Next article FreeAbout the CoverPDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreCoverSponges (Porifera) represent an evolutionarily ancient phylum with a fossil record dating back to Precambrian times. Many sponges harbor dense and diverse microbial consortia extracellularly within their tissues. In contrast to these “high microbial abundance sponges” (HMA), the tissues of the so-called “low microbial abundance sponges” (LMA) are nearly devoid of microorganisms. The sponge on the cover, Ircinia sp., from the Mediterranean Sea, belongs to the HMA category. This scanning electron microscopy image shows the main skeleton structures, which are surrounded by thin, fibrillar, collagenous spongin, making up the bulk of the sponge tissue. Microorganisms are not visible at this magnification.HMA and LMA sponges may occur on the same reef in close proximity. Recent studies have shown that HMA sponges differ from their LMA counterparts in more than just the number and diversity of microorganisms they contain. For example, the HMA sponges appear to be less well irrigated. While the HMA sponge tissue displays fewer canals and smaller choanocyte chambers, the extracellular matrix is greatly expanded. The HMA/LMA dichotomy has long been recognized; however, the reasons for this pattern remain unknown.In this issue, Hentschel and collaborators examined sponge species from three locations for the presence of microorganisms in the sponge mesohyl matrix (pages 78–88). Of more than 50 sponge species investigated, half belonged to the HMA and half to the LMA category. The HMA/LMA dichotomy was also investigated in the taxonomic framework of their animal hosts, but a clear phylogenetic pattern could not be identified. This survey significantly expands previous datasets, serves as a reference database for anyone interested in bacterial abundances in sponges, and contributes to a better understanding of sponge-microbe symbioses.Credits: Cover photo ©S. Diller, Scientific Photography, Wuerzburg, 2014 (see http://www.vimeo.com/100428013 for “nanoflight” videos of sponges); cover design, Beth Liles, Marine Biological Laboratory. Next article DetailsFiguresReferencesCited by The Biological Bulletin Volume 227, Number 1August 2014 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.1086/BBLv227n1cover © 2014 by Marine Biological Laboratory. All rights reserved.PDF download Crossref reports no articles citing this article.