Light Attenuation


Without direct illumination from a strobe, underwater photos take on green or blue hues. These rocks are covered in bright red, orange and yellow organisms, but none of those colors appear unless we bring underwater flashlights.

Shallow coral reefs are bright and colorful.  The deep abyss is pitch black.  What about in between?

Light from the sun is composed of a spectrum of different wavelengths of energy, including visible light.  When white light passes into the ocean, these different wavelengths behave differently.  Some wavelengths are absorbed by the water very rapidly, and some are able to penetrate deep into the water.  Longer wavelenghts of light (red, orange, yellow) are absorbed more rapidly than shorter wavelengths.  If you descend in to clear open-ocean water everything looks blue:


But in coastal water with lots of nutrients, like along the West Coast of the US, there are tons of phytoplankton in the water.  These phytoplankton absorb blue and red light, and reflect green light (the same reason why grass is green).  This turns coastal waters green instead of blue.

The only way to combat this light attenuation is to bring our own light sources.  We carry flashlights and camera strobes to help illuminate all of the colors in the Salish Sea:


This photo was taken at about 20m depth. Without the camera strobe, all of these colors would appear as some shade of green.


Puget Sound King Crab


The largest crab we see in the San Juans.  Not edible like the Alaskan king crabs, or at least I’ve never heard of them being harvested and eaten.


Tim Dwyer, amazed at how large Puget Sound king crabs can get.  Despite the large, heavy claws, these crabs have never pinched us.  When we handle them they tend to tuck all their legs into the body.  This particular crab was scrabbling to find something to crawl away onto, not trying to menace the camera.


Closeup of the face.  The spiky bits are part of one of the two pairs of the antennae.  You can see the other pair of antennae just above the spiky bit on the left.  The eyes are dead center, on either side of the triangular rostrum (looks like a nose).  The blue rectangular pieces are the 3rd maxillipeds, which help chew up food.  There are five other pairs of mouthparts, including the 2nd and 1st maxillipeds, the 2nd and 1st maxillae, and the mandibles.  All are involved in chewing and shoving food back into the mouth.


Lopholithodes mandtii is an Anomuran crab, more closely related to the hermit crabs than to shore crabs or Dungeness and red rock crabs.  How to tell if a crab is an Anomuran or a Brachyuran (the group that includes typical crabs)?  Count the number of walking legs, including the pincers.  If it has 4 pairs it’s an Anomuran.  If it has 5 pairs, it’s a Brachyuran.  Another way to tell is to look at the abdomen (the shield on the ventral side of the crab).  If it’s asymmetrical it’s an Anomuran.


Color in adults tend to be various shades of orange or red, with blue highlights.


Juveniles are bright orange to red.


Despite the bright coloration shown in these photos, Puget Sound king crabs are almost perfectly camouflaged.  The bright oranges and red are nearly invisible at the depths these crabs live (we tend to see them at 20m or deeper).  Without the bright colors, they look like all the other rocks they crawl over.  They only pop out when we use flashlights.

Rockfish Recompression


Images of a bocaccio with severe barotrauma at the surface and in a basket being sent to the bottom for recompression.  Image from NOAA SWFSC

Here is a great PSA put out by NOAA Southwest Fisheries Science Center, all about the effects of barotrauma on rockfish and how to help recompress rockfish once you’ve caught them.  We use the inverted basket method to send our fish back down after we catch them for diet analysis.

The bulging eyes and stomach forced out through the mouth are the result of increased pressure on the internal organs from the swim bladder.

Boyle’s gas law says that as the pressure on a unit of gas decreases, the volume of that gas increases proportionally.  That is, when the pressure is cut in half, the volume doubles.

Rockfish live in deep water, typically 20m or deeper, and often several hundred meters deep.  Standing at sea level, you have all the weight of the gaseous atmosphere pressing down on you.  When you dive into the water, the weight of the water is added to that, at a rate of the equivalent of one atmosphere every 10m you descend.  The pressure at 20m is 3atm.  If a rockfish is brought from 20m to the surface, the pressure on it decreases to 1/3 the original pressure, which means the volume of its swim bladder increases 3-fold.

Unlike salmon and other surface-oriented fishes, the swim bladder of rockfish is not connected to the esophagus.  Salmon add gas to their swim bladder by swallowing gulps of air from the surface.  This limits them to staying near the surface.  Rockfish use a capillary system like that surrounding our lungs to deliver gas to the swim bladder.  The benefit of this system is that rockfish are not tied to the surface, and can colonize deeper habitats.  However, it also prevents them from being able to quickly vent gas as the surrounding pressure decreases.

Since rockfish can’t burp, when they are brought to the surface the swim bladder swells and causes the stomach and eyes to bulge out.

Year of the Serpent

Today marks the beginning of the Chinese New Year – the Year of the Snake.

A few snakey photos from my research were featured in a post about snakes and serpents on the UW Biology Department graduate students’ blog, Science Positive.

Serpula columbiana tube worms are sessile invertebrates that secrete a calcareous tube (Serpula means “serpent”).  There are also brittle star arms snaking out from the crack in the rock (brittle stars are in a group called the ophiuroids, which means “snake-like”).


The scientific name for lingcod is Ophiodon elongatus (Ophiodon means “snake-tooth”).