full salinity. Commonest MTL to MLWS, some up to HW, not sublittoral. On rock and live limpets [13 Chthamalus stellatus] exposed to strong and very strong wave action, where it is may be the dominant littoral barnacle. Usually overlaps with shore zones of Balanus semibalanoides and Chthamalus montagui. In Britain, breeds April to September, cyprids settle July to December.
Masses of Chthamalus near high water mark on shores exposed to strong waves are a favoured habitat of the isopod Campocea hirsuta and juveniles of the gastropod Melarhaphe neritoides [02 Chthamalus stellatus].

DISTRIBUTION
N.E. Atlantic coasts from Shetland to Azores, Canaries and N.W. Africa, and into Mediterranean and Black seas. See GBIF map www.gbif.org/species/2115620
British coasts westwards from Isle of Wight to Scilly and thence north to Shetland. Most Irish coasts, except Larne to Dublin. Southward (2008) wrote, “It straggles down the east coast of Scotland in small numbers, mostly in cryptic situations [deep cracks in rock], as far as Stonehaven. – – It is missing from the north Wales coast east of Holyhead, and from Lancashire, Cumbria, the Galloway peninsula and the Clyde Sea”. Records of it on NBN from these areas of reported absence require evidence of the tergoscutal flap for substantiation as the shell and aperture can overlap in appearance with that of C. montagui which is often mistaken for it. Those in situations of low wave action are especially suspect . See NBN map at species.nbnatlas.org/species/NBNSYS0000174548

Glossary
ala – (pl. alae) side flange of wall plate, overlapped by radius of next plate.
carina – posterior wall plate adjoining the smaller pair of lid plates (terga). carinal – (adj.) of the carina.
cirri – (sing. cirrus) feather-like thoracic limbs used in filter feeding.
cypris – (pl. cyprids) final larval bivalved stage in barnacle development.
euryhaline – tolerant of wide range of salinities.
HW- high water level.
HWS- level of high water spring tide.
lateral plates – wall plates other than end plates (depending on sp., strictly may be rostro-lateral or carino-lateral).
LWN- level of low water neap tide.
micropyle – small breathing/sensing hole between tergoscutal flaps, sometimes visible when operculum shut. Found on spp. that are left out of water for long periods.
MLWS- level of mean low water spring tide.
MTL – mid-tide level.
opercular – (adj.) of the operculum (aperture lid).
operculum – moveable lid-like structure used to close the aperture.
paries – ( pl. parietes) central section of wall plate, flanked by alae and radii.
ppt – parts per thousand (of salt in water).
radius – (pl. radii) side flange of wall plate, overlaps ala of adjacent plate.
right – on right when barnacle viewed from above with rostrum at 12 o’clock.
rostral – (adj.) of the rostrum.
rostrum – anterior wall plate adjoining larger pair of lid plates (scuta). (Lost on Elminius and balanoids; place taken by fused rostro-laterals; referred to as rostrum for simplicity.)
scuta – (sing. scutum) the larger plates of the operculum (lid).
sessile – (adj.) permanently attached to substrate (of immobile organism).
stenohaline – unable to tolerate much variation in salinity.
terga – (sing. tergum) the smaller plates of the operculum (lid).
tergoscutal flaps – membranous “lid seals”, (most visible as valves start to open).

References & links

Rainbow, P.S., 1984. An introduction to the biology of British littoral barnacles. Field studies 6: 1-51.
fsj.field-studies-council.org/media/350581/vol6.1_161.pdf

Southward, A.J., 2008. Barnacles. London, Linnean Society.