Marine Synanthropes


Urbanization has winners and losers. Synanthropes are organisms that live near and benefit from humans and from the artificial landscapes and seascapes that humans create.


Meet the marine synanthropes:

 Close-up of a giant Pacific octopus Urban octopus
Filamentous seaweeds Algal turf takeover
California Sea Lion Sea lion: City slicker


Urban octopus

Giant Pacific octopus in den
Photo by Ed Gullekson

Key Points:

  • Giant Pacific octopus are the largest known octopus species in the world.
  • A recent study suggests that deep-dwelling giant Pacific octopus are positively correlated with urbanization intensity.
  • This may be due to the large amount of junk that ends up on the seafloor in urban areas.

In 1955, Columbia Pictures released the thriller It Came From Beneath The Sea, in which an enormous octopus begins terrorizing the good citizens of San Francisco. Though lacking in certain factual details (for instance, the octopus possesses only six legs in some shots) and decidedly biased against the octopus, the film alludes to one phenomenon that is not entirely fictitious.

It turns out there are giant octopodes lurking in the depths of urban waterfronts in coastal cities on the West Coast of North America. They are giant Pacific octopus (Enteroctopus dofleini), the largest known octopus species in the world. Reaching over 20 feet in length from one tentacle tip to the other, they are veracious predators of crabs, clams, and other invertebrate prey.Close-up of a giant Pacific octopus

In a 2018 study in Urban Ecosystems, Eliza Heery and colleagues at the University of Washington, Seattle Aquarium, and NOAA set out to determine whether giant Pacific octopus distribution and habitat-use patterns were related to urbanization intensity, and if so how and why.

Because of the large size and expansive home range of giant Pacific octopus, characterizing their distribution patterns using traditional ecological survey techniques is not an easy feat. Instead, Heery et al. relied upon citizen-contributed data from the REEF program ( These data provided information about octopus presence/absence over a much larger spatial scale and a much longer time period than would otherwise have been available. Heery et al. used the REEF data in a series of statistical models, which you can read more about here, to ultimately conclude that urban effects varied with depth. In deep habitats (> 24 m), the probability of encountering octopus increased with adjacent land-based urbanization intensity.

Why might this be? They conducted additional field surveys to explore two potential explanations – giant Pacific octopus are considered to be a mesopredator, meaning they are medium sized predators that are at a middle point in the food chain, with some prey and some predators of their own. Past studies in terrestrial urban ecology have demonstrated that urbanization facilitates mesopredators such as raccoons, in part by providing food and shelter resources. To determine whether the increased occurrence of octopus at depth in urban areas was associated with food resources, Heery et al. collected middens – piles of shells leftover from past meals of octopus – from octopus dens throughout Puget Sound. These data indicated there were no differences in the diets of urban octopus and more rural octopus, suggesting that food resources are not the driver of urban-related distribution patterns. Secondly, Heery et al. conducted a series of video surveys in adjacent areas where there was a lot of versus very little anthropogenic debris (junk). Results from these surveys indicated that octopus are far more common where there is a lot of junk on the seafloor, suggesting that this may be one of the mechanisms driving urban-related distribution patterns in giant Pacific octopus.


Algal turf takeover

Filamentous seaweeds

Key Points:

  • Algal turfs are thick, carpet-like beds of seaweed that retain sediment and compete with foundation species like corals and kelps.
  • High nutrient and sediment loads in urban waters may be facilitating the expansion of algal turfs.

Are algal turfs taking over the marine environments as a result of widespread urbanization? That’s the worry of scientists who study marine macroalgae. Algal turfs are expansive beds of sediment-retaining seaweeds, and are common features of urban areas. They have long been black listed by conservationists and many ecologists because they compete directly with reef-building corals and perhaps also canopy-forming kelps, both taxonomic groups of considerable importance for marine biodiversity.

Though the term ‘algal turfs’ is used to refer to a variety of different types seaweeds, those in urban areas are typically formed by filamentous algae that are just a few centimeters tall and grow in dense, intertwined matrices or ‘thickets’. Urban algal turfs are thought to consist of opportunistic, pioneer species capable of colonizing novel habitats rapidly and tolerating of stressful environmental conditions that are inhospitable to other organisms. In particular, urban turfs appear to thrive on elevated nutrient levels (primarily nitrogen and phosphorous from wastewater, sewage, and runoff) and high sediment loads (from coastal construction, dredging, and other sources). Recent studies also suggest urban turfs may be further facilitated by increases in temperature and CO2 associated with climate change. Currently, researchers are working to understand how herbivores and competitors that limit algal turf may be used to control turf growth in urban areas.



Sea lions: City slickers

California Sea Lion

Key Points:

  • Urban sea lions utilize the extensive haul-out opportunities provided by buoys, floating docks, and other urban infrastructure.
  • Dense aggregations of sea lions in many coastal cities suggest they may be synanthropic, yet more research is needed to characterize the patterns and drivers of sea lion populations in urban areas.

sea lion pup in restaurant boothStrange encounters with urban sea lions are a familiar tale that regularly make local news headlines in many coastal cities. For instance, in San Diego, an eight-month old sea lion pup recently took up residence in an ocean-front restaurant and was quite obviously grumpy (pictured) when it came time for her deportation by a marine mammal rescue team (link). In the same area, another sea lion pup allegedly climbed 145 steps from the beach below to hang out in a gift shop and only left after being bribed with salmon (link). In yet another episode, a paddle boarder captured this video as a young sea lion who climbed aboard, bummed a ride, and perhaps overstayed her welcome:

Sea lions are smart and versatile. Though they fit many of the characteristics of synanthropes, and are clearly a common inhabitant of many coastal cities, particularly on the West Coast of North America, their population dynamics with respect to urbanization have yet to be thoroughly studied.

It’s possible sea lions capitalize on urban environments in a manner similar to terrestrial mesopredators (mid-sized consumers) like raccoons and coyotes, that are dietary generalists and behaviorally flexible. However, raccoons and coyotes particularly benefit from the cornucopia of food resources in terrestrial urban environments that’s provided by human trash and/or small domesticated pets. There’s no evidence thus far that urbanization enhances food resources for marine mesopredators. Alternatively, urban sea lion populations may benefit from fewer predators in urban areas – as raccoons and coyotes do – yet this has not been well studied. While some smaller sharks are known to inhabit urban estuaries, urbanization is thought to have negative impacts on many marine predators (link). Yet, it remains unclear whether urban-related patterns in apex consumers are strong enough to drive synanthropy among mesopredators like sea lions.

What is evident is that sea lions utilize the extensive haul-out opportunities provided by urban infrastructure. Sea lions are a common fixture on buoys and floating docks in many cities, often causing quite a headache for port authorities. For instance, in the late 1980s, sea lions took over a major dock at a popular tourist destination in San Francisco en masse and have maintained their territorial claim to the site ever since, effectively barring entry to humans and eliminating the dock as usable infrastructure for the port (though it came with other tourist benefits: link). Some cities attempt to prevent such take-overs by requiring sea lion deterrents at public harbors and waterfronts (link).