Damselfish: Danger (and Sex) on the Reef

I was up before sunrise to go diving in Parque Nacional de Bahía Loreto (link). Driving south from Nopolo on Baja’s Transpeninsular Highway, I stopped at the Mirador (viewpoint) above the village of El Juncalito to check sea conditions. The winds were light from the north and the Sea of Cortés was calm. I turned off the highway where it turns inland and begins to climb. The dirt road to Ensenada Blanca drops down into an arroyo that drains a large canyon stirring up a cloud of dust. I wondered what the arroyo was like during a flash flood. 

Sunrise over Isla del Carmen from the Mirador above El Juncalito

In the village, chickens were foraging in the road and skinny dogs were sleeping in the dirt beside it. A sign outside a casita advertised “Pulpo Se Vende” (octopus for sale). Children in white shirts and navy blue pants and skirts were walking to school and women were hanging up laundry. A school bus was stopped in front of the only tienda in the village blocking traffic in both directions. Nobody honked.

Ensenada Blanca

I passed the fishermen’s camp littered with pangas in various states of repair and broken-down vehicles on cinder blocks. High tide the night before flooded the road, which was now muddy in places. Several fishermen were launching their pangas across the hard-sand beach. I drove to the edge of the launch area, took my kayak off the truck and loaded my dive gear.  

Fishermen's camp at Ensenada Blanca

Launching a panga across the beach at Ensenada Blanca

l paddled across Bahía Danzante to Islote Pardo, a small, uninhabited rock more than a mile offshore. As I passed beyond the protection of the peninsula, I could see and feel a low, long-period swell rolling up the Sea of Cortés, a remnant of a tropical storm to the south. Nearing the islote, I crossed what looked like a river with small standing waves. When a south-flowing current hits the shallow shelf near the islote, the water speeds up and rises to the surface creating turbulence. I was surprised at the speed of the current as it carried me south around the end of the islote. There it joined with the surge from the long-period swell and the swell's reflection off the islote. It was like paddling in a washing-machine; I wasn't diving there.

Islote Pardo

I paddled outside the islote to the north end and checked the visibility and currents. I was looking for damselfish (Pomacentridae) feeding near the surface. I try to dive where the damselfish congregate because that’s where the action will be on the reef – wandering and resident predatory fishes will cruise by looking for an easy meal; schools of smaller, nocturnal-feeding fishes will seek shelter in the shallow eddy created by the current; and solitary fishes like eagle rays and mantas will pass by on their wide-ranging travels.

Panamic sergeant majors feeding at the surface (taken from a kayak)

Panamic sergeant majors feeding at the surface (taken from a kayak)

I found an aggregation of damselfish (Panamic sergeant majors) feeding in the current, which was wrapping around the point creating a large eddy up against the west side of the islote. I've anchored in this eddy before. There’s a large sandy area where the current slacks and dumps its sediment load, a perfect place for my small Danforth anchor. From here, I can swim to the point where the damselfish congregate and, when I get tired from diving in the current, I can drop back into the eddy to rest.

My dive kayak anchored near Islote Pardo

I've spent hundreds of hours floating on the ocean watching damselfishes feeding on rocky reefs in Mexico and kelp beds in southern California. You can tell the direction and strength of the currents by watching them from a boat. To get the choicest plankters, damselfish move up into the water column facing into the current; those at the front of the aggregation see more food than those behind them, so fish at the back move towards the front. Leap-frogging like this moves the aggregation farther from the reef making them more vulnerable predators in the water and in the air.

Panamic sergeant majors feeding aggregation near the surface

Panamic sergeant majors feeding aggregation near the surface

The benefits to small fishes from feeding in large aggregations are twofold: the more fish in the aggregation, the more eyes and (figuratively) ears there are to detect approaching predators, and the lower the probability that any one individual will be consumed. The behavior of the aggregation telegraphs the level of threat they perceive; when it’s loose and fish are scattered throughout the water column, the perceived threat is low. When the aggregation compresses (inter-individual distances decrease), becomes polarized and retreats towards the reef, the perceived threat is high.

