Nemertea and their feeding processes through use of the proboscis
by Matthew Feltes
BIOL/WATR 361, Spring 2015
Nemertean worms have a distinctive feature known as the proboscis which aids in the capturing of their prey. Although all nemertean worms have a proboscis not all of them share the same proboscis characteristics and thus nemerteans can be partially identified by their proboscis. The way the nemerteans feed is also relevant to the purpose of the proboscis. There are some important mechanisms that aid in the functionality of the proboscis that are worth discussing. To understand the importance of the proboscis, we need to understand what it is and how it works.
To comprehend why the proboscis is an important apparatus, we must first understand a little bit about the organism that uses it. Nemertean worms belong to the phylum Nemertea. There are around 800 to 1300 different species of known nemertean worms. The proboscis is so characteristic of the nemertean worms that they are commonly known as proboscis worms. The most notable use of the proboscis is for the capturing of prey but studies have also found alternative uses. It has been noted for being used for defensive purposes, burrowing, locomotion, and for escape responses (Gibson, 1970). Nemertean worms are bilaterally symmetrical. They have an unsegmented body plan which is thin or flattened in appearance. Nemertean worms have a complete, regionally specialized gut which includes an anus for waste disposal (Riser, 1985). They also have a closed circulatory system and a well-developed ladder like central nervous system. Some nemerteans are less than a centimeter long where as others can be as big as 30 meters (Turbeville, 2002). Some nemerteans have a set of simple eyes called ocelli which detect light but do not form an image.
The anatomy of the proboscis can be divided into three sections. In a study conducted by Ling (1971), the proboscis of Lineus ruber was divided into three segments: anterior, middle, and posterior segments with specific focus on what the proboscis was composed of. The retractor muscle was also associated with the proboscis functionality. When the proboscis is extended, the muscles can be stretched to about double the length of the nemerteans body (Gibson, 1970). The length of the proboscis is extremely variable depending on the species and size of the nemertean worm. The anterior section was found to be the thinnest section which contains the stylet if present. The stylet is a firm, anatomical structure that is sharp enough to pierce their prey. Ling (1971) took a transverse section of the anterior section of the proboscis and determined that it was made up of the outer endothelium, subendothelial layer of circular muscle fibrils, outer basement membrane layer, longitudinal muscle layer, inner basement membrane and the inner epithelium.
When observing the middle section of the proboscis, some morphological differences were noticed compared to the anterior section. The middle section of the longitudinal muscle had a larger diameter then the anterior section (Ling, 1971). When looking at a transverse section of the middle proboscis Ling (1971) found that the subendothelial circular muscle fibrils were absent as compare to the anterior section. Other layers such as the outer endothelium, outer basement membrane, longitudinal muscle layer, inner basement membrane, and the inner epithelium were still present. The addition of the circular muscle layer and the nerve plexus were also mentioned to be found in the middle section of the proboscis. From the middle section of the proboscis to the posterior section the diameter decreases in thickness and size. The transverse sections of the posterior proboscis contained the outer endothelium, outer basement membrane, longitudinal muscle layer, basal cells, inner basement membrane, nerve plexus and the inner epithelium (Ling, 1971).
The retractor muscle which aids in the withdrawal of the proboscis after it has been extended is a key apparatus in the functionality of the proboscis. The retractor muscle is a tubular muscle cord which is attached to the connective tissue at the anterior end and also attaches to the dorsal wall of the rhynchocoel at its posterior end (Ling, 1971). The rhynchocoel is a tubular cavity that lies above the nemertean gut that contains the proboscis when not extended. The rhynchocoel is made up of a thin layer of mesodermal cells which forms the proboscis sheath and an epidermal layer which forms the proboscis muscle layers (Iwata, 1985). When the proboscis is triggered, it is inverted or turned inside out. The proboscis is extended using the nemerteans longitudinal muscles and is returned into the rhynchocoel by the reduction of the retractor muscle which pulls the tip of the proboscis back inside the body (Gibson, 1970). This is not always the case depending on which class of nemertean is being observed. Some nemerteans can retract their proboscis by lowering the pressure in the rhynchocoel. This lowering of pressure could be followed by an outflow of the blood from the rhynchocoel villus into the dorsal and lateral blood vessels (Gibson, 1970).
