Seaweeds, lichens and phytoplankton, what’s not to like..?

Rocky Shores, by John ArcherThompson and Julian Cremona 2019. Bloomsbury Publishing Plc.

Rocky shores are one of the most extreme environments on the planet.” [Anon.]

This is a BOTANY blog site!

OK, let’s address the elephant in the room at the outset: Why am I, a Botanist, appraising Rocky Shores by John Archer-Thompson and Julian Cremona?

First, because there’s a good deal of plant-related material in the book [Chap. 4 on lichens (14 pp.), Chap. 5 on seaweeds (38 pp.), and Chap. 10 on plankton (4.25 pp. re phytoplankton), and there’s a good shout-out for fungi in Chap. 12 on decomposition], so it fits within my sphere of broad botanical interest – and, arguably more importantly, that of the readership of Botany One. Fascinating in their own right, seaweeds, lichens, fungi, and phytoplankton are also extremely important as components of the wider ecology of the rocky shore, which is the broader focus of Rocky Shores. And, as with any ecologically-focused text, the botanical contribution cannot – and must not – be overlooked. But, as is also the case in other habitats, too often the plant-related part is under-appreciated, so one is more than happy to put in a good word or two for the plant-relevance of this book. Second, as a UK university lecturer I taught marine biology generally, and rocky shore ecology specifically, for several years. That has given me a good appreciation of that important habitat, and suitable background to express a view on a book devoted to that environment.

Although my comments here are primarily about the plant* parts of Rocky Shores – as befits the botanical nature of the blog site (and my own preferences!) – I can also appreciate the animal component too – especially with my marine biologist hat on – and will make some comments about that as well…

Overall appreciation for Rocky Shores

Rocky Shores is a great read and a terrific resource for any marine biology course in providing essential background to rocky shore ecology (although those not already familiar with the organisms considered will benefit from a decent ID guide to help them name the organisms on the shore…) – and is suitable for anybody who wants to know more abouit this amazing habitat, and the organisms found there.

As you should expect, the text contains science, but it also includes a good deal of historical, geographical, and cultural information – which makes it more of a natural history than a standard ecology textbook, and creates a more satisfying all-round read.

The authors’ stated aim is to produce an accessible guide to rocky shores: This they have achieved in a most admirable fashion. Throughout, Rocky Shores is nicely written, readable, and accessible. It’s also abundantly illustrated with lots of excellent images – almost all of which are in colour, which add a lot of extra value to this already very attractive book.

It’s not just for British shores…

As volume 7 in the publisher’s British Wildlife Collection, Rocky Shores specifically deals with those habitats in Britain. But, international readers should not avoid the book. Because zonation of rocky shores is a universal** phenomenon (Thomas Stephenson & Anne Stephenson, Journal of Ecology 37(2): 289-305, 1949; doi: 10.2307/2256610), a lot of the content will be relevant elsewhere on the planet. True, different species will be found beyond British shores, but the same general organising ecological principles apply; Rocky Shores contains lots of ‘beyond-the-rocky-shores-of-Britain’ relevance.

Very good pedagogic credentials…

One of the most characteristic features of rocky shores is the zonation that is displayed where bands of different organisms are found at different parts of the shore – from the ‘splash zone’ at the top (nearer the land) towards the ‘lower shore’ at the bottom (closer to the sea). The reasons behind that distribution are considered throughout the book, but particularly in Chap. 2 which deals with patterns and zones. Even as one who’s taught rocky shore ecology for 15 years I learnt a lot from this chapter. Indeed, I learnt a lot from many of the chapters*** – which is surely the sign of a good book – as you’ll see below.

There is a pleasingly high number of in-text references (with full citation details provided in a listing towards the back of the book), but much of what is stated in Rocky Shores is unreferenced: You’ll therefore need to take the authors’ word for those statements. Whilst one acknowledges that those words are based on the authors’ many years of teaching rocky shore ecology to students of all ages, and their own researches and writings (e.g. Cremona’s Seashores – An Ecological Guide; Archer-Thompson’s The Chronicles of Larry, Volumes 1 and 2; Archer-Thomson, Field Studies 9: 531–546, 1999; Archer-Thomson, 2016. The Sea Empress incident and the limpets of Frenchman’s Steps, twenty years on), and as educators there’s no intention or suggestion to present false information, it would be nice to have that seal of approval provided by a proper evidence base****. This is especially important where new information – to me at least – is presented [see below for some examples].

