Critters of the Night

By Katie Gerstle |

Alas, it is that time of year. The days get shorter, it rains, we start to lose our summer bodies in anticipation of fall hibernation, it keeps raining… But the fall is definitely not all doom and gloom, especially if you’re a diver.

As cold water enthusiasts who live adjacent to one of the most biodiverse marine ecosystems on the planet, we are privy to witnessing the intricacies of some of the most amazing species. That is, if we are bold enough to venture into the emerald waters of the Pacific Northwest. However, the colder it is, the more we can see through our otherwise murky oceans. The longer that it is darker, the more species and their unique behaviours we are fortunate enough to experience.

Why is night time important in the oceans?

Red rock crabs (Cancer productus) are strong, aggressive, opportunistic omnivores. At night, they migrate into the intertidal zone to forage. Red rock crab larvae typically hatch between march and April, and undergo six different stages of development as they float in the water column before settling down. Larvae dispersal is a crucial aspect of fostering robust, healthy seafloor population dynamics for this species, and for the many others that rely on red rock crabs for their survival.

As a species of decapod crabs, red rock crab planktonic larvae, known as megalopae, float in the water column and feed on phytoplankton and small zooplankton, but are also prey for larger types of zooplankton. Most larvae undergo daily vertical migrations within the water column, ascending at dusk and descending at dawn, in an effort to avoid predation and increase chances of survival. These migrations are theorized to strongly influence dispersion success, or how far the larvae spread through the oceans.

Crabs are cool, but who are likely to be the “big ticket” sightings on a night dive?

Sharks, like the spiny dogfish and the Pacific spiny dogfish (Squalus acanthias and S. suckleyialso fine-tune their movements based on daylight. One study suggested that the Pacific spiny dogfish performs the same type of vertical migrations in the water column as red rock crab larvae, resting closer to the surface at night and then retreating deeper during the day. In another study, 27 bluntnose sixgill sharks (Hexanchus griseus) of both sexes and all sizes showed clear and consistent patterns of year-round movement in Puget Sound, WA. One theory regarding the differences between night and daytime behaviours is that each core process for survival comes at the cost of another. These core processes include (1) opportunistic foraging, (2) predator and/or competition avoidance, and (3) conserving energy efficiency. Sixgill sharks spent most of the day conserving energy in deeper waters before migrating to shallower waters at sunset, where their activity levels appear heightened. Then, at dawn, they return to the depths. 

Their daily movements are hypothesized to be a product of foraging behaviours as they follow their prey up and down through the water column. The researchers noticed that other species of sharks in the same vicinity moved synchronously through the water column in reaction to the same patterns of mass vertical migrations. It appears that the diel vertical migration patterns of the smallest individuals, like phytoplankton and zooplankon, govern a wide range of consumers and predators in the coastal food web here in the Pacific Northwest.

Like ringing a dinner bell, the lack of light triggers a smorgasbord of predators and prey converging in shallower waters. We get that nighttime is an important aspect of the marine food web, but why does this kind of research matter?

As we learn more about where species are likely to be found in the ocean, it improves our ability to fine tune our movements as well. For example, if we know where a pod of orca whales (Orcinus orca) are likely to be found not only on a 2-dimensional map, but in a 3-dimensional model accounting for the depth of the water column, we can improve our conservation practices. These practices could include anything from fishing regulations to local boat traffic.

What role does the average, lowly scuba diver play in all of this?

The oceans are vast and deep, and we know more about the surface of the moon than we do about the depths of our own planet. In other words, we need all of the help we can get. This is where having your PADI Night Diver Certification comes in handy.

Many marine species that are low in population and migration data are considered “Data Deficient” on the International Union for Conservation of Natural and Natural Resources (IUCN)’s Red List. Basically, it means that there is so little research on that species that we have no idea if we (the ever-destructive human race) are helping, hindering, or flat-out demolishing their populations. If you happen to see, or better yet photograph, any unique species or peculiar behaviours, those observations can be submitted to bolster our baseline information about that species, and in some cases, about an individual animal. Sixgill sharks, orcas, humpback whales, dolphins, and sea lions are just some of the many species where markings can identify individuals within a population, and assist in understanding population assessments and migration patterns.

Report your shark sightings, whale sightings, or call 604-258-SEAL(7325) to alert the Marine Mammal Rescue Centre of an animal in distress.


Andrews, K. S., Williams, G. D., Farrer, D., Tolimieri, N., Harvey, C. J., Bargmann, G., & Levin, P. S. (2009). Diel activity patterns of sixgill sharks, Hexanchus griseus: the ups and downs of an apex predator. Animal Behaviour, 78(2), 525-536.

Beerman, A., Ashe, E., Preedy, K., & Williams, R. (2016). Sexual segregation when foraging in an extremely social killer whale population. Behavioral Ecology and Sociobiology, 70(1), 189-198.

Griffing, D., Larson, S., Hollander, J., Carpenter, T., Christiansen, J., & Doss, C. (2014). Observations on abundance of bluntnose sixgill sharks, Hexanchus griseus, in an urban waterway in Puget Sound, 2003-2005. PloS One, 9(1), e87081.

Jessen, S., & Patton, S. (2008). Protecting marine biodiversity in Canada: Adaptation options in the face of climate change. Biodiversity, 9(3-4), 47-58.

Orlov, A. M., Savinykh, V. F., Kulish, E. F., & Pelenev, D. V. (2011). New data on the distribution and size composition of the North Pacific spiny dogfish Squalus suckleyi (Girard, 1854). Scientia Marina, 76(1), 111-122.

Park, W., & Shirley, T. C. (2005). Diel vertical migration and seasonal timing of the larvae of three sympatric cancrid crabs, Cancer spp., in southeastern Alaska. Estuaries, 28(2), 266-273.

Smith, T. E., Ydenberg, R. C., & Elner, R. W. (1999). Foraging behaviour of an excavating predator, the red rock crab (Cancer productus Randall) on soft-shell clam (Mya arenaria L.). Journal of Experimental Marine Biology and Ecology, 238(2), 185-197.

Sorochan, K. A., & Quijón, P. A. (2014). Horizontal distributions of Dungeness crab (Cancer magister) and red rock crab (Cancer productus) larvae in the Strait of Georgia, British Columbia. ICES Journal of Marine Science: Journal du Conseil, fsu065.

Williams, R., Lusseau, D., & Hammond, P. S. (2009). The role of social aggregations and protected areas in killer whale conservation: The mixed blessing of critical habitat. Biological Conservation, 142(4), 709-719.

Images, from top to bottom: (1) Whytecliff by night, from:, (2) Red rock crab, from:, (3) Spiny dogfish, from:, (4) Sixgill shark and diver, from:, (5)