Knowing more about the basic functionality of the proboscis, we can now look into how nemerteans can be organized into a class based on the proboscis. The morphology of the proboscis is used to distinguish two classes of nemertean worms: Anopla and Enopla (Gibson, 1970). These two classes both have unique adaptations to the proboscis which set them apart. The first major difference between the two classes is if the proboscis contains a stylet at the anterior end. Enoplan worms contain a stylet apparatus which they use to stab their prey and inject them with toxins and other digestive juices. Anoplan worms on the other hand do not have a stylet so their proboscis has a sticky coating that is covered in toxins that are used to immobilize their prey. The toxins of most nemerteans are produced and secreted by the posterior proboscis epithelium (Gibson, 1970). Anoplan nemertean worms have a separate mouth and gut opening from the aperture that their proboscis is everted from whereas the nemertean worms in the class Enopla have their gut and proboscis using a common orifice of the rhynchocoel (Gibson, 1970).
When Enoplan worms locate their prey, the proboscis is everted with such a substantial force that the tip strikes the prey on the ventral side which is more vulnerable to the stylet (McDermott & Roe, 1985). The stylet of the proboscis pierces the prey and injects a toxin which immobilizes the nemertean’s prey and can even kill it. Multiple punctures of the target organism with the stylet may be required before the toxins reach an effective concentration to restrain the prey. This allows for the nemertean worm to use its head to feed on the now powerless organism. Nemertean worms also have two to eight accessory stylets which form in stylet pouches that are replacements for the main stylet if damaged or removed (Gibson, 1970). Anopla worms coil their proboscis around their prey and hold onto it until it cannot move anymore or dies from the toxins. Chemoreceptors are believed to be used to locate the nemerteans prey however this is not always the case for all nemertean worms.
Understanding how the nemertean worms feed and what they prey on is also important in understanding how the proboscis functions in the feeding process. Nemerteans worms feed on the soft semi-fluid or partly digested parts of arthropods, annelids and other worm shaped animals along with some molluscs and fish (McDermott & Roe, 1985). Their prey can either be alive or dead when the nemertean worms find their food. Nemertea are mostly carnivorous and hunt or scavenge for their prey. Some species are also opportunistic feeders meaning they feed on whatever they can find or capture themselves. Nemerteans have been observed feeding on inactive or decaying food material without prior use of the proboscis (Wang et al., 2008). The two main feeding patterns that occur among the nemerteans are suctorial and macrophagous feeding (McDermott & Roe, 1985). Nemerteans that use the suctorial action attach to their prey, suck out all of the nutrients, and leave the exoskeleton behind. The macrophagous feeders eat their prey whole. Both patterns of feeding have been observed in Enopla, while the Anopla are mostly macrophagous feeders.
The manner in which the proboscises of the different classes of nemertean worms are mechanically organized may indicate how nemertean worms feed. Enopla can inject their prey with toxins and digestive fluids to help liquefy their prey’s internal organs. This makes it easier for the nemertean of the Enopla class to suck out the nutrients they need. The Anopla do not have a stylet but instead constantly hold onto their prey until their toxins immobilize them so that they can eat them whole. A study conducted by McDermott & Roe (1985), found that the macrophagous category of feeding is more likely to indicate a scavenging lifestyle. There are three steps in the feeding process of nemertean worms. The first step involves making contact with the prey. Observations conducted by Roe (1970), suggested that Paranemertes has little or no distance chemoreception for finding prey and that the prey had to come into contact before the nemertean responded. This would indicate that some nemertean worms need to somehow run into their prey by chance in order to find food. Other studies have shown some evidence that other nemertean worms do in fact seem to use a type of chemoreceptor to hunt their prey. Once nemertean worms find their prey they must capture it. The proboscis is the major component of this step of feeding. As mentioned above the proboscis either penetrates the prey by using a stylet or by coiling around the prey immobilizing it with toxins. No matter how much stronger the prey is than the nemertean it will still become paralyzed within a few seconds to a few minutes after coming in contact with the proboscis (Roe, 1970). The third step in the feeding process is the consumption of the prey by the suctorial or macrophagous patterns described above.
Nemertean worms contain a proboscis which is used to capture prey. The proboscis is a useful tool in identifying nemertean worms since it is unique to nemerteans. The different classes Anopla and Enopla have different proboscis anatomy which sets them apart. The way the nemerteans feed also helps us understand the purpose of the different types of proboscis and how they are used. Some of the important mechanisms that aid in the functionality of the proboscis were discussed in order to better understand how the proboscis works. It is evident that the proboscis is an important feature to the nemertean’s ability to capture their prey and is worthy of being recognized as a significant feature of nemertean worms.
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