How To Use This Resource

Our goal in making this website was to create an electronic field guide with which to identify trematodes as well as provide additonal species-specific information. Trematodes are present in every environment and are readily found as long as you take the time to look through the "catch of the day" or latest hunting prize. The available resources were thus developed for use by scientists and non-scientists alike in the hopes of inspiring laypeople (especially amateur naturalists and teachers) to look for trematodes in their own backyards. The Trematode Field Guide can be split into three main parts:

Backyard Sampling
  1. The Species tab, which includes a list of the current species represented on the site and is searchable by both host and parasite name. Each individual species page includes information on identifying features, hosts infected, life cycle pattern (if known) and images.

  2. The Resource tab, which provides helpful tools for teachers to incorporate trematodes into their curriculum, distribution maps of known sites of Alloglossidium spp. for amateur naturalists, a literature list for futher reading about Alloglossidium spp., and a general image gallery.

  3. The Links page, which provides links to external databases that are useful for general parasite identification as well as further information about host species.

Return to Top

What Are Trematodes?

Trematodes, or "flukes", are parasitic flatworms belonging to the phylum Platyhelminthes. Typically a few centimeters in length (although size can vary greatly), trematodes can be recognized by a flattened-oval shape. The Trematoda, which is comprised of approximately 18,000 species, can be further split into two groups (sub-classes). The aspidogastrea (or aspidobothrea), which can be distinguished by a single large ventral sucker, contain roughly 80 species that parasitize freshwater and marine molluscs, fish, and turtles. The digenea incorporate the remainder of the trematode species and can be identified by the presence of two suckers, one located near the mouth (oral sucker) and one on the underside of the fluke (ventral sucker). The digenea also infect wider variety of invertebrate - including molluscan, crustacean, and leech hosts - as well as vertebrate organisms.

Trematodes can also be classified based on the system they infect within their vertebrate host or even the environment in which their host is found. For instance, tissue flukes inhabit biological tissues including organs, muscle tissue, and the digestive tract. Blood flukes, on the other hand, infect the blood of their host at some stage during their life cycle.

Life Cycles:

Typically trematodes are hermaphroditic (having both male and female organs) and are able to alternate between sexual and asexual reproduction - although some exceptions do exist (the aspidogastrea only reproduce sexually while the digenean schistosomes have separate sexes). For many trematodes, sexual reproduction occurs in the final, or definitive, vertebrate host. Eggs, having been shed along with feces, release free-swimming larva that can infect a first intermediate (molluscan) host. While inhabiting the mollusc, asexual reproduction increases parasite numbers and thus the likelihood of infecting the next link in the cycle. Individuals can then be passed onto the next intermediate host via ingestion (predator eating infected prey) or penetration.

The aspidogastrean life cycle is notably different from that of the digeneans in that there are no multiplicative larval stages; rather, many aspidogastreans incorporate a direct life cycle (in molluscs) with only those species parasitizing vertebrates requiring an intermediate host. The digenean life cycle is more complex, typically incorporating 3-hosts (including a vertebrate final host) and asexual reproduction during larval stages. However, there are always exceptions to the rule. For instance, a number of life cycle patterns are utilized by species in the genus Alloglossidium includng a traditional 3-host life cycle as well as truncated 2-host life cycles whereby the parasite reaches sexual maturity in an invertebrate host. For more information, including life cycle diagrams, browse our tools for teachers section.

Return to Top

Why Study These Worms?

Although parasitism is one of the most common lifestyles among eukaryotic organisms, there remains a significant amount of undescribed parasite biodiversity. Trematodes are not immune to this predicament - one global estimate hypothesizes that there are between 25,000 and 50,000 digenean flukes from fishes, yet to date there are only ~5,000 named digenean species from fishes. Additionally, trematode biodiversity is represented by variation in natural history (including complexity of life cycles and geographic distributions). By striving to answer the deficiencies in our current understanding of trematode diversity we can gain better insight into ecosystem processes as well as environmental health and allow us to better address economic and public health concerns. Furthermore, by creating a more complete picture of trematode species, we begin to use more parasite systems as evolutionary models to address patterns of diversification.

Return to Top



The material is based upon work supported by the National Science Foundation under Grant Number 1145508. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.