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New observations on Dromalia alexandri Bigelow, 1911, a rhodaliid physonect siphonophore from Southern Californian waters



Gillian M. Mapstone, Department of Life Science, The Natural History Museum, Cromwell Road, London SW7 5BD, UK.



Dromalia alexandri is a short-stemmed physonect siphonophore found regularly in bottom trawls from the shelf waters of southern California, including biannual samples taken to monitor sewage outflow from Los Angeles and San Diego. Since the last description of the species in 1983, a greater size range of specimens is here examined, some with particularly well preserved cormidial units and detached zooids. Higher abundance and larger sizes of D. alexandri specimens collected from the Southern California Bight might be due to increased nutrient levels derived from sewage outflow. Small and large colonies are illustrated and described, and two growth zones identified in the zone of proliferation, including the first siphosomal horn to be identified in a rhodaliid. Nectophores of D. alexandri are found to lack a descending surface diverticulum, and nectophoral lamellae were identified in the naked zone below the aurophore in two specimens. In the largest colony studied, a larger number of young cormidial units are present at the beginning of the first whorl than previously reported, giving a total of c. 57 in whorl one, and in the smallest specimen whorl one has only 10 units, fewer than previously found. Much larger and more complete cormidial units of D. alexandri are figured and described than previously published, as are also a detached mature bract and mature male and female gonophores. Cormidial units are found to comprise only three full cormidia (composed of a gastrozooid, bract, and a gonodendron with gonophores and gonopalpons) when mature, and typically one or two additional bracts. Descriptions of the colony and zooids are based on axes for siphonophores standardized by recent authors and applied here for the first time to D. alexandri. A revised distributional map of the species is included covering a larger geographical area than earlier maps, and based on a greater number of records than previously available. The paper concludes with a comparison and discussion of the main characters of D. alexandri and those of five other species in the family Rhodaliidae.