This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Editor's Note: Marine biologist William Gilly is on an expedition to study Humboldt squid on the University-National Oceanographic Laboratory System research vessel New Horizon in the Gulf of California. He and other scientists will spend the coming weeks learning about the giant squid, their biology and ecology on this National Science Foundation-funded expedition. This is his first blog post about the trip.
GULF OF CALIFORNIA—We are back in the Gulf of California…on a boat…searching for Humboldt squid. This may sound familiar if you read our blogs in May. Those posts were written on our Stanford Holistic Biology teaching cruise. This is a more forceful outing aboard a UNOLS research vessel, the 160-foot New Horizon out of Scripps Institute of Oceanography that is presently home to 18 scientists, students and technicians plus the crew. We will be carrying out field work for our NSF-funded collaboration, "Hypoxia and the ecology, behavior and physiology of jumbo squid, Dosidicus gigas."
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Our project has an overall focus on how Humboldt squid utilize and cope with a dark, cold, low-oxygen, acidic midwater-world called the Oxygen Minimum Zone (OMZ). It is a prominent oceanographic feature in the southern half of the Gulf of California, including the Guaymas Basin, where we will work on this expedition, and over most of the Eastern Pacific Ocean wherever there is deep water and high surface productivity. Dead phytoplankton and other organic matter sink in the water column and are consumed (metabolized) by microbes that take oxygen out of the water and expire CO2. This leads to a stable, low-oxygen/high-CO2 environment that is hostile to most of the fish and marine creatures we are familiar with. Oxygen goes back up at great depth, because this deep water is derived from polar waters that freeze, excluding the water as ice and sinking the cold, salty, oxygenated brine to great depth where it slowly, slowly makes its way around the world’s oceans.
But certain fish, crustaceans and mollusks have evolved adaptations that allow them to survive in the OMZ, especially near its upper boundary. Although species diversity in the OMZ is typically low, the numbers of creatures can be enormous. During the daytime these planktonic organisms tend to form a dense layer of life at a depth of several hundred meters. This layer can be detected with sonar, an acoustic method that uses transmitted and reflected sound waves to "visualize" animals under water. The dense layer of plankton is therefore called the acoustic deep scattering-layer (DSL). In the Gulf, daytime depth of the DSL is typically close to that of the OMZ (where the oxygen concentration is about 10 percent of that at the surface). At night, many DSL organisms migrate to near-surface waters and feed to smaller zooplankton and phytoplankton. The next morning, they return to the depths, where they can hide from visual predators, primarily fish and birds that cannot cope with the cold, hypoxic OMZ.
Previous studies using pop-up satellite tags that record depth and temperature have shown that Humboldt squid spend nighttime hours actively foraging in near-surface waters, but (like the plankton) they spend most of their daytime hours in the upper reaches of the OMZ, where they remain quite active. Our group has proposed that the squid can feed all day long in this hostile environment. In support of this hypothesis we find that the most abundant prey items of large Humboldt squid in the Gulf are small, mesopelagic organisms associated with the daytime DSL/OMZ—several species of lantern-fish (myctophids), small squids, pteropods (swimming mollusks with tiny shells) and sometimes crustaceans (krill, shrimp, swimming crabs, etc.). This ability probably gives the squid a distinct advantage over other pelagic predators such as tunas, sharks and marine mammals, all of which hunt in the DSL but must do so during relatively short dives, because they are limited by the harsh conditions in the upper OMZ.
But we also know that Humboldt squid are highly aerobic predators that utilize oxygen at a rate equal to that of a yellowfin tuna at a given temperature. So we are left with a puzzle. How can squid tolerate the OMZ and remain active there for extended periods of time? What adaptations allow them to do this? And what are they really doing in the OMZ? We think they are foraging, but that is just a hypothesis. Finally, what does this ability cost them, both physiologically and ecologically? These are the basic questions we will address in the next two weeks of field work.
Our group from Hopkins Marine Station of Stanford University will be carrying out shipboard behavioral and physiological experiments designed to test how OMZ conditions impact strong swimming and escape responses of squid. Both behaviors use jet-propulsion—water is expelled at high pressure from the body cavity through a small outlet port (the siphon) by the powerful mantle muscle. Is daytime foraging in the OMZ accompanied by compromised escape speed or reaction time? How does that relate to increased susceptibility to their main predators in the Gulf, sperm and other toothed whales?
A team from Oregon State University led by Dr. Kelly Benoit-Bird will use active acoustics (sonar) to study behavior of free-swimming squid in the water column under the ship. Individual squid as well as groups can be followed in real time, revealing behaviors in a non-invasive way. Acoustics can also provide a large-scale view of the entire population of squid in a given area, as well as allowing calculation of squid biomass—a critical piece of information for both ecologists and fishery scientists.
Dr. Brad Seibel’s group from the University of Rhode Island will investigate the biochemical and physiological mechanisms that allow squid to cope with cold, hypoxia and hypercapnia (high CO2)—the combination of which characterizes the OMZ. They will also examine the ability of important prey organisms to tolerate these conditions and carry out midwater trawls to identify what prey organisms are actually most abundant in a given area.
Finally, Dr. Unai Markaida (Colegio de la Frontera Sur, Campeche) and Dr. Cesar Salinas (CIBNOR, La Paz) and graduate student Jorge Ramos are joining us from Mexico. They will be sampling squid to document size, sexual maturity, stomach content and age. This should be especially interesting because (as we speculated in our blog in May) a warm El Niño-like winter has been accompanied by a movement of large Humboldt squid from their usual haunts in the Guaymas Basin. Commercial fishing for squid normally is going strong by this time of the year in Santa Rosalia, the main port of landings in Mexico, but the fishing this year has shifted 100 miles or more to the north. What is going on? And what might this year’s anomaly teach us about the amazing range expansion of Humboldt squid in the last decade along the Pacific coast of North America to as far north as southeast Alaska?
Many questions and not much time…
Image of New Horizoncourtesy of William Gilly; map of the Guaymas Basin, Gulf of California courtesy of W. Gilly and A. Booth; oxygen and depth illustrations (OMZ typically between 200-300 and 1200 m depth and is bounded by 10 percent oxygen saturation) courtesy of W. Gilly and A. Booth; image of typical mesoelagic prey items for Humboldt squid in the Guaymas Basin from a daytime midwater trawl throught the DSL: two myctophid lantern-fish, two types of shrimp and a small squid, courtesy of W. Gilly; image of basic squid anatomy relevant to jet propulsion (water drawn into mantle along sides of head, flows over gills serving respiration, then is ejected by mantle muscle out of siphon providing jet propulsion, which can vary from very gentle to extremely powerful—maximum pressure capable of being generated by a large Humboldt squid and maximum swimming speed are unknown) courtesy of Shane Anderson.