|
|
Alexander Archibald-Binge 2016
|
|
|
|
Summary | |
Nemerteans (ribbon worms) are colourful flatworm-like predators that lack segmentation and inhabit marine environments world-wide, as well as some terrestrial and freshwater habitats (Ruppert et al. 2004). Ribbon worms vary from microscope creatures to some of the longest animals in existence. The focal species of this description is unknown but is of the Lineus genus (figure 1). This is a primitive member of the anoplan class within nemertea, which has approximately 500 species known to date (Gibson 1995). The defining characteristics of this species is an unarmed proboscis, rhynchocoel, and a long slender body form with horizontal slits at the sides of the head. This species has a complete digestive system, a coelomic circulatory system and uses protonephridia for excretion. The respiration system occurs across the body surface and no gills are required (Ruppert et al. 2004). Nemerteans are bilateral protostomes that are sister taxa to the trochozoans. Nemerteans are organisms that are fortunate to continue to thrive and aren’t considered to be under threat or endangered.
|
 |
| Figure 1 |
|
|
|
Physical Description |
Identification | |
Due to difficulties with identification after this Nemertean was preserved, the species was unable to be identified. The following range of identification techniques were used to narrow the species of focus to the family of Lineidae. A lack of stylet on the proboscis along with the mouth being posterior to the brain demonstrated that this species belonged to class Anopla. The order of Heteronemertea was then determined as the organism displayed four layers of body-wall musculature. To determine the family the organism then had to be analysed further. The presence of a horizontal slit down each side of the head indicated that it was part of the Lineidae family. Once the family was known, the genus was determined using a reworked key (Loosanoff & Engle, 1943). The key showed that a long and slender body form (though this species wasn't too long) that was also very contractile placed this species in the Lineus genus.
|
|
|
Morphology | |
The focal organism of this species description was approximately 10-12cm long, 0.5cm wide and was a light orange/yellow colour, with a darker orange area visible at the anterior end of its body (figure 2). This anterior section was rounded. As previously mentioned, a closer look at the anterior end revealed horizontal slits, along with the mouth being posterior to the brain. The nemertean somewhat resembled an annelid however no segmentation could be observed. Analysis showed an absence in a cuticle which is common across the whole nemertea phyla. There was also an absence of ocelli, despite ocelli being present in most species of the Lineus genus (Loosanoff & Engle, 1943). Previous studies suggest that the most useful morphological features that assist in identifying genera in lineidae include the anatomy of the proboscis, rhyncocoel wall and dermis, the presence of neurochord cells and a caudal cirrus and also the type of musculature that can be found near the foregut (Gibson, 1981). Another morphological feature of ribbon worms is that they are often covered in mucus.
|
 |
| Figure 2 |
|
|
|
|
Ecology | |
Generally nemerteans are seen as predators that ambush they’re prey with a proboscis that is either venomous, penetrating or sticky (Ruppert et al. 2004). Despite this dominant display of predatory interactions in their natural environment, nemerteans do still play a role as prey to other creatures that are higher up the food chain. This was evident in a study in which an analysis of various fish stomachs was undertaken. Although low, there was a small percentage of stomachs that held nemertean species (McDermott, 2001), thus demonstrating nemerteans may still act as prey for other species. When considering the ecology of a nemertean species, it is important to consider interactions not only between it and other animals, but also the interaction between the species and the environment it lives in.
Nemerteans are mostly marine invertebrates, however there are a few species that are found in freshwater and some that can be found in damp terrestrial habitats. The fact that most nemertean species are found in a marine environment is reflected in their apparent widespread distribution across marine habitats. Ecological niches that nemerteans can inhabit include interstitial, commensal, parasitic and pelagic forms (Gibson, 1995). They are most commonly found in benthic environments as is the case in this example. The focal species of this description was sourced from coral fragments taken from Heron Island, and is therefore a marine species. Ribbon worms can be found in most marine habitats, although usually in a low abundance (Thiel & Kruse, 2001). This species of the Lineus genus was found in a cryptic, dead coral environment. It appeared to be using the dead coral as a shelter by burrowing into it, possibly to avoid predators or to catch prey off guard.
|
|
|
Life History and Behaviour |
Reproduction | |
Nemerteans possess the ability to reproduce asexually (e.g. clonally) and also sexually (Ruppert et al. 2004). The ability to reproduce in multiple ways increases the complexity of the reproduction cycles seen in the life history of nemerteans. Fragmentation is an asexual form of reproduction that has been found to occur particularly in the Lineus genus (Ruppert et al. 2004). This is a method that can be routine, and is likely to happen when an organism is irritated. Sexual reproduction has also been observed in the Lineus genus, with one species being found to reproduce annually in successive years (Lineus viridis). In this example the mating system was found to be polyandric, due to the female creating a cocoon in which herself and several male mates reproduce through internal fertilisation (von Doehren et al. 2012). It also should be noted that some nemertean species are hermaphroditic, however majority of these are found in terrestrial or freshwater environments (Jenner, 2004).
|
|
|
Respiration/Gas Exchange | |
The nemerteans are a phyla that all lack specialized gills, with respiration and gas exchange occurring across their body (Ruppert et al. 2004). A characteristic that helps to make this process possible is their body shape – if they are a larger form then they are often flattened out so that this gas exchange can still occur. They have a coelomic circulatory system consisting of rhyncocoel and peripheral vessels (Ruppert et al. 2004). These vessels are responsible for whole body circulation as well as specialised local circulation.
