Electra pilosa is a native encrusting Bryozoans found in the Gulf of Maine co-habitating with the cosmopolitan, MIS species Membranipora membranacea. Both of these encrusting colonies have a lacy appearance and are typically found on brown algae such as Laminaria sharing space on the same blade. Upon inspection, established colonies may look similar but they can be distinguished from each other by specific differential features.
Colony growth pattern is an important clue to differentiating Electra from Membranipora. Electra in its first stages of growth forms star-shaped colonies whereas Membranipora grows in a uniform, radial pattern (what Silén called "unitary multiserial", see LINK below). In contrast, the colony of Electra is separated into sectors divided by radial axes that extend beyond the margins of the colony forming a multi-pointed star (what Silén called "composite multiserial" growth). Each sector has a central growth axis composed of 2 or more parallel rows of rectangular zooids (approximately 0.33 by 0.5 mm in dimensions) flanked on each side by wings of oval or ovate zooids which fill in the area between the axes. The side walls of the calcified enclosure do not directly abut each other like in Membranipora (see February 12, 2012 post), but are separated by a translucent, calcareous surface membrane. Each enclosure is surrounded by several spines that vary in length. However, the median, proximal spine is larger than the rest and can be easily seen with a magnifing lens.
The bottom line when identifying encrusting Bryozoans in New England is to check for growth pattern (composite vs. uniform), boundary morphology (jagged vs. smooth), zooid skeletal structure (ovate vs. rectangular), arrangement of spines (larger pointed, proximal spine vs. 6 short, blunt spines, 4 at each corner of the rectangular enclosure) for Electra vs Membranipora, respectively.
Growth Pattern of Electra vs. Membranipora
Comparison of growth pattern of Electra vs. Membranipora in small colonies where the axial structure of Electra is most pronounced. Modified image from Silén, 1987 (see LINK below).
Axial Growth Pattern of Electra pilosa
Outer border of Electra pilosa colony growing on Laminaria collected from Hawthorne Cove, Salem Sound, MA. The colony illustrates composite multiserial growth. At the bottom, two axial strands of rectangular zooids curve toward each other encircling inter-axial ovate zooids. At the top, several axial strands join each other to form a single 5-zooid wide strand. Stereozoom 1.0 x 10x objective.
Calcareous Enclosures of Electra pilosa
Calcareous Enclosures of Electra pilosa
Electra pilosa zooids showing axial (left and right) and inter-axial zooids (center) with conspicuous proximal spines and translucent calcareous membranes between walls of the enclosures. Stereozoom 3.0 x 10x objective
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