Dive Dry with Dr. Bill

#549: Munching & Mating... But at the Same Time?

Every so often I have to remind my readers that there are three fundamental activities undertaken by almost all species. One is respiration, aka breathing. However producing a video or writing a column about that could be pretty boring. The second is munching... all critters need nourishment to continue their existence so they can partake in the third function, mating. Without that, or more accurately reproduction (since many critters don't actually "mate" as we know it), the species itself cannot persist and extinction results.

Like previous summers, I've spent a lot of time underwater at night to film the M & M activities in our dive park. Tuesday night I descended with hopes that the surge, which has affected my ability to film, might have diminished a bit. Indeed after three weeks it had. I knew this would be an interesting dive because the first unusual thing I observed was a barracuda about 10 feet away from me looking for a tasty morsel. I think that was the first time I'd seen one on a night dive in our waters.

The usual munching suspects were out in force... morays and kelp bass attacking the sheltering blacksmith, black sea urchins gnawing on kelp holdfasts, lobster scavenging out and about. Plenty of activity to film. I was amused when one moray chased after a blacksmith through a tight crevice... and got stuck! The morays head popped out upside-down at the other end and I filmed as it twisted and squirmed to free itself. Maybe it had already tasted a few too many blacksmith if you know what I mean. However there weren't as many spawners out there... I only saw a few worms up in the water column.

I tried to avoid a group of divers who descended after me. They were "horizontally challenged," moving through the water vertically with their fins kicking up sediment everywhere they went. Clouds of silt and sand are not conducive to good filming. Once they surfaced things started to settle down a bit.

I was on my way toward the stairs near the end of my 72 minute dive when I spotted a southern kelp crab munching away on algae. With its left claw it pulled the seaweed loose and brought it to its mouth. However, its right claw remained fixed and in the dark. At first I thought it might have lost the claw in battle or to a hungry predator. Then I realized why it was not being used to feed... it had a firm grip on the crab's latest love interest (maybe they met on the dating site Plenty of Fish)!

It is not uncommon to find the males of some species to be much smaller than the females. After all, masses of eggs take up much more space than male gametes. In this case it was the male that was MUCH larger than the female (verified by the shape of the telson or "tail" on each crab). Now I like younger women too... after all, any cougars out there interested in this old geezer would probably be in nursing homes by now. But I've observed several male southern kelp crabs dragging relatively tiny females around with them like cavemen.

I researched the southern kelp crab (Taliepus nuttallii) in my copy of Intertidal Invertebrates of California and discovered (or probably "rediscovered" given my creeping senility) that the males of this species are generally about twice the size of the females. This one was at least four times the size of his beloved.

After reading this column, one of my Harvard roommates (Skip Pugh) reminded me of the probable reason these male crabs hold onto their ladies. If they have already mated with them, dragging them along for a few hours may give his gametes the time they need to fertilize her eggs. By preventing other males from mating with her, he increases the chances that his genes will be passed on to the next generation. There are even a number of invertebrates which will scrape sperm from previous matings out of the female's receptacle to ensure his has a better chance of success.

This species is known from Santa Barbara south to Magdalena Bay in Baja. It has a close relative, Taliepus marginata found in northern Chile and Peru. This distribution suggests an interesting possibility. During the Ice Ages when cooler waters from the northern and southern hemispheres compressed the tropical belt near the equator, there may have been a single species found on both sides of those warmer waters. Once the glaciers receded and the tropical and subtropical belt widened, they may have been left isolated from one another.

When I'm diving during the day, I'm quite accustomed to snorkelers and new divers who do not understand proper etiquette in terms of exiting on the stairs. They all-too-frequently cut in front of me forcing me to abort my landing. A few well-placed cuss words are occasionally my response. However, I wasn't prepared for the critter that cut me off as I approached the stairs at the end of this dive... it was a round stingray that cut in front of me and then swam around on the lower stairs before heading back out to see. I thought it wouldn't be prudent to read it the riot act!

© 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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Three stills of the hungry and amorous male with his lady and the round stingray that butt in line to exit the stairs.

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