Spearfishermen this behavior to hunt pelagic fishes, hence the hundreds of hours watching damselfishes. Where you find thousands of damselfishes feeding in a current, you'll eventually see large fishes looking for a meal. In the Sea of Cortés, I've seen pelagic predators, like dorado, roosterfish, wahoo and Pacific crevalle jacks, bigeye trevally and reef-based predators, like cabrilla, dog snapper and pargo, stalk aggregations of Panamic sergeant majors, the commonest damselfish.

Jurel Voraz (bigeye trevally)

Cabrilla, which shelter in caves in the reefs, slowly patrol the lower edge of damselfish aggregations singly or in small groups. When the cabrilla are facing them, damselfish move towards the reef; eye contact is important. The cabrilla are looking for unwary individuals and make sudden charges into the aggregation to pick them off.

Cabrilla (leopard grouper) on the edge of a damselfish aggregation

A pair of cabrillas (leopard grouper) cruising through a damselfish aggregation

Last year I was spearfishing at El Califin, a small reef west of La Paz, following a large aggregation of Panamic sergeant majors feeding near the surface. At seemingly random intervals, the damselfishes swam rapidly in unison towards the bottom, but not back to the reef. I thought they were reacting to large needlefish cruising near the surface. Then I realized that magnificent frigatebirds, which roost on the large cardóns near the point, were flying low overhead. The damselfish were reacting to the predation threat from the frigates. 

El Califin west of La Paz

Magnificent frigatebirds roosting on cardóns at El Califin

I saw this behavior every time I dove at El Califin. I also saw wounded damselfishes in the feeding aggregation – mostly recent punctures of the skin in the middle of the body above the lateral line, consistent with being grabbed in a bird's talons from above, and less frequently diagonal slashes near its tail, which could have been created by a bird’s bill.

I haven't seen birds attack damselfish while I was in the water, but I've witnessed attacks from my kayak. I've seen ospreys, magnificent frigatebirds, blue-footed and brown boobies, and brown pelicans take fish in shallow water around rocky reefs. Ospreys crash into the water feet first to grab a fish in their talons; frigates pick fish off the surface with their bills while on the wing; boobies and pelicans dive head first into schools of small fishes.

Osprey carrying a fish that it captured on a reef near Nopolo

Osprey eating a fish on a cardón

I've seen wounded damselfish on other reefs in the Sea of Cortés. The majority of the injuries have been puncture wounds or tears above the lateral line between the middle of the body and the tail. I estimate that one out of 1,000-2,000 fish in some aggregations have recent wounds. I've seen as many as four recently-wounded damselfish in one aggregation, which was directly below an osprey’s nest on an islote near El Juncalito where the parents were feeding several chicks.

Osprey nest with young (right peak) and adult perched on a cardón (top middle) on an islote near El Juncalito

I believe that the puncture wounds are made by ospreys, which are common in southern Baja and often nest on cliffs above rocky reefs. Ospreys probably approach from behind; when the damselfishes see a bird, they dash downward. This is communicated instantly among hundreds of fishes in the aggregation and they dive in unison. The wounded fish probably escaped from the talons of an osprey that didn't have a good hold. Those that survive the wound, carry the scars of the encounter. 

Panamic sergeant majors with recent wounds

Panamic sergeant majors with healed wounds

Panamic sergeant majors are one of the most abundant fishes on rocky reefs in the Sea of Cortés, which means that they are reproducing quite successfully. Spawning in Baja California Sur occurs in the summer and I've observed their reproductive behaviors at close range. The male chooses a nest site among rocks and cobbles, which he spruces up for the female and defends once she has laid her eggs. 

