Fenestrulina malusii is a subtidal bryozoan found throughout the NE Atlantic and Mediterreanean. Colonies form bright white encrusting patches around 1 cm in diameter, on a wide variety of substrata.

Blankenberge, British Isles, De Panne, European waters (ERMS scope), Gulf of Mexico, Irish Exclusive economic Zone, Northumberland, Oostduinkerke, Oosterschelde, Portuguese Exclusive Economic Zone, Spanish Exclusive Economic Zone, United Kingdom Exclusive Economic Zone, Wemeldinge, Westhinder, WestHinder lichtschip, Westkapelle, Zeebrugge

This bryozoan is common on all British coasts, and throughout the temperate NE Atlantic and Mediterranean. Colonies have been reported from California between 5 -25 m (Nielsen, 1981)

Colonies form circular encrusting sheets which are bright white in colour. The colony shape may vary due to irregularities in the substratum and the presence of other organisms. Older colonies often loose the central area, such that they appear as a ring or crescent.

F. malusii zooids are oval, with a calcified body wall. The individual zooids are separated by distinct grooves and have a number of round pores present around the periphery. A large central pore (the ascopore) is situated on the frontal surface, proximal to the orifice which is semicircular in shape. Two to three spines are present on young zooids and no avicularia are present

Zooids are typically 0.4-0.6 by 0.3.-0.4 mm. Colonies typically 1 cm in diameter.

F. malusii commonly grows on a wide variety of substrata including stones and shells. Algal fronds and holdfasts, particularly large kelp species such as Macrocystis pyrifera and Agarum fimbriatum, may also be colonised by F. malusii.

The species ranges from the subtidal kelp forest to at least 50 m. Gautier (1962) reported it from 300 m in the western Mediterranean, but stated that it was rare beyond 100 m. F. malusii has been recorded from brackish conditions in the Lagoon of Orbatello, in the Mediterranean (Apolloni, 1931) and Maizuru Bay in Japan (Rucher, 1969).

Depth range based on 37 specimens in 3 taxa.
Water temperature and chemistry ranges based on 14 samples.

Environmental ranges
  Depth range (m): 0 - 219
  Temperature range (°C): 9.215 - 24.377
  Nitrate (umol/L): 0.146 - 8.636
  Salinity (PPS): 31.235 - 35.563
  Oxygen (ml/l): 4.869 - 6.794
  Phosphate (umol/l): 0.176 - 1.030
  Silicate (umol/l): 1.521 - 20.289

Graphical representation

Depth range (m): 0 - 219

Temperature range (°C): 9.215 - 24.377

Nitrate (umol/L): 0.146 - 8.636

Salinity (PPS): 31.235 - 35.563

Oxygen (ml/l): 4.869 - 6.794

Phosphate (umol/l): 0.176 - 1.030

Silicate (umol/l): 1.521 - 20.289
 
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Like all bryozoans, F. malusii is a suspension feeder. It feeds on small phytoplankton using ciliated tentacles of the lophophore.

F. malusii frequently grows attached to the frond and holdfasts of sublittoral aglae, including the species Macrocystis pyrifera and Agarum fimbriatum.

The founding zooid (ancestrula) develops into a young colony, and later  into an adult colony through asexual budding. Sexually produced embryos  are brooded within the colony and larvae are released after 10-14 days.  Larvae settle soon after liberation (often within an hour) and  metamorphose into an ancestrula.

Sexually-produced embryos, which are yellow-orange in colour, are brooded throughout the year, but they are particularly abundant in late summer. The ovicells (brood chambers) are conspicuous and present in large numbers on zooids towards the periphery of the colony. The larvae of F. malusii are large non-feeding coronate larvae, which lack a shell and have a densely ciliated belt (the corona) for locomotion. The larvae are pale yellow when fully formed and settle quickly after liberation.

Colonies grow through asexual budding of new zooids.