Dive Dry with Dr. Bill

#557: They Get So Crabby at Night

I love a good mystery, and that is one of the reasons I love to dive and try to make sense out of what I see and film underwater. I also read voraciously when I'm topside, and almost all the books are detective stories or CIA thrillers. Of course I also like to solve both those mysteries... whether it be researching scientific publications to help explain what I see beneath the sea or figuring out the end of the book (which I too often do before I reach the last page).

A while back during the Blue Moon I was on yet another night dive, all alone in the park exploring the rocky reef's nooks and crannies. About half way into my 80 minute dive the current kicked up and I had to fight the rest of the dive as the kelp submerged into horizontal position making forward progress difficult and restricting what I could see or film. Before that I had been thrilled with all the spawning worms in the water... and especially the fact that the kelp bass were going after them when the wiggly critters were highlighted in my video lights. I guess they needed a change of diet from blacksmith and kelp surfperch.

The following night I again felt the "urge to submerge" in the dark, but entered the water a bit earlier than usual hoping to avoid the strong current that developed the night before when high tide started ebbing. I succeeded, but unfortunately a fair bit of surge had kicked up and there wasn't much to see. The silver lining in that is there wasn't much video to edit late at night after I surfaced so I had time to read before falling asleep.

However, the dive was not entirely without benefit. I went to one of my usual moray holes and voila... there were two crabs at the opening and no moray. One of the crabs was a yellowish-brown with black claws and I'd seen them occasionally on other night dives (and hidden deep in the reef on a few day dives). Previously I had thought this was the yellow crab (Metacarcinus anthonyi. formerly Cancer anthyoni). There was another similar looking crab with black claws, but it was purple in color. Ah, a new mystery to solve! Hurray!!

Looking through my field guides, the only crab that seemed to fit was the black-clawed crab Lophopanopeus bellus. However the field guide listed its southernmost distribution as Pt. Sur in north central California. I posted two still images of it on Facebook and a Canadian friend of mine, Liz Bryk, said she thought it was indeed that species as she and her boys saw it up in Brtitish Columbia frequently. I did some further research and discovered there were actually two subspecies of this crab. The southern one, Lophopanopeus bellus diegensis, is known from Monterey Bay to San Diego! Liz also said that although they are a brownish-yellow color when immature, when they reach the age of consent they turn purple! They are in the crab family Xanthidae, commonly referred to as pebble crabs. Another mystery solved!

Most landlubbers on the island only see our striped shore crab (Pachygrapsus crassipes) skittering along the rocks above the tide line. Divers can add the much larger sheep crab (Loxorhynchus grandis) which stays below the waves. However, most crabs in our area are smart and hide during the day to avoid predators like morays and octopus. At night they may venture out of their shelters to feed on such tasty treats as seaweed or rotting fish carcasses. Yummy. Recently I've seen several other species at night like the southern kelp crab (Taliepus nuttallii), the tiny and cryptic Hemphill's kelp crab (Podochela hemphilli) and now the black clawed crab.

The black clawed crab has a trapezoidal to oval carapace that is wider towards the head region. Apparently they may range in color from white to brown to gray to reddish or purple. I found nothing in my research that suggested a color change at sexual maturity, so these may just be color variants, possibly dependent on what their food sources have been. The claws are black as the name suggests, but there may be white on the tip as there was in the purple individual I filmed. The northern subspecies may be intertidal, but the ones in our region are subtidal down to about 250 feet. They hide under rocks, especially on gravel bottoms, but I occasionally see them deep in the reef.

Like me, these crabs are omnivores and will eat seaweed (algae), small molluscs such as clams, and barnacles. Well, I guess they aren't exactly like me... I avoid barnacles and only occasionally eat seaweed when I'm at a sushi restaurant. When faced with a predator, or a nosy marine biologist, they like to play dead. The one in the upper right is doing just that as I film.

Modified appendages on the abdomen of the male transfer sperm to the female. Apparently the males don't believe in courtship or "pre-nuptial" activity... they go right to it with force. In fact, if these crabs were human, the males should be arrested! At least in the southern kelp crabs there is some courtship behavior prior to actual mating... if you call grabbing your woman with your claws, and dragging her all along the reef while you feed... foreplay. Mating may last as long as several hours.

Females may mature at a very small size as some a mere 1/3" across have been observed carrying eggs. The number of eggs is dependant on the size of the female, and ranges from 1,000 to 6,000 based on one scientific study. The eggs are carried under the broad telson or tail-like structure. It is reported that many females may carry two broods during the year with the breeding season lasting perhaps from February through October. Minbe lasts year-round... if I get lucky. Eggs are carried for 25-30 days and generally hatch at night into an early pelagic larval form.

The larvae undergo several different stages as they drift with the plankton for about five weeks. Feeding during these stages may focus on microscopic plant plankton (phytoplankton) and later on small zooplankton and the larvae of other species. Anything smaller than it might be fair game... but anything larger than it might constitute a predator! Larval mortality is very high as is the case with most species that do not brood or rear their young, but cast them adrift to fend for themselves. I'm glad Mom and Dad took good care of me when I was a young whippersnapper (now I'm just an old one).

© 2013 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Images of the purple colored black-clawed crab.

This document maintained by Dr. Bill Bushing.
Material and images © 2013 Star Thrower Educational Multimedia