Male (l) and female (r) Panamic sergeant majors spawning in a nest (the eggs are red)

The nest site can be a cup-shaped depression, a vertical section of a large rock, a space among cobbles or a small cave under a boulder. It’s only a couple of body lengths across, which makes it easier to defend. The male cleans the surface of algae and invertebrates down to bare rock where the female lays her adhesive eggs. Males fight among themselves for nest sites (territoriality), but will nest close to other males. Colonial nesting increases their defenses against egg predators and attracts more females than solitary individuals (link). Among damselfish, size matters. The largest males (5-7 inches long) control all of the nest sites in the colonies. Large individuals tend to be more solitary and stay closer to the reef; I rarely saw them in the feeding aggregations in the water column.

Two males (dark blue) and females spawning in nesting colony while other damselfish swim by

The male turns dark blue and advertises his prowess by repeatedly swimming up to the largest females passing by and back to his nest site. One area at Punta Coyote near Puerto Escondido approximately 15 ft by 8 ft had about 20 nest sites in cup-shaped depressions on an open, rocky shelf, suggesting that it was a high-quality, colonial nest site. Hundreds of females and smaller males and juveniles were swimming over and around the area. 

Part of a colonial nesting area at Punta Coyote (at least 11 males occupy nests in the picture)

A female selects a male and follows him into his nest site. If acceptable, she deposits her eggs in a single layer with the male following beside or behind releasing sperm. She makes many passes pressing her abdomen against the cleared surfaces in the nest, including the roof of small caves where she releases her eggs upside down. Egg laying lasts many minutes and the male will occasionally dart out to chase a rival or an egg predator. Both the male and the female have genital papillae, small conical tubes behind the anus that allows the female to direct eggs to the rock surface, and the male to direct sperm to the eggs, with accuracy. The eggs are less than 1 mm diameter and are attached to the rock by a multi-strand egg stalk (link).

Genital papillae on male (l) and female (r) Panamic sergeant majors

Once the eggs are laid and fertilized, the female leaves and it’s the male’s job to tend the nest. He'll fan the eggs or brush them lightly with his fins so they don't become covered with sediments and smother. He'll pick off and eat egg-eating invertebrates that wander into his nest. And he'll chase away fishes that would consume the fat-rich eggs. It’s an exhausting 24/7 responsibility, and the males don't stray far from the nest, even to feed. Males may consume eggs incidental to their nest cleaning activities, or purposely when they're hungry (partial clutch cannibalism) (link).

Damselfish eggs [color is a function of diet (e.g., carotenoid pigments) and stage of development]

The following videos follow a pair of Panamic sergeant majors in one nest over a span of 40 minutes. Spawning behavior is almost continuous at the beginning and slows near the end. The male in this video is not blue, but striped; you can tell the male by his larger size and aggressive behavior.

Do males spawn with more than one female? I inspected several nests and it appeared that the first female covered most or all of the cleaned surface with a single layer of eggs. A second female could deposit her eggs on top of the eggs already in the nest smothering them and wasting the male’s efforts with the first female. However, if the male has a large nest site, he could attract a second female to lay her eggs and increase his chances of reproductive success. Multiple females spawning in one nest has been suggested for other damselfishes (link). 

Do females lay eggs in more than one nest? Females probably have thousands of eggs in one clutch and they probably lay an entire clutch in one nest. If they can produce additional clutches during the spawning season, which is several months long in the Sea of Cortés, they could mate with multiple males. Like some other damselfishes, female sergeant majors may prefer to lay eggs in an existing nest, which would in effect reduce egg predation (link). Once the eggs hatch, the larvae spend 3-4 weeks in the plankton before they return to the reef (link). 

The more time I spend floating with damselfishes, the more interesting things I see. For example, males guarding egg nests distinguish among fishes that would eat their eggs (e.g., other damselfish species, triggerfishes, surgeonfishes and parrotfishes), which they chase with vigor, and fishes that are not egg-eaters (e.g., young groupers, snappers and grunts), which they let pass by unmolested.

A lot of what we know about the behavior of marine fishes, especially commercially unimportant species, comes from capturing lots of individuals and statistically summarizing what was measured. Fish behavior is determined by interactions between a fish's genes and its environment, which happens at the level of the individual is lost in statistical summaries. Nothing replaces going out into the ocean and taking a look.