Squid Trophic Ecology
We are constantly interacting with other living organisms: socializing with other humans, eating plants and animals, or seeking shade under trees. Because Humboldt squid (and oceanic organisms in general) spend most of their time far beneath the ocean surface, there are limited opportunities to observe their behavior. We can learn a lot about how an organism interacts with its environment by learning what it eats and what eats it. The study of these relationships between organisms and their interactions with the environment is called ecology. Trophic ecology ("trophic", derived from the Greek word referring to food, or feeding), is specifically the study of interactions between predators and prey. Humboldt squid play a major ecological role in their open ocean environment. They are eaten by many marine animals, but are also voracious generalist predators, feeding on almost anything they can catch. By studying the trophic ecology of Humboldt squid, we will better understand how they influence the ecosystems they inhabit.
Humboldt squid grow rapidly, some reaching 6ft long and over 100lbs in less than two years. This amazing growth rate is fueled by their success as predators. Young hatchlings have an internal yolk sac that supports them for a few days, but they must quickly learn to hunt on their own. We are currently unsure what these tiny squid eat, but we suspect they eat planktonâ€”small drifting plants and animals. As Humboldt squid grow larger and become more skilled at hunting, they devour larger fishes, crustaceans, and other species of squid. Relative to their body size, Humboldt squid have a fairly small mouth, and their esophagus runs through their donut-shaped brain. This means their food must be very small or they must use their parrot-like beak and grinding radula (tongue) to tear their food into small pieces.
Figure 1: A) The fingers of a scientist pinch the mouth of a squid. The white sphere is the muscular buccal mass that allows the squid to control its beak, the black "V" at the center of the buccal mass. B) A squid radula magnified ten times under a microscope. The rows of teeth along the surface of the tongue allow the squid to break their bites onto smaller pieces. C) The upper (left) and lower (right) beak halves of a squid beak. Credit: E. Portner
Post-larval squid (See page on squid reproduction for distinction) can extend their arms to grab and hold their prey. This strategy allows them to restrain and eat animals bigger than their mouths. However, creatures like whales and humans are far too large to grab ahold of. Adult Humboldt squid tend to eat creatures less than half their own body size.
Humboldt squid live in the eastern Pacific Ocean, usually between Chile and northern California. Though they mostly eat micronekton - small swimming fishes, cephalopods, and crustaceans - throughout their range, Humboldt squid diets vary depending on where they live. In Mexico, they mainly eat Myctophid fishes. Commonly called lantern fishes, these small, open-ocean fishes emit light to camouflage themselves in dim waters. This camouflage strategy, called counter-illumination, is only effective at mid-depths (between 600 and 3000ft). These depths are where there is enough light that swimming organisms still cast shadows, but the amount of light is small enough that organisms can emit light of a similar brightness from their light organs. Mexican Humboldt squid also eat swimming red crabs and small squids. In California, they eat less micronekton and incorporate larger fishes such as flatfish, rockfish, hake, and even salmon into their diets. This worries fishermen and fishery managers because humans also like to catch and eat these large fishes. If Humboldt squid compete with humans for these resources, will the fisheries be in trouble? Scientists have yet to answer this question.
Figure 2: Examples of micronektonic prey items of the Humboldt squid. A) An Enoploteuthid squid. B) The red swimming crab, Pleuroncodes planipes. C) A large myctophid fish. The black "freckles" on the silvery portion of the mytophids body surround some of its light organs, photophores. Most of its photophores are located on its belly. The scale bars in A and C are in centimeters. Credit: E. Portner
Because they often feed far below the surface during the day, or near the surface under the cover of darkness at night, it difficult to witness predation in the wild without the use of highly specialized equipment. Underwater robots, called remotely operated vehicles (ROVs), can be used to get a camera to the depth that squid feed at. Unfortunately, ROVs are expensive to operate and there is no guarantee that you'll see squid feeding. Most of our knowledge Humboldt squid diet comes from inspecting many squid's stomach contents. Some scientists specialize in identifying animal body parts from stomachs using partially digested tissues including fish bones, crustacean legs, and squid beaks. This technique has also been a useful tool in the identification of predators that feed on Humboldt squid.
Throughout their short lives (less than two years), the Humboldt squid are hunted by predators of all sizes. Though a single female Humboldt squid can spawn millions of eggs, only a few will survive to adulthood. Many squid eggs and hatchlings become a significant, high-protein source of food for small predators. Squid that survive to adolescence and adulthood are hunted by increasingly larger predators such as tuna, sharks, dolphins, and especially sperm whales. Sometimes whole squid are found inside the stomachs of these predators and can be easily identified, but more often only their beaks remain undigested (figure 1C above). Squid beaks have unique structural features that can be used to identify the kind of squid they came from.
In the last 15 years, humans have become important predators of Humboldt squid, catching and eating hundreds of thousands of tons every year. As we become more influential in the trophic ecology of Humboldt squid, research in this field becomes increasingly necessary to support the sustainable exploitation of the Humboldt squid fishery. Some scientists think these squid may become more abundant and occupy a larger range, in part because they have such a flexible diet. The exact impacts of changes in Humboldt squid distribution and population size are difficult to predict. But, because they have many trophic interactions in their current environments, it is likely they will alter newly infiltrated food webs in many complex ways.
FAO 2005-2014. Fisheries and Aquaculture topics. State of world marine fishery resources. Topics Fact Sheets. Text by Michel Lamboeuf. In: FAO Fisheries and Aquaculture Department [online]. Rome. Updated 4 September 2012. http://www.fao.org/fishery/topic/426/en
Field JC, Baltz K, Phillips AJ, Walker WA (2007)Â Range expansion and trophic interactions of the jumbo squid,Dosidicus gigas, in the California Current. California Cooperative Oceanic Fisheries Investigative Report 48:131-144. http://www.calcofi.org/publications/calcofireports/v48/Vol_48_Field.pdf
Markaida U, Sosa-Nishizaki O (2003) Food and feeding habits of jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae) from the Gulf of California, Mexico. Journal of the Marine Biological Association UK 83:507-522. DOI:10.1017/S0025315403007434h, pdf
Pardo-Gandarillas MC, Lohrmann KB, Valdiva AL, Ibáñez CM (2009) First Record of parasites of Dosidicus gigas (d' Orbigny, 1835) (Cephalopoda: Ommastrephidae) from the Humboldt Current system off Chile. Revista de biolog&iactue;a marina y oceanografía 44(2): 397-408. DOI:10.4067/S0718-19572009000200013
Rosas-Luis, R., Salinas-Zavala, C. A., Koch, V., Luna, P. D. M. & Morales-Zárate, M. V. (2008) Importance of jumbo squid Dosidicus gigas (Orbigny, 1835) in the pelagic ecosystem of the central Gulf of California. Ecological Modelling 218, 149-161. DOI:10.1016/j.ecolmodel.2008.06.036
Ruiz-Cooley RI, Gendron D, Aguíñiga S, Mesnick S, Carriquiry JD (2004) Trophic relationships between sperm whales and jumbo squid using stable isotopes of C and N. Marine Ecology Progress Series 277:275-283. DOI:10.3354/meps277275, pdf
Xu, J, Gordon, JI (2003) Honor thy symbionts. Proceeding of the National Academy of Science, USA 100(18):10452-10459. DOI:10.1073/pnas.1734063100