Balanus glandula has an irregular and pyramidal in shape shell which is usually dirty white in colour. They are small in size with a diameter of 10-18mm but they may grow up to 30mm. It has a combination of six shell plates, a calcareous base, an M-shaped opening and six pairs of cirri (Picker & Griffiths 2011; Hiebert and Jarvis, 2015). Barnes & Barnes (1956) found that Balanus glandula appears to have a variety of forms depending on its environment, from conic or convexly conic with a small aperture, more cylindrical, much less ribbed with larger aperture a mixture of strongly ribbed and weakly ribbed cylindrical ones with large aperture; shorter obliquely conic elongated columnar.

Reported colonizing the Argentinian coast and nowadays it is also registered successfully established on the rocky shores of Japan (Noda and Ohira, 2020) and south of South Africa (Mendez et. al.,2014; Robinson et al., 2017). Also distributed in northern Alaska to just south of the border between the United States and Mexico (Wares & Cunningham 2005).

B. glandula was recorded over more than 400 km of coastline between Elands Bay and Misty Cliffs (near Cape Point)(Laird & Griffiths 2008).

The acorn barnacle, Balanus glandula Darwin 1854, is sessile (Sotka et al., 2004) and to attach to substrates, it uses the flat calcareous base of its shell, movable opecular valves and the 6-plated wall (Hiebert and Jarvis, 2015). This invasive barnacle is observed fouling the endemic N. granulata and Neohelice (Chasmagnathus) granulata, hence its fast spread (Mendez et al., 2014).

The acorn barnacle is a sessile suspension feeder (Sotka et al., 2004). They feed by extending or beating their cirri to capture plankton or detritus from incoming currents (Hiebert and Jarvis, 2015). Nauplii larvae of B. glandula is planktonic and therefore feeds on diatoms while cyprid larvae is non-feeding (Pfeiffer-hoyt and Mcmanus, 2005).

B. glandula sexually matures at 1 year of age (Noda and Ohira, 2020) carry embryo broods that hatch into naupliar larvae within their mantle cavity. They produce 2 to 6 broods per year during winter and spring and the number of nauplii per brood may range between 1000 and 3000 depending on the size of the parent (Simon-Blecher et al., 2008; Hiebert and Jarvis, 2015; Lamont and Emlet, 2021). After the naupliar larvae has been released into the water column (released as stage 1 or stage 2 nauplii) they go through 6 developmental feeding stages which take between 2 and 4 weeks. Through the process of metamorphosis, stage 6 nauplii larvae transform into non-feeding cyprid larvae. At this stage they are specialized for seeking suitable settlement habitats in intertidal zones. After finding a habitat and attaching to a substrate, cyprid larvae metamorphose into sessile juveniles (Pfeiffer-hoyt and Mcmanus, 2005; Lamont, 2017; Lamont and Emlet, 2021). The attached hermaphroditic adults become reproductive within a year of settlement. The recruitment of B. glandula according to Elias & Vallarino (2001) occurs during winter months.

Intertidal