A Day for Worms on BHL

The Biodiversity Heritage Library (BHL) is replete with publications which have had a seminal influence on their field. A highly-credentialed member of this category is John H. Day’s A Monograph on the Polychaeta of Southern Africa, published in 1967 in two volumes by the (then) British Museum (Natural History) in London. It is timely to celebrate this book now: on 25 August 2019 the author John Day would have had his 110th birthday. And, as of this month, the author’s own copy of the book has been digitised for the BHL.

Years ago, this copy was loaned to me by John Day’s son, Professor Rob Day (also a marine biologist) from the The University of Melbourne — just down the road from my office and from the Australian node of BHL, both at Museums Victoria in Melbourne.  When my librarian colleagues discovered I had the copy, annotated in pencil by the author with a 2nd edition in mind, they were keen to digitise it, even though BHL had previously digitised another copy. (A new version of Day’s monograph was later prepared but never published, perhaps due to lack of funding.)

Polychaetes are important to marine biologists due to their weight of numbers: in nearly all seafloor environments anywhere in the world, polychaetes are just about the most numerous and the most species-rich invertebrate. Therefore, many marine ecologists spend a lot of time identifying polychaetes — usually a difficult task. Which is why John Day’s monograph was so influential. For decades before, and for decades after, there was nothing like it, and so the two volume work, known to most researchers simply as Day, was indispensable.

Day was a comprehensive synthesis of descriptions, diagnoses and drawings of this rich and diverse component of the marine fauna (currently there are about 12,000 described species of polychaetes). And because most polychaete families and genera occur worldwide, and because there was no other equivalent publication anywhere, Day was a vital reference for anybody anywhere who needed to identify their local marine fauna.

Day is still a model for anyone who sets out to write such a monograph, because of the simplicity and clarity of both text and illustrations. In particular, the introductory pages provide a visual overview and key to the numerous families.


Page 23. Day, John H. A Monograph on the Polychaeta of Southern Africa. Volume 1, 1967. Author-annotated copy. Digitized in BHL by Museums Victoria.

These handful of pages encapsulate what would otherwise be an overwhelming variety; even beginners can begin to think that they may one day be able to interpret the myriad forms and structures of polychaetes. Thus, although the documentation of polychaete diversity has marched on, well-thumbed 50 year old copies of Day are still in regular use in marine laboratories world-wide.

It seems to me that John Day must have honed and improved his diagrams over many years of teaching (he was Professor of Zoology at The University of Cape Town) and that he must have given a lot of thought to the layout and presentation. Day had begun annotation of his own copy with species to be added, but he didn’t touch these introductory pages, suggesting that he too was satisfied with the published version. Even today, for me and many of my colleagues, there is nothing quite like it for introducing novices to the mysterious world of polychaetes — there have been numerous revisions and name changes in intervening decades, but the worms themselves still look the same.


John H. Day (third from right) showing students a tray of specimens collected from an estuary mudflat.  Durban Bay, South Africa, 1951.  Photograph supplied by Rob Day, 2019.

John Day was ahead of his time in other respects too. Our understanding of evolutionary relationships of polychaetes, and how they should be classified within the phylum Annelida, has long seemed an intractable task. Many alternative classifications having been offered up in publications both before and since the publication of Day. The author himself took a purely pragmatic approach and followed earlier workers, arranging the families in two major groups, one per volume: Errantia (mostly mobile worms with similar segments and often with jaws) and Sedentaria (mostly tube dwellers often with a crown of feeding tentacules and modified body regions).

Even though Day only ever claimed this as an arbitrary arrangement, he would have surely been pleased to know that this practical solution has a lot in common with the most recent phylogenetic analyses based on DNA sequence data, which recognise many elements of the old Errantia / Sedentaria scheme and even retain the names! Nor, I think, would he have been dismayed by another discovery confirmed by DNA data: there is no such thing as “Polychaeta”, which is now known to comprise a variety of forms which do not all belong on a single branch of the annelid phylogeny. Furthermore, several other kinds of worms previously known as separate Phyla are now known to belong within the Annelida — a possibility also canvassed by John Day in the introduction to his monograph.  (I’ll nevertheless keep using polychaetes as an informal term.)

I never met John Day, but his no-nonsense attitude was evident in other ways. He had lost a leg when hit by flak on a bombing raid while flying as a navigator for the RAF during the Second World War (he was still able to provide a course before passing out and the tail gunner saved his life by limiting loss of blood until they returned to base). There are unconfirmed stories that after the war, on trips between Cape Town and London, he would bring preserved polychaete specimens for study through customs in his prosthetic leg.

Taxonomists nowadays are wont to criticise Day, and other publications of the era, for accepting as valid many “cosmopolitan species”.


Page 146. Phyllodoce malmgreni and related taxa in the family Phyllodocidae. Page 146 from Day, John H. A Monograph on the Polychaeta of Southern Africa. Volume 1, 1967. Author-annotated copy. Digitized in BHL by Museums Victoria.


Phyllodoce malmgreni was described from the Red Sea, but has been much more widely recorded throughout the Indo-Pacific. This museum specimen of Phyllodoce cf. malmgreni is from Australia. Photograph Robin Wilson.

Typically, these were originally described from European localities but subsequently recorded (or rather very similar forms were found) from more widespread regions. If they could not be distinguished, specimens from widespread localities were assigned to a single species.

In recent decades, taxonomists have split many “cosmopolitan species” into a number of new species, each from a different ocean or region.  At first, this seemed an almost routine tidy up task. The advent of molecular systematics has shown that the situation is far more complex and interesting. DNA sequence data has repeatedly demonstrated that even within a region (for example northern Europe), what was thought to represent a single species is often discovered to comprise many species. Usually, these newly recognised species can barely, or not at all, be distinguished morphologically, yet they show very deep genetic divergences.  Understanding of how these taxa have evolved and dispersed (for example, is each instance a single global, monophyletic radiation?) is arguably among the most interesting questions in marine diversity studies today.

Whatever the answers turn out to be, it seems clear that many of these cryptic species will only be distinguishable through genetic analysis. Until such time as rapid genetic diagnoses become routine, ecologists and other non-taxonomists are likely to have to resort to some kind of species-complex definition to cope with messy nature. It seems that such species-complexes might look a lot like the often-maligned “cosmopolitan species” accepted by the pragmatic (and prescient?) John Day.

Photograph of a man on a beach
Written by

Robin Wilson is Senior Curator of Marine Invertebrates at Museums Victoria, Melbourne, Australia. His work on the systematics of marine segmented worms or polychaetes (more correctly known as Annelida) is aimed at better documenting the Australian polychaete fauna, describing new species, and especially developing versatile identification tools linked to online databases.