Bryozoans get rooted
Published 11 September 2022

Life as a sessile organism comes with its challenges. Whereas your mobile compatriots have the option to change their surroundings if conditions become difficult, you need to weather things as they come. You need a firm, broad foundation to achieve optimum growth. You need to fend off competitors attempting to occupy the same space. The Petraliellidae are one group of bryozoans who have these challenges covered.

Preserved specimen of Petraliella from the Yale Peabody Museum of Natural History.

Petraliellids are found in tropical and subtropical waters at a range of depths from the sublittoral to the deep sea. Colonies include large feeding zooids with evenly perforate frontal walls. Avicularia with robust, hooked, serrated mandibles are often present alongside the opercular openings. Eggs and larvae are brooded in globular, finely porous ovicells that develop at the front of the maternal zooids (Cook & Chimonides 1981; Tilbrook & Cook 2005). In life, colonies are brilliantly coloured due to pigments in the soft tissues of the zooids.

Colonies of petraliellids can vary significantly in overall form, with examples of a single species potentially differing in habit due to environmental conditions of stage of development. Habit can vary from encrusting to semi-erect or lunulitiform (lens-shaped). They are able to develop the last two forms due to the production of rhizoids, elongate root-like structures (formed from modified non-feeding zooids) that emerge from openings in the basal plate of the colony and anchor it to the underlying substrate. Semi-erect colonies may be unilaminar, bilaminar or tubular (‘bilaminar’ presumably refers to two layers of zooids growing back to back, one above and one below).

SEM image of Petraliella dentilabris zooids showing the developed ovicells, from Tilbrook & Cook (2005). Scale bar = 500 µm.

Production of rhizoids allows petraliellid colonies to establish themselves on unstable substrates such as sand beds or rubble, anchoring the colony in place and lifting it above the substrate surface. The capacity for raised growth also allows the colony to potentially overgrow competitors such as sponges. For encrusting bryozoans, sponges and the like may present a significant limitation to colony growth, but petraliellids are capable of turning the tables.


Cook, P. L., & P. J. Chimonides. 1981. Morphology and systematics of some rooted cheilostome Bryozoa. Journal of Natural History 15 (1): 97–134.

Tilbrook, K. J., & P. L. Cook. 2005. Petraliellidae Harmer, 1957 (Bryozoa: Cheilostomata) from Queensland, Australia. Systematics and Biodiversity 2 (3): 319–339.

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