Copepod Neuroecology

(updated February 15, 2013)

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About this page

The study of specializations of the nervous system and the effector systems it controls, in relation to the particular ecological niche an organism inhabits, may be termed "neuroecology." This covers a broad area from sensory input and central processing to motor output and behavior, studied in an ecological and evolutionary context. Because of the relative simplicity of copepod nervous systems, the similar body plan for all species and the great diversity of ecological niches they have penetrated, and barring only their small size, they make very appropriate animals for neuroecological studies. The following information is biased toward the expertise of the page creators, but we welcome input from others on content as well as links to other pages on related topics.

Mechanosensory Mechanisms in Copepods

Nerve impulses can be recorded extracellularly from both mechanoreceptors and putative chemoreceptors (for details of methods, see Gassie et al 1993). "Giant" impulses (mV) with unusual characteristics are found in Augaptiloid and Centropagoid superfamilies, but not in more recently diverging calanoid groups (Yen et al. 1992). Among the properties exhibited by these giant antennal mechanoreceptors (GAMs) are:
  1. There are two reidentifiable mechanoreceptor units, "A" and "B", in each antenna (Hartline et al 1996).
  2. These originate in the sensory setae of each distal tip (Lenz and Yen 1993).
  3. They are sensitive to very small (<10 nm) hydrodynamic signals, including abrupt displacements and sinusoidal vibrations.
  4. Their frequency range for sensitivity to oscillatory stimuli is unusual for aquatic arthropods, extending up to and above 2 kHz.
  5. They can fire impulses at exceptionally high rates (up to several kHz).
  6. They can follow (phase-lock to) oscillatory stimuli to 2 kHz or so.

The figure above shows the electrophysiological responsiveness of two copepod mechanoreceptor neurons, designated "A" and "B", in a copepod's first antenna. The stimulus is a "tone pip" produced by vibrating a small bead up and down in the water in front of the antenna (bottom trace). The 300 Hz vibration builds up over several cycles and then decreases again. As the vibration builds up, cell A begins to fire nerve impulses producing the rapid "spike" deflections in the electrical recordings of the top three traces. Note that the time of each impulse corresponds to a peak or a trough of the bead displacement. When the bead oscillation becomes big enough, cell B begins to fire impulses. The three top traces represent repeats of the stimulus shown on the bottom trace. The calibration bar for the bottom trace is the calculated water movement at the sensory hair on the copepod's antenna.

The figure above shows the high sensitivity of the copepod receptors. Sensitivity was determined by changing the strength of the stimulus shown in the previous figure to determine how strong (peak velocity of the bead) it has to be to elicit a single impulse from one of the cells, termed the "threshold" sensitivity. Other frequencies are tested in the same way until a range of frequencies from 50 to 2000 Hz has been tested. The results are plotted on logarithmic scales above. The different symbols correspond to different units and different animals. Threshold sensitivities of around 10 micons per second for frequencies ranging from 100 to 1000 Hz are illustrated here.

Structure-Function Studies of Mechanosensory Mechanisms in Copepods

Morphological studies with scanning and transmission electron microscope (TEM) have shown that the mechanoreceptors of the distal tip of calanoid first antennae are packed with unprecedented numbers of microtubules (Weatherby et al. 1994). Furthermore, the actin-containing scolopale surrounding the sensory dendrites of the mechanoreceptors is developed to an extreme. These features paint a picture of exceptional rigidity in this receptor, undoubtedly contributing to its high mechanical sensitivity and possibly its frequency response. Setal receptors are differentiated along the length of the calanoid antenna, with the spiniform setae of the distal tip more adapted for mechanoreception while those of the antennal shaft seem adapted for gustatory (contact mechano-chemo) reception (Lenz et al. 1996)
The figure above is a schematic of a longitudinal section through a spiniform mechanoreceptive seta along with TEM cross sections of the sensory dendrite (llight blue in the diagram) at different levels (lines a, b, c, d and corresponding images). In addition to the sensory cell process, there are cell bodies and cytoplasmic processes of a cell that contains a dense packing of stiffening actin filaments, forming the "scolopale" surrounding a pair of distal dendrites. In adition, three other types of supporting cells are found (red, gren and blue in the diagram). Legend -- bb: basal body (a ciliary component); bl: basement lamina; cu: cuticle; dd: distal dendrite; lc: liquor cavity; mt: mitochondrion; nu: nucleus; pd: proximal dendrite; sc: scolopale