|
|
|
Feeding | |
As mentioned earlier, nemerteans are predators that capture their prey using a proboscis apparatus (Ruppert et al. 2004). The Lineus genus is within the anopla class, thus having an unarmed proboscis (stylet absent) which is often covered in sticky papillae. Nemerteans in the enopla class however have a proboscis that is armed with a weapon-like barb called a stylet (Ruppert et al. 2004). Species of nemertea within the order of heteronemertea (including Lineus) are known to prey predominately on polychaetes, whereas hoplonemerteans prey more commonly on small crustaceans (Thiel & Kruse, 2001). Many nemerteans roam about at times of the day that are appropriate for hunting and have the ability to take down a high variety of prey using powerful neurotoxins.
|
|
|
Locomotion | |
A high percentage of ribbon worms are able to move across a surface using epidermal cilia, while gliding on a slime trail that’s partially secreted by cephalic glands on the head (Ruppert et al. 2004). Species of the Lineus genus have been found to have the ability to extend up to 10 times the resting length of their body. An example of the Lineus sp. moving around can be seen in the video below.
|
|
|
Lineus sp. filmed through a microscope demonstrating its method of locomotion.
|
|
|
|
|
Anatomy and Physiology | |
The defining characteristics of nemertea include the proboscis and rhynchocoel (figure 3). As mentioned above, the proboscis of heteronemerteans are unarmed and are found in the rhynchocoel which is a coelomic cavity filled with fluid (Ruppert et al. 2004). Ribbon worms have a digestive system that consists mouth, foregut stomach, intestine and anus. Excretion in this phyla occur through two or more protonephridia that are generally restricted to the anterior foregut part of the body (Ruppert et al. 2004). The circulatory system of nemerteans is coelomic. In this system there are two cephalic vessels that join at the ends to form a U-shape, and there is no heart needed to pump these vessels (Gibson 1981). Nervous system structure is generally similar across lineid heteronemerteans, with a well-developed brain that has dorsal and ventral portions that are separated from ganglionic cell layers by connective tissue (Gibson 1981). A sagittal cross section of the focal species demonstrated anatomical features expected of heteronemerteans. There was evidence of ciliated epidermis, circulatory vessels can be seen and coelomic cavities are also visible (figure 4).
|
 |
| Figure 3 |
|
 |
| Figure 4 |
|
|
|
Biogeographic Distribution | |
Nemerteans are distributed world-wide (figure 5) due to their ability to live in a variety of marine environments. These environments range from intertidal to deep see environments and include marine, benthic or littoral species (Wright 2014). The focal species of this description was found in the tropical location of Heron Island and was displaying a benthic lifestyle.
|
 |
| Figure 5 |
|
|
|
Evolution and Systematics | |
Nemertea and Platyhelminthes were long believed to be sister taxa due to the characteristics of; a common flat shape, absence of a cuticle, ciliated epidermis, protonephridia and also an eversible proboscis (Ruppert et al. 2004). It is now commonly believed however that these characteristics were all convergence throughout the two phyla’s evolutionary history, and that nemerteans are actually the sister taxa of trochozoans, chaetognathans and gnathiferans (figure 6). The presence of an anus along with the occurrence of a coelom are the two major characteristics that unite these taxa (Ruppert et al. 2004). There is also evidence that sacular gonads are a synapomorphy for Nemertea and Trochozoa (Jenner 2004).'
Within the phyla of Nemertea members of the anopla class are believed to be primitive whereas the enoplans are seen as more derived organisms (figure 7). This means that the species in focus is considered to be a more primitive organism. The main characteristic that makes enopla more derived is the development of nerve cords internal to body-wall musculature (Ruppert et al. 2004).
|
 |
| Figure 6 |
|
 |
| Figure 7 |
|
|
|
Conservation and Threats | |
There have been no studies that suggest that the nemerteans are in danger of becoming threatened or endangered (Wright 2014). Their ability to survive a variety of different environments reflects the fact that they don’t seem to be under any immediate threat, however alteration of environments is bound to somehow effect their behaviour.
|
|
|
References | |
Gibson, R. (1981). Nemerteans of the Great Barrier Reef: 3. Anopla Heteronemertea (Lineidae). Zoological journal of the Linnean Society, 71(2), 171-235.
Gibson, R. (1995). Nemertean genera and species of the world: an annotated checklist of original names and description citations, synonyms, current taxonomic status, habitats and recorded zoogeographic distribution. Journal of Natural History 29, 271-562.
Jenner, R. A. (2004). Towards a phylogeny of the Metazoa: evaluating alternative phylogenetic positions of Platyhelminthes, Nemertea, and Gnathostomulida, with a critical reappraisal of cladistics characters. Contributions to Zoology, 73(1-2), 3-163.
Loosanoff, V. L., & Engle, J. B. (1943). Polydora in oysters suspended in the water. Biological bulletin, 85(1), 69-78.
McDermott, J. J. (2001). Status of the Nemertea as prey in marine ecosystems. Hydrobiologia, 456, 7-20.
Ocean Biogeographic Information System. Available from: http://www.iobis.org/mapper/?dataset=1591
Ruppert, E. E., Fox, R. S., & Barnes, R. D. (2004). Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition.
Thiel, M., & Kruse, I. (2001). Status of the Nemertea as predators in marine ecosystems. Hydrobiologia, 456, 21-32.
Von Doehren, J., Beckers, P. & Bartolomaeus, T. (2012). Life history of Lineus viridis (Muller, 1774) (Heteronemertea, Nemertea). Helgoland Marine Research, 66(3), 243-252.
Wright, J. (2014). "Nemertea" (On-line). Animal Diversity Web. Available from: http://animaldiversity.org/accounts/Nemertea/
|
|
|
|
|