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Organisms with a wide range of temperature tolerance are termed eurythermal; “eury” is from the Greek for “wide” or “broad.” Eurythermal species have a broad geographic range or live in a habitat subject to wide swings in temperature, or both. Most intertidal species, particularly those that are sessile like barnacles and mussels, are quite eurythermal and tolerate temperature changes of >20 °C in a single tidal cycle. Geographic ranges for intertidal species can be quite broad. On the eastern seaboard of the United States, intertidal species often extend from Cape Cod in Massachusetts to Cape Hatteras in North Carolina and sometimes a great deal further north and south. Eurythermal pelagic species typically have a more modest temperature range. The widely distributed Pacific krill Euphausia pacifica has a temperature range of about 10 °C, inhabiting the northern half of the great Pacific Gyre from the Sea of Japan across the northern Pacific and down the US coast to southern California. Pelagic species that vertically migrate from depths >200 m to the surface and back each day can encounter large temperature swings, particularly in the tropics. Vertical migration is an interesting and widespread lifestyle that will be discussed at length later in the book.

The opposite of eurythermal is stenothermal, “steno,” the Greek word for “narrow.” Stenothermal species have a narrow range of temperature tolerance and are typically found in habitats with small daily and annual temperature deviations. An example of such a habitat for pelagic species is the Antarctic, where the cryopelagic fish Pagothenia borchgrevinki has been documented to succumb to heat death at temperatures above 4 °C but can readily tolerate the low temperatures (−1.86 °C) found beneath Antarctic ice.

The next two terms, briefly introduced in Chapter 1, are also opposites: ectotherm and endotherm. They describe organismal body temperature and what determines it. The body temperature of ectotherms is nearly identical to the ambient temperature. Once again, the term is derived from the Greek: “ecto” meaning “outer” or “outside.” In layman’s terms, the ectotherms are “cold‐blooded.” Other than mammals and birds and some highly adapted fishes like the tunas, all animals are ectotherms. The particular subjects of this book, the micronekton and macrozooplankton, are all ectotherms.

Endotherms utilize the heat generated by metabolism to maintain a constant body temperature. “Endo” is the Greek root for internal or inside; an endotherm’s body temperature results from heat generated within. Mammals and birds modulate the amount of metabolic heat lost to the environment (or gained from it) by a variety of mechanisms, including insulation (fur, feathers, blubber) and adjustment of blood flow to limit or facilitate heat exchange with the external environment. Endotherms, in layman’s terms, are “warm‐blooded.”

The terms endothermy and ectothermy were created to precisely define how a species’ internal body temperature comes to be the way it is: by virtue of internally generated heat or by interaction with the external environment.

Another pair of terms formerly used to describe species’ body temperature, homeothermy and poikilothermy, are still quite useful though not as widely used as they once were. A homeotherm (from the Greek “alike” or “constant”) has a body temperature that is closely regulated around a constant set point. The trick here is that achieving a constant body temperature may be done in a variety of ways. Mammals and birds regulate their internal temperature precisely by controlling loss of metabolic heat. However, a nearly constant body temperature can also be achieved behaviorally, as lizards do by regulating their time spent in sun and shade. In the deep ocean, nearly every species is a homeotherm because temperatures vary little below 1000 m. Some species, e.g. sockeye salmon, have thermal preferenda or optima that they will seek out in a thermal gradient, giving them a nearly constant body temperature as well.

Poikilotherm (from the Greek “poikilo” or “varied”) may be considered as the older version of ectotherm. A poikilotherm has a body temperature that changes with the external environment, so it is certainly an ectotherm. However, as we just discussed, ectotherms dwelling at a constant temperature are also homeotherms. So, using the old terminology, a poikilotherm could also be a homeotherm when living at constant temperature, which is confusing at best.