Escape Behavior in Copepods

Behavioral studies in the mesopelagic Augaptiloidean Pleuromamma xiphias and the shallow-water Centropagoidean Labidocera madurae, show that rapid escape reactions can be triggered by the same types of disturbances as elicit firing in the GAMs. Behavioral sensitivities over a range of frequencies are similar to those measured physiologically. This suggests that firing of the GAMs is capable of triggering escape behavior. It may be that even a single nerve impulse can elicit the reaction in these animals. Similar results are obtained from the Megacalanoidean Undinula vulgaris, except that reaction times are much faster than for the more "primitive" species. In animals tethered to a force transducer, forces in excess of 100 dynes can be exerted during a single power stroke of an escape jump. Multiple kicks are typically given in each jump, up to 30 or more at up to 100 Hz repetition rates in some species. The muscle power output during these kicks is close to or above the peak performances thus far reported for muscle in other animals (Lenz and Hartline,1999).

In high-speed video studies of free-swimming Acartia tonsa and Acartia lilljegorgia, accelerations of 100 meters per second squared can propel the animals to peak speeds of 500 millimeters per second (500 body lengths per sec) in a few milliseconds. In relative terms, this outperforms most other animals: the famous stoop speeds of falcons (300 mi/h) come to some 70 BL/s; humming birds do better at 300 BL/s. Only some terrestrial insects moving in air (rather than the very viscous medium water represents) outperform copepods at 2000 BL/s (click beetles; fleas)(see "Animal Olympians" for more examples). Escapes often begin with a rapid reorientation away from the source of disturbance, animals turning at rates of 30 degrees per millisecond (Buskey, Lenz and Hartline, 2002). Interestingly, when escapes in Acartia are elicited by sudden dimming of the light falling on them, their reaction time is much longer (30 milliseconds) than to mechanical stimuli, and their escape jumps cover much more ground (several centimeters compared to just a few millimeters). This difference in escape response may relate to photic stimuli (shadows) being more likely to represent a threat from a visual predator than mechanical stimuli (a visual predator cannot see a copepod in the dark when it can't cast a shadow that might warn the prey). Presumably the copepod must put more distance between it and the predator if the predator might track it visually. (Buskey and Hartline, Biol. Bull. 204: 28-37, 2003).

Myelin and Copepod Escape Behavior

Morphological studies of the substrates of sensory-triggering of the escape reaction have revealed that some, but not all, copepods have their nerve fibers ensheathed in a fatty multi-layered wrapping called "myelin." In fact, the taxa that possess myelinated axons correspond very nicely to those with very small sensory neuron impulses. Myelin is the feature that speeds nerve impulse propagation in vertebrates and allows a large animal to react rapidly enough to survive. The copepods that have myelin react much more rapidly to sudden water movements, such as those a predator would make, than those lacking myelin. The copepods that possess myelin are found in a greater diversity of ecosystems, including in more risky oceanic environments, than those that don't. Part of the explanation for this seems to be their faster reaction to predators. Copepods lacking myelin are largely restricted to predator-evasion strategies such as diel vertical migration (migrate up several hundred meters at night to feed then migrate back down to dark regions to avoid visual predators during the day), bioluminescence, or inhabiting coastal zones where environmental variability is a major survival factor. To date, all representatives examined from the more recently evolved superfamilies Clausocalanoidea, Eucalanoidea and Megacalanoidea possess myelin , whereas members of the more primitive Augaptiloidea and Centropagoidea do not.

The electronmicrographs below show non-myelinated axons from Candacia on the left and myelinated Euchaeta on the right.

There are several interesting implications of these findings:

  1. In terms of basic biology, they erode the basic dogma that myelin is primarily a vertebrate invention. Although myelin has been known in a few invertebrates (and it was reported once in a copepod and overlooked for almost 20 years), it was not realized that so many copepods have it or what its significance is. Given the vast numbers of calanoid copepods in the ocean, it may even be argued that there are more invertebrates with myelin than vertebrates. Because it seems to be absent from more primitive copepods, it would appear that it has evolved independently by the more advanced forms.
  2. In terms of ecology and phylogeny, they provide insight into the ecology and evolution of one of the more important plankton groups, one on which much of the ocean's productivity depends. They thus have ultimate importance to such practical matters as fisheries management, understanding the impact of global environmental change on the oceanic food web, etc.
  3. In terms of nervous system mechanisms, it used to be thought that mechanisms for increasing nervous system communication speeds are unimportant for small animals, since all parts of the animal are in very rapid communication anyway. Finding myelin in copepods (as well as giant nerve fibers in copepods and Drosophila) shows that every millisecond counts. Myelin potentially shortens the reaction time for a 2 mm copepod by reducing nerve-conduction time about 2 msec. Compared to ca. 6 msec for an unmyelinated species, this is a significant fraction of an already very fast reaction.