Although not really relevant to zonation on rocky shores, there is very good coverage of rock pools (and with a decent mention of seaweeds…) in Chap. 3, which contributes to a nicely rounded appreciation of the rocky shore habitat more generally, and therefore provides an educational bonus.

Rocky Shores includes a great example of fieldwork – and its value – in Chap. 12 on challenges, threats and the future of rocky shores, specifically with respect to the Sea Empress oil spill in 1996. It argues well for the importance of long-term data-sets (especially those that provide base-line information on an area before an ‘event’ for the purposes of sensible and meaningful comparison). And, if we acknowledge that the students who collected the data are not professional scientists, that example is also a great advertisement for the value of ‘citizen’ (which now should probably be termed ‘community’) science.

There is also a good rebuttal of the chromatic adaptation hypothesis of seaweed distribution (Matthew Dring, Limnology and Oceanography 26(2): 271-284, 1981; So often one sees this hypothesis included in marine ecology books without challenge; nice work John and Julian.

Finally, and whilst of educational relevance, one does wonder if the several pages of illustrated seaweed life-cycles [2 pages for browns, 2 for reds, and 1 for greens] are really necessary. There are no corresponding diagrams for the animals mentioned, and some of those – especially taxa with planktonic phases – have complicated life histories. Whilst it’s nice to see special consideration of plant-related matters, maybe the 5 pages devoted to seaweed life-cycles could have been better used…

Moments of joy, and sadness…

Rocky Shores contains some unexpected discoveries. Very much a bonus is the rather unusual, but very nice, touch on p. 276 which is a photograph of an example of the all-but-lost phenomenon of ‘diatom art’. To produce that image – which was created especially for the book – ‘diatomist’ Klaus Kemp***** arranged 49 diatoms on a microscope slide. This is no mean feat considering that these single-celled algae are only approx. 20-200 microns long. Such an exquisite thing of great beauty – and so unexpected.

At the other end of the emotional spectrum, I was saddened to read that the Bristol Channel had been demoted to be the location which now only had the 3rd highest tidal range in the world. Although no reference(s) were provided in-text for this local-pride-in-one’s-birthplace-shattering assertion, it’s there, in black-and-white, in print, on page 30, so it must be ‘official’. I’d’ve continued to be happy with 2nd place (Allen Archer & Mary Hubbard, Chapter in Special Paper of the Geological Society of America 370: 151-173, 2003; doi: 10.1130/0-8137-2370-1.151). OK, it’s a UK phenomenon that’s clearly not world-beating, and 2nd was a very creditable performance, but, 3rd!?? Hey-ho, sad news. Still, there’s plenty of good things to discover in Rocky Shores, so it’s not all ‘bad’…

Lots of lovely lichen stuff…

Maybe because I’m more familiar with seaweeds, I found Rocky Shores’ Chap. 4 on lichens to be one of the most interesting.

It was good to be reminded of the somewhat heretical nature of the notion when Schwendener proposed his ‘dual hypothesis’ (Rosmarie Honegger, The Bryologist 103(2): 307-313, 2000;[0307:SSATDH]2.0.CO;2) that a lichen is composed of a fungus plus an alga – which latter may be a green alga, or a blue-green alga (i.e. a , cyanobacterium).

Although the dual nature of lichens is probably familiar to all who know anything about these organisms, new to me was the fact that some lichen symbioses were three-partnered – including bacteria other than cyanobacteria. Again, there was no in-text reference for this, but see Scott Bates et al. (Applied and environmental microbiology 77: 1309-1314, 2011; doi: 10.1128/AEM.02257-10) and Ines Aschenbrenner et al. (2016, Front. Microbiol. 7:180; doi: 10.3389/fmicb.2016.00180) for more information.

I was aware of the cited research which found that some lichens had a yeast component in addition to the usual fungus + alga/cyanobacterium arrangement (Toby Spiribile et al., Science 353(6298): 488-492, 2016; doi: 10.1126/science.aaf8287). Importantly, the included in-text citation – of primary research published within the last 4 years – supports my view that Rocky Shores is probably as up-to-date as you can expect for a rocky shore text.

I was intrigued by the authors’ notion of a ‘reverse lichen’ in describing channel wrack (Pelvetia canaliculata, a brown seaweed found towards the top of the shore) when containing a fungus (Mycophycias ascophylli) within its tissues. Although no reference was provided in-text, sources here and here provide necessary support for that statement.

A great revelation to me is that the odour of TCP [a mild antiseptic whose initialism comes from its original chemical name and formulation of trichlorophenylmethyliodosalicyl] is associated with Lichina pygmaea (a small, tufted, black lichen). Apparently, when crushed/bruised/damaged the lichen emits that signature fragrance, which helps to distinguish it from Catenella (a small, dark red/black, seaweed), which is often found at a similar shore level as, and can be confused with, the lichen. Although no reference is given for this, a bit of Googling turns up the following confirmations: L. pygmaeasmells of embrocation [“a liquid that is rubbed onto the body to reduce pain or stiffness in muscles”, which is presumably where the TCP comes in…] when crushed”; and “when scratched this species produces a slight chemical or pharmaceutical smell”. Sadly, for one such as I whose sense of smell is pretty poor, it’s not a lot of use, but what a great field ID characteristic this is for one’s students who could benefit greatly from this information.

And, as if a chapter devoted to lichen biology is not enough, there is also a good shout-out for fungi on pages 313/4.

Let’s not forget the seaweeds…

Even as a lecturer reasonably well-versed in marine biology I learnt lots of new stuff from this book, especially relating to the ecology of seaweeds, which only enhances an appreciation of those tough little botanics – and underlines further the usefulness and pedagogic value of Rocky Shores (the book).

The notion of seaweed-seaweed communication via exudates from Ascophyllum and flat-topped periwinkle grazing experiments as demonstrated in the work of Carla Flöthe & Markus Molis (Journal of Phycology 49: 468–474, 2013; was both fascinating and new to me. However, although the reference for this is included in-text, it’s in relation to a consideration of the role of tissue-residing anti-herbivory chemicals and direct attack by molluscs; it’s not made clear that it relates to this ‘macroalgal communication’ idea as well. So, for completeness here, and as acknowledged by Flöthe & Molis (2013), their study corroborates the earlier work of Gunilla Toth & Henrik Pavia (Proc. Natl. Acad. Sci. USA 97: 14418–20, 2000;

As with most other habitats I had assumed that all parts of the rocky shore’s plant component would have been substantially grazed by the numerous herbivores that frequent that habitat. How wrong that assumption was. And a really important nugget of information in the book was the revelation that green seaweeds are the favoured food of herbivores on rocky shores; the brown and red seaweeds are largely unpalatable, and uneaten. The substantial store of carbon – and other elements – in the tissues of most rocky shore algae only goes back into marine system – i.e. becomes recycled – when those uneaten algae are decomposed.

The authors are also very keen to emphasise human uses of seaweeds, and devote several pages to that topic, thereby emphasising the importance of rocky shores and the need to conserve this habitat.

Some animal thoughts…

As a fair-minded Botanist, I’m happy to give a bit of a boost to that lesser-known branch of science that studies animals, so, here are a few of my animal ‘highlights’ from the book…

Although it’s not really a rocky shore organism, there are some fascinating insights into the biology of the piddock (a soft-substrate-boring mollusc), and the authors share the intriguing tale of the naming of Golfingia (a sipunculid worm). Rocky Shores gives a big ‘shout-out’ for the importance of the larvae of chironomid midges (so-called non-biting midges), and mentions use of the wrack fly (or kelp fly) as an alternative to the fruit fly for biology research (Nicholas Tolwinski, J. Dev. Biol. 2017, 5, 9; doi:10.3390/jdb5030009).

