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Dive Dry with Dr. Bill

#389: Munched Mola Molas

A few weekends ago I was down at the dive park and noticed two dead juvenile ocean sunfish (Mola mola) floating near the breakwater. I was not diving so I had no wetsuit, but a diver from the Bay Area named John kindly jumped in and snorkeled over to retrieve one of them for me. A few days earlier H.T., a SoCal diver, posted a picture of himself holding a dead juvenile in the waters off Lion's Head Point here on Catalina Island. Phil and Merry Garner reported six dead Molas from Ship Rock. All were all missing their dorsal, anal and pectoral fins.

This did not surprise me since California sea lions frequently attack the younger Molas and bite off these fins, rendering the fish totally disabled. The sea lion may feed on the carcass, but often abandons it. It then sinks to the bottom where scavengers like bat stars and lobster may feed on them. Yes, nature has a cruel side to it where some species will kill for what appears to be "pleasure" or "play." Most of us have seen cats play with a live mouse or lizard, some have seen killer whales toss juvenile sea lions about, and we all are aware that some humans will kill others of our own species for "sport."

My suspicion about sea lions being the guilty party in this case was substantiated when, just before I left the dive park, I saw one out near the boundary line tossing a third ocean sunfish about like a frisbee. Up in Monterey Bay this is common behavior during the fall when large numbers of young sunfish appear, and in the past week I've heard numerous reports of it along our own island coast. Some scientists believe the sea lions do this to tenderize the fish for consumption, much as we used to pound abalone steaks prior to frying them. The pinnipeds usually attack the softer belly region. Sea gulls and other marine birds will peck the eyes out.

Molas are a very unusual fish, and although as a scientist I'm not supposed to judge "natural" behavior I do find it sad to see them disabled and dying. I've written previously about ocean sunfish being cleaned by seagulls and halfmoon. My first sighting of what the Germans call the Schwimmender kopf ("swimming head" in English) was about 1970 when I was pulling a kelp raft in over the swim step of Toyon's dive boat, the K.V. Looking down in the water I saw what I thought was a large great white shark swimming up towards me... and jumped back. When it turned, I realized it was just a curious Mola mola. They frequently follow drifting kelp, where some scientists believe they get cleaned by fish hiding under the kelp raft or paddy. Molas have incredible parasite burdens that make tasty tidbits for these cleaner fish... and seagulls.

Primitive ocean sunfish first appeared in the world's oceans about 65 million years ago, shortly after the dinosaurs and many other critters went extinct, and just before I began diving. Based on DNA and embryological studies, scientists believe they evolved from tropical puffer fish who escaped the confines of the coral reefs and went pelagic (into open water). They are placed in the same fish order as the puffers, porcupine fish, boxfish and triggerfish. Sunfish are found in all tropical and temperate seas of the globe.

The Mola should be in the Guinness Book of World Records. They are the heaviest bony fish known to man. An individual 10 feet long and 14 feet from the tip of the dorsal fin to the tip of the anal fin weighed in at just under 5,000 pounds. That makes the more average adult sized fish of about 6 feet, like the one caught off our waters by angler Zane Grey, seem somewhat puny at a mere 2,200 pounds. Another record is that these fish are known to have the largest number of eggs of any vertebrate species. A small mature female a mere four feet long was estimated to contain about 300 million eggs in her ovaries.

How do these a fish reach such prodigious sizes? Why, just like Dr. Bill... exercising too little and eating too much! Molas are lazy swimmers, relying mostly on ocean currents to propel them along. The adolescents school which is why so many of them are appearing de-finned in our waters. Unlike the good doctor, the sunfish munch on gelatinous animal plankton including jellyfish, siphonophores (such as the Portuguese man-of-war), comb jellies and salps. Not exactly my preferred menu items, but I'd be willing to try jellyfish sushi if one of my Japanese friends introduced me to it. Food is taken in through the small mouth and ground up using hard, bony plates on the upper and lower jaw just like their puffer and porcupinefish relatives. An individual kept in the Monterey Bay Aquarium gained 800 pounds in just fourteen months. I have no intention of trying to match that!

Prior to researching this column, I was unaware that in addition to feeding in the near surface waters, some ocean sunfish also make deep dives to feed on squid, sponges, brittle stars, crustaceans and small fish near the bottom. Molas are called sunfish due to their habit of basking in the sun while lying horizontal at the surface. Some scientists believe that their sunning may be to warm up following the deep feeding dives. Others feel it is to allow them to see prey drifting beneath them, since their body shape and placement of the eyes makes it difficult for them to see to the front.

Sunfish in the San Pedro Channel are often mistaken for sharks due to their large, protruding dorsal fins when swimming near the surface. And now you know that both sunfish and sharks are finned... by different species but with very similar results. Like sharks, they are also decimated by fishing activities. It is estimated that up to 26% of the by-catch from drift nets off California is made up of ocean sunfish, and as much as 90% of the by-catch from the swordfish fishery in the Mediterranean. At least in Taiwan and Japan they are not considered by-catch since they are targeted and eaten there. Imagine the size of a steak from a ten footer!

© 2010 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 and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. 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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Pair of ocean sunfish swimming above Farnsworth Bank, H.T. with the finned Mola off Lion's Head Pt.
(image courtesy of H.T. and Gabe Lu); diver John retrieving dead juvenile from the dive park,
and close-up of finned juvenile.

This document maintained by Dr. Bill Bushing.
Material and images © 2010 Star Thrower Educational Multimedia