For more information on invertebrate myelin generally, click here. Many many web sites give information on vertebrate myelin, but if interested, you might start with Wikipedia.

REFERENCES: The initial report appeared in the Scientific Correspondence section of Nature April 15, 1999 ( Davis et al. 1999). Details may be found in (Lenz et al. 2000) and (Weatherby et al. 2000)]. More recent publications include the findings that copepod myelin have a very different cellular origin from that of vertebrates and other myelinate invertebrates (Wilson and Hartline, 2011a & b).

Bioluminescent Copepods

Bioluminescence is a mechanisms used by many organisms dwelling in the depths of the oceans where little light penetrates. Many of the great variety of copepods dwelling at depth (>200 m) apparentlyemploy bioluminescence as a means of startling or decoying visually-sensitive predators. Studies on tethered Pleuromamma xiphias have shown that abrupt hydrodynamic disturbances are capable of triggering bioluminescent discharge. The threshold for triggering such a response is some 100 times stronger than that required to trigger a weak escape jump. Evidently, the bioluminescent response is more of a "last ditch" measure than is a jump. See Hartline et al. 1999 for a recent report.

Copepod Nauplii

Copepods are among the several crustacean goups with free-swimming nauplii as larval stages. The image below is a scanning electronmicrograph of a nauplius of Bestiolina similis. It swims using its three pairs of appendages as paddles to row itself along in the water. For an organism that size, it is similar to what you would experience swimming in very heavy syrup! However, when startled by a sudden mechnosensory cue, it is capable of a very rapid escape response, propelling itself forward at over 500 body-lengths per second with its "paddles." The anterior pair of appendages (top of image) will be transformed into long sensory first antennae of the adult when the nauplius metamorphoses into a "copepodid" stage. Note the tiny mechanosensory setae at the very posterior tip of the animal.

Scale bar: 50 microns. Photo by Jennifer Kong

Value of Research

Research described on this page is focused on understanding the physiological and morphological substrates of the behavior of copepods in the context of their evolutionary and ecological adaptation to their environment. Many people draw a blank when they encounter the term "copepod." Why should anyone care, and further, why should taxpayers want any of their hard-earned money to go to such abstruse studies? Although the most valuable end results of any "basic" research such as that treated above are almost impossible to predict (and hence the justification can only be based on past experience of consistent, remarkable and unexpected benefit), there are general grounds motivating the particular lines currently being pursued and funded under rigorously peer-reviewed grants. As it happens, copepods are among the more numerous and diverse of zooplankton groups in aquatic ecosystems. In fact, it is often said that they are the single most numerous group of animals on earth (although hard to estimate reliably, probably numbering in the quintillions). They are a key link in the food web upon which the health and productivity of marine and freshwater ecosystems depend. Study of their behavioral ecology is of long-term importance for understanding and managing aquatic food sources for human consumption and for shedding light on issues related to basic physiology, morphology, evolution, ecology, biodiversity and environmental health. Inhabiting a pelagic environment with little cover, they are subject to intense predation pressure. Thus their ability to detect and escape from potential predators is a major factor in their success. Owing to their small size (mm), their sensory capabilities and other aspects of nervous system function have not, until recently, been investigated with the tools of behavioral neuroscience. Much might be learned about copepod survival mechanisms and evolutionary pressures through application of such techniques. Given the severe selective pressures experienced by the group, adaptations in sensory mechanisms, in other parts of the nervous system (the "neuroecology" of the organisms) and in the motor output mechanisms should be expected.

Other Web Links

Web pages on zooplankton sensory and motor systems.
Link to general pelagic crustacean sensory-motor web page.
To Lenz Pages. To PBRC Home Page. To Hartline Pages.