This latter pair of organisms has been singled out for special mention because “Marine insects have generally been ignored in standard entomological books on the one hand, and in marine invertebrate books on the other” (Lanna Cheng (1976). Marine Insects, p. 3. UC San Diego: Library – Scripps Digital Collection). Even when marine insects have been considered, it is still a widespread view that their role in marine biology is of little relevance as very few of the hundreds of thousands of insect species occupy marine habitats such as the open sea (e.g. Simon Maddrell, Journal of Experimental Biology 201: 2461-2464, 1998).

As one who was brought up with that marine-insect-irrelevance notion, I now realise that it really only applies to fully marine habitats. As is so often the case, the inter-tidal zone (“the area that is exposed to the air at low tide and underwater at high tide (the area between the low and high tide lines”) that is the focus of Rocky Shores, is a special situation. And, courtesy of Archer-Thompson and Cremona, I now appreciate the role played by insects to, and in, the inter-tidal zone.

The important – and probably irreplaceable? – contribution of insects to the economy of the shore is summed up in the authors’ own words: “What the rocky-shore herbivores cannot do, the seaweed flies have achieved: they consume huge amounts of  algal biomass and convert it into usable material to re-enter the ecosystem” (p. 325). It’s always good to have one’s ‘knowledge’ challenged – and corrected where necessary(!) And here it is worth applauding more generally the books’ Chap. 11 that’s devoted to the strandline – “a mark left by the high tide or a line of seaweed and other debris washed onto the beach by the tide”. It’s a wonderful section dealing with a topic that’s so easy to overlook as one wades through it to see the ‘more interesting stuff’ on the rocky shore proper.

Finally, I think the authors have missed an opportunity when considering dinoflagellates (single-celled algae). Whilst it’s good that Rocky Shores mentions the endosymbiotic role of some species within the snakelocks anemone (an anemone often found in rock pools on the shore), it makes no mention of the contribution of these algae to the coral symbiosis. I’m aware that intertidal purists may cry “foul!” here since coral isn’t a rocky shore inhabitant, and therefore is beyond the scope of Rocky Shores. But, this is the sort of gem of information that the authors otherwise like to slip in as a bonus elsewhere in the text. What a great educational angle to play here if you were to suggest that the snakelocks anemone on British shores can be viewed as a much-enlarged example of a single coral polyp allowing that amazing warm-water mutualistic symbiotic relationship to be studied on one’s own doorstep, so to speak…

It’s all about the names…

Knowing how challenging some of the scientific names of rocky shore ‘plants’ can be, it’s nice to see that the authors have opted to use common names for lichens and seaweeds throughout the text – after having given the scientific name on first mention for accuracy. Should any confusion arise when reading common names in the text, there is a list of Species names at the back of the book – in common name order. Both common and scientific names appear to be listed in the Index.

Coincidentally, I only spotted two ’typos’ throughout Rocky Shores, both relating to scientific names. The first is in connection with the brown seaweed Laminaria hypoborea (p. 152) – the specific epithet of which should be hyperborea. The second is the fungus genus Alternoria (p. 314), which I believe should be Alternaria (Malamatenia Arvanitidou et al., Microbiological Research 155: 49-51, 2000;; Xiaoping He et al., Journal of Separation Science 42: 2510-2516, 2019; doi: 10.1002/jssc.201801284).

And don’t forget the plankton…

Chap. 10 on plankton is rather zoo-biased – at least 15 of its 20 pages are devoted to animals. But, why a section on plankton – “very small plants and animals that float on the surface of the sea and on which other sea animals feed” – anyway? What does this have to do with rocky shores?

One word: Meroplankton – organisms that “spend only the larval or early stages of their life as part of the plankton”. For those organisms that are otherwise fixed in location on the rocky shore, having a mobile planktonic phase is extremely important. It allows their young to travel to and settle upon new rocky shores, thereby – potentially – avoiding competition with parents and other established individuals on already-crowded shores. It also helps to ensure that new individuals are brought onto shores that may have suffered loss of populations – e.g. as a result of oil spills such as that from the Sea Empress in 1996, where the influx of plankton-derived young limpet larvae helped to recolonise limpet-depauperate, oil-damaged sites.

