Tag Archives: Inachus

Scorpion Spider Crabs

Posted on by under anemones, crustaceans, diving, EMWL, photography, Silver Steps
I recently posted on the Crustaceans inhabiting my local shore dive site Silver Steps (see here), but left out one species: the Scorpion Spider Crab (Inachus spp.), which deserves a post on its own. I have blogged about this crab before (see here), specifically in the context of ‘macro-wide angle’ for which this species, perched on- or under a snakelocks anemone, is ideally suited. My previous efforts were based on an affordable inon wetlens, but I have since bought a super-duper nauticam lens. Although very good at macro-wide angle, this lens lets in very little light, which basically gives you three options: 1) lie on some bright white coral sand in the Maldives and shoot upwards to the sun at midday (so actually not really an option), 2) increasingly compromise the exposure triangle to keep a blueish/greenish background (higher ISO>more noise, lower shutterspeed>more camera shake, greater aperture>smaller depth of field), or 3) keep all settings in a ‘normal’ range, resulting in a foreground lit up by strobes on maximum output, but a black background (only if you are shooting critters in rock crevices you can get both fore- and background well-exposed, as strobe light has something to bounce back from – see for some examples the Crustaceans post). I quite like the clean black background and dislike bluer, but grainier images, so have mainly tried option 3. Hopefully, next year in summer I can try going for some blue background shots. Sorry if this blog is occasionally turning into a photography borefest by the way, but I have been really getting into that stuff! ;-). Anyway, these scorpion spider crab pics have turned out quite nicely.

Inachus

Posted on by under anemones, cnidarians, crustaceans, diving, Macro, Silver Steps

This seems like a familiar way to start a blog post but here I go again: ‘the weather has been terrible lately and I have not been in the water!’. March used to be my favourite time for snorkeling because the seaweeds look at their best, but the last three years it has been windy and wet, surely because of climate change…

To keep the blog going (a bit), I have dug out some images from a few dives last year featuring Inachus spider crabs. There are three species, I. phalangium, I. dorsettensis and I. leptochirus, which can be told apart by the arrangement of tubercules on their carapace, but these are often obscured by epiphyte growth, and so I am keeping it to Inachus sp. All species are associated with Snakelocks Anemone (Anemonia viridis) hosts. The photo above is taken using a weird ‘wide angle macro’ wetlens (INON UFL-M150 ZM80). This perspective always fascinated me but it is difficult to achieve. This lens does the job, although it is incredibly ‘soft’, especially around the edges. The close focusing also makes it difficult to direct the strobe light on the subject. So there is a tradeoff between getting the surroundings in view and sharpness. Below first some sharper shots using my 60mm macrolens, followed by some macro wide angle shots (they happen to be each of the different Anemonia colour morph):

I will leave you with the following interesting bit on the biology of these crabs by Diesel (Ethology, 1986):

I. phalangium females are site-constant, and live in the protection of one anemone or group. Males travel frequently between anemones harbouring females due to spawn; they copulate and guard the females until spawning, after which the male leaves again. A male operates in a patrol area containing 3-8 anemone groups and up to 8 females, visiting each female in turn repeatedly just before it is due to spawn. Patrol areas of different males may overlap, with resulting competition to fertilize a female’s next brood. Large males have higher reproductive success than small ones. Females live up to 8 months after the moult of puberty and hatch up to six broods, and males live up to 7 months as adults. A male could fertilize a calculated 26,000 eggs, whilst a female’s reproductive potential is ca. 4,200 eggs. Mortality risks are higher for males than for females, probably because of increased predation while leaving the protection of anemones in order to visit females. Males learn the positions of anemones harbouring females in their patrol areas, and when these are due to spawn. This allows a male to travel with a target and arrive punctually to fertilize the next brood due in his circuit. I. phalangium is the first marine invertebrate reported to use a “schedule” of localities and times for visiting prespawning females. In this way males minimize searching time and mortality risk, and maximize the number of broods fertilized.