Dive Dry with Dr. Bill

#125: What a "Swell" Shark

Over the holidays I did a number of dives with my friend Xiaoyan from Tokyo. Although she usually dives tropical waters (her last trip was Guam and Yap), she had dived the northern Channel Islands a year ago. After that chilly experience, she came prepared with her dry suit to survive our cold winter waters! Much of what we saw was new to Xiaoyan and she enjoyed taking pictures of even our common fish. She also turned out to be a good spotter, sighting two important species I had yet to videotape.

Among the two was the subject of this week's column, the "swell" shark. I don't know how Xiaoyan spotted the head sticking out from under a rock covered with brown seaweed. I completely missed it despite being even closer. Perhaps her vision is better than that of my well-aged eyes (which have not matured like a fine wine)! I just knew I was grateful to get a chance to videotape another shark species up close and personal. Look for it (and the great whites) on my "Sharks and Rays" DVD to be released in early spring. Unlike the great white sharks I filmed recently, the swell shark didn't require a cage. In fact I could film it from just inches away without the least bit of fear. In fact, about the only "harm" these sharks cause is by entering lobster traps and eating the bait (or the lobster).

The swell shark is sometimes referred to as the balloon or puffer shark. My Spanish-speaking friends may know it as gato hinchado, pejegato hinchado or tiburón inflado. This shark's scientific name (ventriosum) means "large belly" in Latin. When these sharks are threatened, they will take water or air into their stomachs and swell up. This is an interesting defense strategy, prevents them from being dislodged from their hiding places in the rocks. Sometimes the shark will grasp its tail in its mouth, forming a U-shape, before inflating. For some reason the smaller individuals are not able to inflate themselves. Since these individuals are more likely to need protection from predators, this seems strange.

Swell sharks have a very flat head. Their bodies are quite distinctive making them easy to identify. The only shark in our waters resembling them in coloration is the leopard shark, but the dorsal fins are quite different. The background color is red- to yellow-brown with large dark blotches and small light colored spots. This coloration also serves as defensive camouflage. Individuals larger than 3 feet are uncommon. Although the swell shark's mouth is larger in proportion to its body than that of the great white, they have 50-60 very small teeth (whew).

These unusual solitary fish are found as far north as Monterey south to Chile, but are more abundant south of Pt. Conception. They frequent shallow water, especially among rocks and kelp beds, but may be seen as deep as 1,500 feet (although not by me, at least on SCUBA!). Swell sharks are most active at night when they feed, and may be seen in the open over sandy bottoms then. During the day they usually hide in the protection of caves and rock crevices as the one we observed did. I rarely see swell sharks in the Dive Park, probably because the constant presence of divers forces them to seek less frequented areas of our coast.

Under the cover of darkness these sharks venture out to feed on small fish. Their prey may be quickly sucked into the mouth, or the shark may remain still and slowly open their mouth as the fish approaches. Like other sharks and rays, swell sharks have the ability to sense the weak electrical fields associated with the prey's muscles and movement, and don't need to rely on vision to locate their food. They will also eat bottom-dwelling molluscs and crustaceans. In turn they are eaten by the large northern elephant seal, sea lions and larger sharks. Because their flesh is mildly poisonous, they are not eaten by humans (at least those with intelligence!). Certain boring snails may break into the egg case and eat the developing embryo.

Most divers in our dive park encounter these sharks in the form of their eggs, known as "mermaid's purses." This name comes from the legend that eggs cast on beaches were mermaid's purses. Unfortunately I've never encountered a mermaid underwater to confirm this legend (but I do keep looking)! The eggs (see photo) are soft and light colored when laid, but quickly harden and turn dark green to black in color. Dr. Milton Love states that eggs produced on the mainland have long tendrils at the ends, and those from Catalina do not. This may be due to genetic differences or possibly related to the calmer leeward waters compared to the exposed mainland coast. It would be interesting to see if there is a difference between eggs laid on our protected leeward coast and those laid on the exposed windward side. Females lay just two eggs in each batch. They take as long as 10 months to hatch, although development is faster in warmer water. The small (6") young use a series of short spines on their back to break through the egg casing.

© 2005 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.

Swell shark head sticking out of hiding place under rock overhang, and swell shark
egg case or "mermaid's purse" showing egg yolk inside and tendrils on end.

This document maintained by Dr. Bill Bushing.
Material © 2005 Star Thrower Educational Multimedia