Or, to quote the authors: “The relevance of healthy plankton communities to the wellbeing of rocky-shore organisms – and indeed human welfare – cannot be overstated. The biodiversity of our shores is completely dependent on the twice-daily delivery of plankton brought in by the tides” (p. 287).

Chap. 10 also adds educational value with a nicely-summarised history of plankton research. But, given the small size of many members of the plankton, scalebars would be useful additions to the pictures of plankton.

A reminder: Why study rocky shores anyway..?

Let’s allow the authors to make that case:

“No other environment in the British Isles can match the rocky shore for biodiversity” (p. 171). With such a tremendous range of life forms within a reasonably small area, rocky shores are ideal sites for experiencing a wide variety of the diversity of the tree of life.

“No two rocky shores are the same” (p. 16). This is dramatically underlined by Chap. 1 which takes the reader on a rocky-shore-based tour of Britain and reveals the diversity of habitat that exists in this rather small part of the world.

“Unexploited rocky shores are one of only a small number of habitats left in the British Isles that might be considered natural and unmanaged” (p. 18). Surely, there’s incentive enough to encourage you to visit a shore.

With Rocky Shores to guide and inspire you, a whole world of discovery and adventure is pretty much on everybody’s doorstep and awaits your visit…


John Archer-Thompson and Julian Cremona have produced an informative, authoritative, and entertaining book about the ecology of rocky shores, and it’s a wonderful read. All things considered, Rocky Shores deserves to become a ‘classic’. At the very least it ought to be on the reading list for every course about rocky shore ecology!

[My apologies for such a long piece, which appears to have morphed from a book appraisal into an essay on why rocky shore ecology is so interesting. But, this book is such a revelation; there was so much to say and praise. Maybe, consider this blog item your lockdown library’s long read of the week..?]

* For convenience, lichens and seaweeds are treated as plants in Rocky Shores (p. 14), which further strengthens the botanical interest in this book. Although I’ve also used this phrasing in teaching marine biology, it’s not without its dangers. For example, it was always rather disappointing to see that it had not been appreciated for the imprecise, if convenient, taxonomic shorthand that it is when lichens and seaweeds were actually classified/categorised as plants in students’ rocky shore fieldtrip write-ups…

** This is an unfortunate phrase, and one of those words that should probably be banned from science speak – and everyday usage as well. Fortunately, it is understood by most who read it to mean the much-more-geographically-restricted sense of Earth-wide, or global, since – to my knowledge – there is no evidence that this phenomenon occurs everywhere – or indeed anywhere – in the Universe. Nevertheless, there are those who probably aren’t aware of this planetary limitation, so caution is advised.

*** For example, a word new to me was ‘nurdles’ (p. 337).

**** I should here stress that it’s not a criticism specifically of Rocky Shores, but an observation that applies to many plant biology textbooks – several of which provide fewer or no in-text references. It’s all too easy to just accept the printed word because it’s published, and in a scholarly text. True, if you want to check a statement it’s usually easy to find the necessary evidence source on-line these days – as I’ve demonstrated above (which act may help to fix the information in the reade’s head, and which may have been the authors’ subtle pedagogic intention all along..?). Ease of ‘Googling’ may be a reason why in-text references are omitted or restricted in number; that, and a desire not to alienate would-be readers [I haven’t been able to discover the intended readership of the publisher’s British Wildlife Collection that this book is part of] by filling the text with citations in a way that might appear off-putting. But, it’s important not to lose sight of – or connection with – the original sources, if only to emphasise that continuity of progress element – and priority – that’s integral to science. If we don’t do this – even occasionally – we run the risk, however unintentionally or inadvertently, of perpetuating mis-statements. We’re not going to solve that conundrum here, but I’ve at least raised it as something to be mindful of. I suppose what I’m saying is: always be a little sceptical when reading and check statements to satisfy yourself of their veracity.

***** More examples of Klaus Kemp’s diatom artwork can be found here and here.

    • Absolutely, Darrel!
      If the teaching of marine biology is revived at your venerable HEI it would be a great addition to that module’s reading list. And, if all students read its Chap. 2 in advance of the field trip, would save AN Other from having to dliver a 2.5 hour briefing lecture on the rocky shore upon arrival(!)
      Be safe, well, and alert!

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