Hi Pieter

The bioturbative effects of the shore crab Carcinus maenas are limited, since many live in hard-substratum environments. But indeed, in sandy areas it shows either locomotion (small footprint but many legs!) or temporary burrowing activity. Burrowing may be either for finding food with chaelae or for hiding. In sandy areas, the shore crab may temporary hide in the sediment completely by digging in backwards. It does not dig deep as it keeps contact with the sediment surface.

This being said, following Kristensen et al. (2012), I would consider the shore crab an 'epifaunal biodiffusor'. C. maenas is not well adapted for living in and moving through the sediment like the examples mentioned for 'surficial biodiffusors' (e.g. burrowing sea urchins). Also following Kristensen et al. (2012) it cannot be considered a 'regenerator' as does not maintain burrows continuously.

I agree with you:

" a measure of sediment reworking (bioturbation in the wide sense, see Kristensen et al., 2012) was entered in the analyses. This measure includes four categories for the present fauna: surficial modifiers, upward conveyors, downward conveyors and biodiffusors (Table 1). The categories were defined by Solan (2000) in the following way; surficial modifiers being species whose activities are restricted to the surficial layer (0–2 cm) of the sediment profile... " Article Structural and functional changes of soft-bottom ecosystems ...

And of course they play important roles as predators, scavengers, detritivores ... and people are also looking into their plastic uptake:

" Uptake and Retention of Microplastics by the Shore Crab Carcinus maenas " ( Andrew James Russel Watts et al, 2014)

Hi Pieter

The bioturbative effects of the shore crab Carcinus maenas are limited, since many live in hard-substratum environments. But indeed, in sandy areas it shows either locomotion (small footprint but many legs!) or temporary burrowing activity. Burrowing may be either for finding food with chaelae or for hiding. In sandy areas, the shore crab may temporary hide in the sediment completely by digging in backwards. It does not dig deep as it keeps contact with the sediment surface.

This being said, following Kristensen et al. (2012), I would consider the shore crab an 'epifaunal biodiffusor'. C. maenas is not well adapted for living in and moving through the sediment like the examples mentioned for 'surficial biodiffusors' (e.g. burrowing sea urchins). Also following Kristensen et al. (2012) it cannot be considered a 'regenerator' as does not maintain burrows continuously.

I suggest that you contact my colleague Inga Nordhaus. She is working for many years on crabs.

"Dr. Inga Nordhaus"

Hi Pieter,

I fully agree with Godfried. During my experiments/surveys at Viane (Eastern Scheldt) I found many shore crabs dug into the sand in the first 2.5-5 cm or so. Therefore, they definitively have an impact, but more as biodiffusor I think.

Hi Pieter,

I think both Godfried and Jim have their money on the right horse. It is about the net effect on sediment bulk properties and geochemistry. In that sense, regenerators bring sediment with typically reduced properties, back up into oxygenated conditions, so that the sediment and porewater- associated compounds enter another redox cycle: the sediment that was "tucked away" is regenerated again ! Fiddler crabs (Uca spp.) do exactly that, they bring sediment from deep down to the surface. Shore crabs are in another line of sport. Indeed, their net effect on the sediment is on the top 2.5 - 5 cm as they either walk through it, pick their food morsels from it while disrupting the sediment matrix, or burrow in it temporarily to escape predation and/or desiccation. Gerino et al. (Vie et Milieu, 2003) provided a comprehensive set of bioturbation categories. Solan et al. (MEPS, 2004; http://www.int-res.com/articles/meps2004/271/m271p001.pdf) provides a nice visual investigation on the bioturbation effects of crab legs, using luminophores (fluorescent particles). The primary net effect of decapods with such a particular niche and life history (macrobenthic sedimentary epifauna) is that the top sediment layers become [more or less] uniformly mixed. It may be a bit counter-intuitive, but this effect classifies them as "biodiffusors", and in fact places them at par with shallow-burrowing bivalves, such as cockles (Cerastoderma edule).

Hi Pieter, I agree with previous answers but would not lay emphasis on sand bottoms alone. Shore crabs live in great numbers in tidal areas where juveniles live on the flats for most of the year. Their role as biodiffusor will be small compared to turbulation by water movement. Older crabs in these areas move to gullies where they can will have more impact on the bottom. I wouldn't be surprised if in general shore crabs move to harder substrates with age.The impact of their bioturbulation will change with age not only because of their size but also because of changes in depth and substratum.

Thank you all very much for your nice answers!

So to conclude,

The crab C. maenas should be classified as a 'biodiffusor' and not as a 'regenerator'.

And, as for sediment reworking intensity classification, after considering your answers, I think that it should be considered a "surficial biodiffusor", as it definitely reworks the sediment deeper (up to 5 cm as Jim and Francesco mention) in comparison to many other epibenthic species that only modify the very topmost layer (1 cm).

Hi Pieter

In ecology, it is often difficult to use strict boundaries: filter feeders sometimes act as deposit feeders, r or K strategy depends on the size range of study objects, and depth zones vary with fluctuating light levels. In using bioturbation categories, our point of view - geophysical or biological - is important, as well as considering spatial scale of the mechanism.

When the deepest effect in the sea bed is considered most relevant to assign a species as a surficial biodiffusor, e.g. down to 5 cm, it is simple indeed and I agree to classify C. maenas as such, especially large crabs. Likewise, long-billed wading birds, seals and even humans wearing boots have surficial effects.

However, from the perspective of a species' life style, I prefer to call C. maenas an epifaunal biodiffusor. In sandy areas, in terms of scale, I believe bioturbation activities by the crab are limited and, as pointed out in my first reaction, the morphology of this species is not well adapted for burrowing.

So it is nice to agree on biodiffusion, apparently our perspective how to deal with it in detail differs, no problem!

To all: thanks for this fruitful discussion.

Hi Godfried,

Thought about it a bit more, and I think your right, especially in my case (sand flat, small specimens), and based on its life style, it may be more appropriate to classify it as called an epifaunal biodiffusor.

I also believe that for bioturbation classification it is important to consider where they spend most of their active time. As you mention, it is not well adapted to constantly life beneath the sediment surface. Yes, they may occasionally dig themselves in (up to 5 cm) for hiding, however, the bulk of their time will be spend on top of the surface and reworking the topmost layer (0-2 cm) searching and digging for small bivalves and such.

Nice discussion indeed, thanks for that!