Ecology of Branchiobdellida (Annelida: Clitellata) in North America
by Josh Schulze
BIOL/WATER 361, Fall 2012
Branchiobdellids, commonly known as crayfish worms, are a subclass of annelids (Annelida: Clitellata) that spend their entire lives on other freshwater invertebrates. They are found commonly across the northern latitudes in almost any body of water that contains suitable hosts. Despite this wide distribution, branchiobdellids often go unnoticed because they are of little economic concern. They do however play an interesting ecological role in the freshwater ecosystems they inhabit. The following essay examines branchiobdellid ecology in North America with emphasis on their life history, distribution, taxonomy, and the nature of their host relations.
Branchiobdellids have a unique life history among annelids in that there are no known free-living forms — they spend every stage of their lives on other freshwater invertebrates (Williams et al., 2012). They most commonly colonize crayfish, but are not host specific. In fact it is not uncommon for many different branchiobdellid species to occur together on a single host (Brown & Creed, 2004). The only factor that seems to influence a branchiobdellid’s willingness to occupy it is how frequently it molts. Any eggs still attached to a molted exoskeleton will be lost, as will any juvenile/adult branchiobdellids that are unable to move from the shed exoskeleton back onto the host. Aside from crayfish, branchiobdellids have been known to colonize isopods and even crabs and shrimp when they occur in relatively fresh waters (Keller, 1992).
The branchiobdellid body plan resembles that of other annelids, albeit very small in scale. Branchiobdellids range in size from approximately 1-12 mm, and have a segmented body that lacks appendages or chetae. The first 4 segments are fused to from the head region, while the remaining 11 make up the trunk. Often the last few segments are spaced so closely that they are indistinguishable from one another. Both the anterior and posterior ends of the animal have adhesive discs for attaching to their hosts. Locomotion is therefore achieved via looping, or “inching” along similar to the movement of an inchworm. Their feeding structure is of a pair of sclerotized jaws that support a very generalized diet. Branchiobdellids feed on any other invertebrates small enough to fit into their jaws such as zooplankton. They also graze on unicellular algae and other organic detritus that collects on the exoskeleton of the host (Gale & Procter, 2011). Reproduction is accomplished sexually, with hermaphroditism being extremely common. Fertilized eggs are placed in cocoons and then attached directly to the exoskeleton of their host (Keller, 1992).
Branchiobdellids have a Holarctic distribution and occur widely in waters across North America. Because of the obligate nature of their host relationship, they can only occur in bodies of water that support other invertebrate communities (Gelder et al., 2002). They are primarily associated with crayfish, but have been observed colonizing Blue Crabs in the estuarine environments of the Gulf Coast and Chesapeake Bay. In these areas the ranges of euryhaline crayfish species overlap with that of the Blue Crab when they migrate into low saline estuaries. In some cases the crayfish inadvertently transfer some of their branchiobdellid load to the crabs. The new partnership is short-lived however, as the branchiobdellids must either transfer back to a crayfish or die when the Blue Crabs migrate back out into the ocean. Branchiobdellids have never been observed in salinities greater than 3.3 ppt (Gelder & Messick, 2006).
The taxonomy of branchiobdellids in North America is a field that is in need of further study. The current higher classification system is as follows:
These 4 North American families represent 16 genera and 105 species, with the Family Cambarincolinae being the most speciose by far. This morphology-based phylogeny has recently come into question in light of molecular data. The most complete molecular analyses to date indicate that none of the above families are monophyletic (Williams et al., 2012). The primary reason classification based on physical traits fail is because branchiobdellids are generally very morphologically cryptic. Positive identification at the species level is often only possible via dissection and examination of the reproduction organs.
The nature of the branchiobdellid-host relationship is presently very controversial as experimental evidence exists in support of several hypotheses. The first hypothesis is that the relationship is commensal, whereby the presence of the branchiobdellids has no effect on the host and they in turn do not derive significant benefits from their host. The second hypothesis is that both the branchiobdellids and hosts benefit in a mutualistic relationship. The third hypothesis is that of parasitism, where the branchiobdellids are feeding on or otherwise decreasing the overall fitness or their hosts.
The commensalism hypothesis was supported originally by observational data, but has gained support from experimental data as well in more recent years. Branchiobdellids have long been observed living on crayfish with no apparent decrease in host fitness or increase in host mortality. This notion has been tested experimentally by many independent groups that have compared the overall fitness of hosts with branchiobdellids present verses control groups of hosts without branchiobdellids (Keller, 1992). Very often there is no statistically significant difference in fitness of the two groups of hosts. These findings indicate that the relationship between branchiobdellids and their hosts in truly commensal.
Other experiments with the same basic design (monitor host fitness under varying branchiobdellid loads) have yielded strong support for the mutualism hypothesis. In these scenarios, the branchiobdellid densities reached a critical level where their feeding behavior actually benefited the hosts. By consuming micro-invertebrates and other organic debris from in and around the host’s gill chambers, the hosts showed increased fitness levels (Brown et al., 2002). These experiments demonstrate how the provision of food for the branchiobdellids and a cleaning service for the hosts strongly support the mutualism hypothesis.
In support of the parasitism hypothesis, branchiobdellids have been documented parasitizing their crayfish hosts to some degree in certain experimental settings. Parasitism was seen primarily under circumstances of extreme branchiobdellid density and/or during specific life stages in their life cycle. In one experiment, the effect of the branchiobdellids on the host became parasitic under extremely high densities (Brown et al., 2012). It was hypothesized that under these conditions the branchiobdellids ran out of their typical food sources and would resort to consuming small amounts of the host’s gill tissue. Branchiobdellids at high densities also appeared to restrict the hosts locomotion and ability to feed normally as well as make them easier targets for predation.
Further support for the parasitism hypothesis comes from branchiobdellid gut analysis. Host tissues have been found in the stomachs of juvenile worms but not adults, indicating that they may be parasitic only in the early stages of their development. In another study, researchers injected a radioactive chromium marker into the host crayfish to determine if the marker would subsequently be found in the gut of the branchiobdellids. The results found that only those that colonized the host’s gill chamber had the chromium marker in their stomachs, where as those living elsewhere on the crayfish exoskeleton did not. These findings indicate the possibility of parasitism occurring only when the branchiobdellids occupy specific parts of the host’s body (Jennings & Gelder, 1979).
The relationship between branchiobdellid worms and their invertebrate hosts is clearly dynamic and defies swift classification. Both observational and experimental evidence can be obtained in support of several hypotheses, including commensalism, mutualism, and parasitism. After considering all the data collected by various researchers, it appears that a single type of relationship does not exist all the time, but rather there is a continuum between mutualism and parasitism that is dictated by the density at which the worms colonize an individual host, and also on which parts of the host the branchiobdellids happen to occupy.
The information in the preceding essay encapsulates the primary aspects of the ecology of branchiobdellids in North America. They have a unique life history that is directly tied to other freshwater invertebrates, ubiquitously occupy the waters of North America, and exhibit rather varying effects upon the hosts they colonize. It can clearly be seen that the importance of the branchiobdellid subclass of annelids cannot be stated in economic terms, but rather in terms of the unique role they play in our freshwater ecosystems.
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