In the open sea, animals can often find food reliably available in particular regions or seasons (e g., in coastal areas in springtime). In these circumstances, animals are neither constrained to get the last calorie out of their diet nor is energy conservation a high priority. In contrast, the food levels in the deeper layers of the ocean are greatly reduced, and the energy constraints on the animals are much more severe. To survive at those levels, animals must maximize their energy input, finding and eating whatever potential food source may be present.

In the near-surface layers, there are many large, fast carnivores as well as an immense variety of planktonic animals, which feed on plankton (small, free-floating plants or animals) by filtering them from currents of water that pass through a specialized anatomical structure. These filter-feeders thrive in the well-illuminated surface waters because oceans have so many very small organisms, from bacteria to large algae to larval crustaceans. Even fishes can become successful filter-feeders in some circumstances. Although the vast majority of marine fishes are carnivores, in near-surface regions of high productivity the concentrations of larger phytoplankton (the plant component of plankton) are sufficient to support huge populations of filter-feeding sardines and anchovies. These small fishes use their gill filaments to strain out the algae that dominate such areas. Sardines and anchovies provide the basis for huge commercial fisheries as well as a food resource for large numbers of local carnivores, particularly seabirds. At a much larger scale, baleen whales and whale sharks are also efficient filter-feeders in productive coastal or polar waters, although their filtered particles comprise small animals such as copepods and krill rather than phytoplankton.

Filtering seawater for its particulate nutritional content can be an energetically demanding method of feeding, particularly when the current of water to be filtered has to be generated by the organism itself, as is the case for all planktonic animals. Particulate organic matter of at least 2.5 micrograms per cubic liter is required to provide a filter-feeding planktonic organism with a net energy gain. This value is easily exceeded in most coastal waters, but in the deep sea, the levels of organic matter range from next to nothing to around 7 micrograms per cubic liter. Even though mean levels may mask much higher local concentrations, it is still the case that many deep-sea animals are exposed to conditions in which a normal filter-feeder would starve.

There are, therefore, fewer successful filter-feeders in deep water, and some of those that are there have larger filtering systems to cope with the scarcity of particles. Another solution for such animals is to forage in particular layers of water where the particles may be more concentrated. Many of the groups of animals that typify the filter-feeding lifestyle in shallow water have deep-sea representatives that have become predatory. Their filtering systems, which reach such a high degree of development in shallow- water species, are greatly reduced. Alternative methods of active or passive prey capture have been evolved, including trapping and seizing prey, entangling prey, and sticky tentacles.

In the deeper waters of the oceans, there is a much greater tendency for animals to await the arrival of food particles or prey rather than to search them out actively (thus minimizing energy expenditure). This has resulted in a more stealthy style of feeding, with the consequent emphasis on lures and/or the evolution of elongated appendages that increase the active volume of water controlled or monitored by the animal. Another consequence of the limited availability of prey is that many animals have developed ways of coping with much larger food particles, relative to their own body size, than the equivalent shallower species can process. Among the fishes there is a tendency for the teeth and jaws to become appreciably enlarged. In such creatures, not only are the teeth hugely enlarged and/or the jaws elongated but the size of the mouth opening may be greatly increased by making the jaw articulations so flexible that they can be effectively dislocated. Very large or long teeth provide almost no room for cutting the prey into a convenient size for swallowing, the fish must gulp the prey down whole.

在开阔的海洋中，动物通常能在特定的地区或季节找到可靠的食物（如春天的沿海地区）。在这种情况下，动物既不用尽力消化吸收食物，也不用节约能源。相比之下，在更深层次的海洋中的食物数量大大降低，对动物的食物约束更严重。为了在这些地方生存，动物必须最大限度地提高他们的能量输入，去寻找和食用任何食物。 在海平面附近，有很多大型的、敏捷的食肉动物以及大量的浮游动物，浮游动物以浮游生物为食（小的浮游植物或动物）的。这些动物有专门的身体结构过滤海水，吃掉其中的小浮游生物。在有可见光的海平面附近，滤食性动物非常繁盛，因为海洋有这么许多很小的生物，从细菌到大型藻类到甲壳类幼体。甚至有些情况下鱼类也是滤食性动物。虽然绝大多数海洋鱼类是食肉动物，但滤食性沙丁油鱼和凤尾鱼靠吃海洋表面区域丰富大量的浮游植物（浮游生物的植物成分）而大量繁殖。这些小鱼使用它们的鳃丝滤食他们生活区域内的藻类。沙丁油鱼和凤尾鱼成为当地大规模渔业的基础，并且是大型肉食动物，特别是海鸟的食物。规模更大的须鲸和鲸鲨也是沿海或极地水域的高效滤食动物，虽然它们的滤食对象不仅限于浮游植物，还有桡足类、磷虾等小动物。 过滤海水来获取其中的微小营养成分是一种非常费力的摄取食物的方法，尤其是当被过滤的水流需要生物体自己来选定时（正如浮游动物所做的那样）。每立方米的海水中，必须要含有至少2.5微克的颗粒有机物质，才能保证滤食浮游生物的净能量摄入。大多数沿海水域都很容易超过这个值，但是在深海，有机物的浓度从几乎没有到每立方米7微克都有可能。即使平均水平可能忽略了局部可能有机物密度很高这一点，但依然说明很多深海动物面临的条件很恶劣，一般的滤食动物可能会饿死。 因此，深水中的滤食性动物很少，这部分滤食性动物中有一部分有较大的过滤系统来应对有机物的稀缺。还有一些滤食性动物会在特定的有机物密度高的水层中觅食。许多浅海区的典型滤食性动物都可以在深海区找到代表，但是它们已经演化为捕食性动物。在浅海区形成的特别发达的过滤系统，在深海区都退化了。相对的，它们进化出主动或者被动的捕猎方式，包括诱抓和缠住猎物，长出粘性的触须。 在更深的海域，动物更倾向于被动等待食物颗粒或者猎物的来临，而不是主动去寻找它们（这样一来减少了能量消耗）。于是就产生了一种更隐秘的觅食方式，这种方式强调诱捕和/或身体需要有很长的附属物，以便增加动物控制或者监测的活动水量。猎物数量有限的另外一个后果是，很多动物经过进化都可以处理相对于自身体积来讲更大的食物，比相应的浅海区动物能处理的食物要大得多。这些鱼类有一个共同的趋势，它们的牙齿和颌骨明显增大。这些动物不仅是牙齿变得很大和/或下巴变长，而且它们的下颌关节可以随意的脱臼，所以嘴巴能够张得很大。由于牙齿特别大或特别长，这些鱼没有办法把猎物咀嚼成合适的大小，只能把猎物整个地吞下去。