Feeding ecology and diet

Hydrozoa are usually thought to feed on planktonic crustaceans such as copepods. Under laboratory conditions, they mostly survive with a diet based on Artemia nauplii. The medusae are mostly voracious carnivores and, when feeding on fish eggs and larvae, can be considered as being at the apex of trophic chains. The polyps are more varied in their food preferences. Recent investigations showed that the few species studied thoroughly feed on a great variety of prey, ranging from gelatinous plankton for the medusae to phytoplankton for the polyps. Some species have symbiotic zooxanthellae and are functionally photosynthetic.

Some medusae can remain immobile in the water, with their tentacles outstretched across the water column, performing ambush predation, whereas others can move across the water to contact prey, performing cruising predation. Polyps can simply extend their tentacles to catch passing prey, but they can also use special sense organs to perceive approaching prey and grab it actively, or they can form currents by moving their tentacles to direct food particles toward the mouth. Symbiotic species are very specialized in their feeding behavior; the most extreme cases are Halocoryne epizoica that feed on bryozoan tentacles, and Polypodium hydriforme that feed on sturgeon eggs from the inside, being the only in-tracellular parasitic metazoan.

Hydrozoa use cnidocysts as the main organelles to catch their food. The superclass has the richest variety of cnidocyst types of the whole phylum, with a fine range of adaptations to catch from tiny prey like the single cells of phytoplankton to the crustaceans and larvae of the zooplankton to the animals that live in the sand and mud, such as nematodes. Many species have very restricted diets, being specialized for just one type of food.

Both polyps and medusae are mostly carnivorous, feeding on almost all animals of proper size. In no other metazoan group (with the possible exception of parasitic trematodes) is the lifecycle of such paramount importance in defining the properties of a given species, and this is valid also for the type of prey. Tiny medusae that feed on fish eggs and larvae, sometimes impairing the success of recruitment, can be the most voracious predators of fish. The propensity of feeding on almost all types of larvae (both for polyps and for medusae) includes the Hydrozoa within the predators of almost all Metazoa with an indirect lifecycle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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