Dive Dry with Dr. Bill

#191: Something In The Way She Moved

I seem to have a sixth sense when it comes to locating marine life to film underwater. Actually, it may only be a fifth sense... divers often lose some of their hearing after decades in the briny deep. Whatever sense it is, I have an uncanny history of looking in the right direction at the right time (at least underwater). It's as if someone is guiding me in my attempts to capture interesting footage to share with the readers of this column and the viewers of my cable TV show.

I was diving Eagle Reef near the Isthmus, filming a swell shark egg case (mermaid's purse) at about 70 ft., when that "something " compelled me to turn around. As I turned, I had my camera ready to film whatever it was... a beautiful mermaid or (gulp) the "landlord" closing in. Unfortunately it wasn't the first, and fortunately it wasn't the second. I raised my camera housing to my eye and began filming something I've only seen briefly once before, and never filmed... a pair of bat rays in conjugal bliss!

By instinct my camera was rolling and my eye at the viewfinder to follow the action. My first thought was... how strange, a bat ray with two sets of "wings" (pectoral fins). Perhaps I'd discovered a new species, the bi-plane bat ray. As they passed in front of me, I soon saw the reason for my "double vision" (another sense I'm losing?). The couple was in flight, or rather swimming along the edge of the rocky reef. The larger female was on top, while the smaller male swam just beneath her. Since bat rays are in the eagle ray family, Eagle Reef was a very appropriate place for this observation!

Like many of the subjects and behaviors I film, I couldn't really focus on the activity. My mind has to be centered on ensuring that the subjects are well framed and that I follow them with the camera. My tiny 1/2" viewfinder doesn't reveal much else. It isn't until the evening, when I capture the footage to my computer and edit it into something I can share with you, that I really observe what I film. I don't trust my brain much since it is an older model, manufactured at a time when RAM (memory) was very expensive and therefore limited. By committing what I "see" underwater to my computer, I benefit from its much greater capacity and speed.

Looking at the 25-second video sequence of this event, it appeared that the mating had just been initiated. The male did not appear to have completed the coupling. My knowledge of the actual mating behavior of bat rays is limited, so I thought this might not be necessary for fertilization. It was incredible to have the opportunity to play this footage back several times to analyze exactly what I had filmed.

For some strange reason I've never seen really small bat rays. My diver friends on the mainland have reported them along the southern California coast. A year or two back, a diver reported seeing a female bat ray give birth right in the Casino Point dive park, so they must be present here. I decided to do a little on-line research using my new laptop computer with its wireless connection, purchased with a gift many years ago from my Mom & Dad. I saved it for a special need and now I can return to the beach with a laptop capable of editing video, just as I did while writing my books more than 20 years ago with the first laptop ever to hit Catalina, a CP/M-based Epson Geneva.

Bat rays mate in the spring and summer so mine were right on schedule. Females are sexually mature at about five years (about 50 pounds), while the males mature earlier at 2-3 years (about 10 pounds). The male carefully selects his mate by swimming behind her to sense chemicals she releases indicating her reproductive status. I guess it's a little like the human female's use of perfume, although the bat rays use natural sex pheromones. Bat rays often congregate during mating season. I have read that already mated or immature females may be "buried" under the sand by unmated ones, or have the other females rest on top of them to prevent the males from selecting them. Now girls!

Males use their "claspers" to transfer sperm to the female's cloaca as they swim. The claspers are normally held out in a trailing position, but rotate upwards during mating. The eggs are fertilized internally and develop within the female. Gestation may take 8-12 months. When ready to deliver, the females enter shallower bays and estuaries where predators are fewer in number. A litter of up to 10 pups are born live, with their 9-12" wide wing-like pectoral fins wrapped around their bodies. The stinging spine is flexible and wrapped in a sheath to protect the mother during delivery, but the sheath falls off soon after birth.

So now you know more than you ever thought you would (or wanted to) about bat ray reproduction. Keep in mind how important this process is. "Munching" allows an individual critter to grow and survive. If it doesn't, it can't reach the next level. "Mating" allows the population and species to grow or maintain itself, and survive. With this column finished, it's time for me to head to Vons for my morning pan dulce. I guess my mind is more on "munching" this morning!

© 2006 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles" or "Gentle Giants: Giant Sea Bass" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Still frames from the spectacular footage of two bat rays mating off Eagle Reef.
The female is on top and the male swims just below her.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia