Williams Lab
Hermit crab symbioses
Hermit crabs are some of the most conspicuous and ecologically important scavengers of intertidal and subtidal marine habitats worldwide. They compose an important link in the food chains of these habitats where they often attain large population sizes and act as food for other crustaceans and some fishes. The use of discarded gastropod shells by hermit crabs for protection of their abdomens (through a long evolutionary process leading to their soft, non-calcified abdomens) has led to a variety of interesting marine symbioses. In spite of the extensive behavioral studies completed on hermit crabs, much work remains in the exploration of the communities of hermit crab symbionts found living on, within and burrowing into inhabited shells.
Among hermit crab symbionts are examples of obligate, facultative and accidental associations. Hermit crabs provide a stable but motile substrate and thus provide access to important resources (e.g., oxygenated water, food) in addition to providing protection from predators and burial. In return, some mutualist symbionts (e.g., anemones) provide protection from predators for their hermit crab hosts or grow with their hosts and alleviate the need of hermit crabs to seek new shells as they grow. Although burrowing polychaete worms and barnacles were previously considered to be commensals of hermit crabs, we have found that some species prey upon the eggs of hermit crab hosts and should be considered periodic parasites. Finally, many true parasites, such as bopyrid isopods, pierce the body of host hermit crabs and feed on their blood. These ectoparasites can draining energy resources of their hosts to the point where the hermit crabs cannot reproduce. In total, there are over 550 species of commensal and mutualists associates of hermit crabs (see Williams and McDermott, 2004) and approximately 120 species that parasitize these hosts (McDermott, Boyko & Williams, in prep). Research in my lab is focused on the biology and taxonomy of these symbionts as well as some of their free-living relatives.
Figure showing the common epibiotic, endolithic, and free-living associates of the hermit crab Pagurus longicarpus from the east coast of the United States. Some symbionts such as the scaleworm Lepidonotus sublevis (top right), and the boring worm Dipolydora commensalis (bottom right) potentially prey upon the eggs of host hermit crabs but their impacts on host reproduction needs to be examined.
Polychaetes
The Spionidae is one of the largest polychaete families, occupying a variety of marine habitats worldwide. Within this family, many species of the genera Dipolydora and Polydora are known to bore into calcareous substrates, including gastropod shells inhabited by hermit crabs. Although polydorids in temperate and subtropical regions of the Atlantic and Pacific have been studied in detail, knowledge of these worms in the tropics (e.g., Caribbean and in the Indo-West Pacific) is limited. For example, only one polydorid species had been identified from the Philippines prior to my research that led to the identification of eight new species of polydorids and eight first records (Williams, 2000, 2001a, b, 2004, 2007). Similarly, in Jamaica there was only one previously described polydorid; however, preliminary research conducted in Jamaica has shown a new species of Polydora is found with the hermit crabs. The new Jamaican species exhibits a form of sexual dimorphism never before documented within this group. Males of this species are dwarfed, being much smaller than females and lacking accessory spines found in females (Orensky & Williams, in review).
Feeding biology
Certain species of Polydora and related genera of the family Spionidae are shell-burrowing polychaetes forming U-shaped burrows in calcareous substrates including commercially important bivalves. Research in my lab has focused on polydorid species which have been identified as facultative or obligate commensals of hermit crabs. While the association between hermit crabs and polydorids is well represented in the fossil record and the prevalence of these worms is often high, few studies have examined the ecology and symbiotic relationship between polydorids and host hermit crabs. My studies have attempted to more fully evaluate this symbiotic relationship by examining the feeding biology of these worms. To date I have found three species of polydorids that prey upon the eggs of host hermit crabs, including two species from the Philippines and one from the east Coast of the United States.
Feeding behavior of Polydora robi - predation on hermit crab embryos
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Pre-contact of embryo, palps of worm extended. |
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Contact and transport of embryo by the palps. |
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Engulfment of embryo. |
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Ingestion of embryo following breakage of the attachment stalk. |
Click here to see feeding behavior of Dipolydora commensalis (from Williams & McDermott, 1997).
Reproduction
We are also interested in the reproduction and larval development of polydorids. Polydora spp. produce elongate spermatozoa which are packaged into spermatophores. These spermatophores are transferred to females for fertilization of eggs during production of egg capsules. Investigations of Polydora robi Williams, 2000 from the Philippines showed this species exhibited the highest fecundity of any polydorid to date (over 8000 eggs were produced by the largest female examined). Fertilized eggs are deposited in capsules produced by females and deposited within the burrow. Larvae develop within these capsules until the three segment stage at which time they are ready to be released. The larvae break free from the capsules and will develop in the plankton until approximately the 20 segment stage. They then settle on a hermit crab shell and move to the apex, eventually producing a burrow.
Life-cycle diagram of Polydora robi, from cover illustration of Invertebrate Biology.
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Schematic diagram of the life-cycle of Polydora robi. In the center is a gastropod shell inhabited by an ovigerous hermit crab, Calcinus gaimardii. A large female of P. robi occupies a burrow in the apex. Egg capsules are shown at bottom left, three-segment stage larvae are shown in upper left, and a juvenile settling on the shell is shown at bottom right. |
Other polydorids such as members of the genus Polydorella, reproduce asexually. Worms in this genus are small, obligate commensals of sponges. Polydorella kamakamai Williams, 2004 is associated with sponges from coral reef areas of the Philippines and undergoes asexual reproduction via paratomy. In this mode of asexual reproduction the worms develop a growth zone following segment 10 and ultimately produce a chain five or more individuals. Division of these chains allows for colonies of these worms to quickly utilize the surface of sponge, on which they produce mucus tubes. Sexual reproduction is rare in this genus although a few individuals of Polydorella kamakamai have been documented with eggs in segments 13–15.
Parasitic Isopods
Two subfamilies of bopyrid isopods (Isopoda: Bopyridae) are known to infect hermit crabs. Members of the subfamily Pseudioninae infect the branchial cavities of hermit crabs while the subfamily Althelginae are abdominal parasites of hermit crabs. Among these subfamilies there are over 185 species worldwide. Dr. Christopher B. Boyko (Molloy College and Research Associate at the American Museum of Natural History) and I are completing studies on abdominal and branchial parasitic isopods associated with hermit crabs. Specimens collected from the Bahamas, Easter Island, Indonesia, and Polynesia have led to the description of new species of these parasites from hermit crabs and other hosts. We are currently working on a review of bopyrids from the Philippines, including phylogenetic analyses to examine the shared evolutionary history between the parasites and host hermit crabs.
Dorsal view of female Asymmetrione sallyae Williams & Schuerlein, 2005 from Singapore, specimen 4.0 mm in length. While males are easily recognizable as isopods, females are highly modified (and asymmetrical as the genus name implies) for their existence as ectoparasites of crustaceans. Many undescribed species of bopyrids exist in the Indo-West Pacific.
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Pseudostegias macdermotti Williams and Boyko, 1999 The specific name is in honor of Dr. John J. McDermott of Franklin and Marshall College. |
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Pseudionella akuaku Boyko and Williams, 2001 The specific name akuaku is derived from the Rapa Nui word for supernatural beings who sometimes assumed animal shapes. |
Burrowing Barnacles
The burrowing barnacles (Crustacea: Cirripedia: Acrothoracica) inhabit a wide range of calcareous substrates, including corals and mollusc shells. Within the acrothoracica, the family Trypetesidae contains seven extant species in two genera that are unique in being obligate commensals of hermit crabs (Williams & Boyko, 2006), forming burrows in the gastropod shells occupied by these crab hosts. Acrothoracican borings in gastropod shells inhabited by hermit crabs date back to the Miocene.
The feeding behavior and nutritional resources of trypetesids remain a mystery, although it has been suggest that trypetesids may feed on food particles dropped by hermit crabs or the feces of hosts. While previous researchers have indicated that trypetesids could not engage in a macrophagous feeding mode, our studies conducted in Jamaica and the Philippines have led to the documentation of hermit crab egg predation by these barnacles. Williams & Boyko (2006) examined the microstructure of the cirri in the genus Tomlinsonia and showed that the protuberant “cushion” on the second segment of cirri is composed of rows of minute bladelike denticules. These denticules possibly serve a function in manipulation of food particles. Future research will explore the feeding biology of trypetesids through gut content analysis of specimens and examination of feeding behavior to determine their mode of feeding and impact on hosts as egg predators.
The burrowing barnacle,Tomlinsoniamaclaughlinae Williams & Boyko, 2006. A) T.maclaughlinae in a Drupella cornus shell cut away to show the burrow. Aperture of the barnacle shown by arrowhead. B) The female barnacle removed from its burrow (the small male barnacle is attached to the large female barnacle toward the upper left). C) Lateral view of terminal cirrus of female barnacle, arrowhead shows position of cirral “cushion” that is covered with blade-like denticules of unknown function, perhaps used for manipulation of food particles.
Described Species
Polydora neocaeca Williams & Radashevsky 1999
Pseudostegias macdermotti Williams & Boyko 1999
Polydora robi Williams 2000
Pseudionella akuaku Boyko & Williams 2001
Polydora umangivora Williams 2001
Polydora mabinii Williams 2001
Minimathelges nanus Boyko & Williams 2003
Pseudione quasimodo Boyko & Williams 2004
Hemiarthrus surculus Boyko & Williams 2004
Polydorella kamakamai Williams 2004
Schizobopyrina bombyliaster Williams & Boyko 2004
Cabirops bombyliophila Williams & Boyko 2004
Asymmetrione sallyae Williams & Schuerlein 2005
Pseudione kensleyi Williams & Schuerlein 2005
Tomlinsonia maclaughlinae Williams & Boyko 2006
Hemioniscus pagurophila Williams & Boyko 2006
Scolelepis alisonae Williams 2007
Scoelelepis magnicornuta Williams 2007
Scoelelepis villosivaina Williams 2007
Dispio latilamella Williams 2007
Current Students
Isah Dualan (M.S., anticipated 2008) – Palp growth and regeneration of the hermit crab symbiont, Dipolydora commensalis.
Kristina Fittipaldi (M.A., anticipated 2008)– The relationship between Toxoplasmosis and Schizophrenia.
Drew Lynford (M.S., anticipated 2008) – Biology and treatment of flatworm pests important in the marine aquarium trade.
Asma Madad (M.S., anticipated 2008) – Branchial parasitic isopods (Isopoda: Bopyridae: Pseudioninae) associated with hermit crabs from the Philippines.
Angela Marucci (M.S., anticipated 2008) – Taxonomy and feeding biology of burrowing barnacles (Cirripedia: Trypetesidae) associated with hermit crabs.
Juliann Ryan (B.S., anticipated 2008) – Invertebrates in the diet of gulls from Long Island, New York.
Past Students
Beth McGuire (M.S., 2007) – Use of partially predated gastropods shells, shell choice behavior, and symbionts of Pagurus longicarpus from Long Island, New York.
Lara Orensky (Luzak) (B.S., 2007) – Morphology and ecology of a new sexually dimorphic species of Polydora (Polychaeta: Spionidae) associated with hermit crabs from Jamaica, West Indies.
Diana Connolly (B.S., 2004) – The ecology and morphology of the boring worm Polydora websteri associated with the slipper shell Crepidula fornicata from the Long Island Sound.
Frank Suppa (B.S., 2004) – Feeding biology and ecology of Lepidonotus sublevis (Polychaeta: Polynoidae), a polychaete commensal of hermit crabs.
Lauren Schuerlein (B.S., 2004) – Two new species of branchial parasitic isopods (Crustacea: Isopoda: Bopyridae: Pseudioninae) from hermit crabs collected in Singapore.