Tantulocarida

(Tantulocaridans)

Phylum Arthropoda

Subphylum Crustacea

Class Maxillopoda

Subclass Tantulocarida

Number of families 4

Thumbnail description
Tiny parasitic crustaceans that spend most of their lives attached to the external body surface of their hosts, a wide range of other marine crustaceans

Evolution and systematics

The subclass Tantulocarida is currently classified in the class Maxillopoda, and it is regarded as most closely related to the barnacles (the Thecostraca), with which it shares a similar body plan and a similar position of genital openings in both sexes. The Tantulocarida comprises four families, 22 genera, and about 30 species. No orders have been established for this group.

Physical characteristics

The adult asexual female consists of a minute head, a neck of varying length, and a sac-like trunk full of eggs or developing tantulus larvae. This is the largest stage in the tantulocaridan life cycle and may attain lengths of up to 0.08 in (2 mm). It is attached to the exoskeleton of its host by a tiny oral sucker, about 0.000472–0.000591 in (12–15µm) in diameter. This stage has no limbs at all and no genital apertures, and it appears to release mature larvae by rupturing the trunk wall. The sexual female is less than 0.02 in (0.5 mm) in length and consists of a large cephalothorax and a five-segmented postcephalic trunk. The cephalothorax carries a pair of sensory antennules but no mouthparts. A small number of large eggs lie within the cephalothorax, and it also carries a conspicuous median genital opening, interpreted as a copulatory pore. The first two of the trunk segments each carry a pair of biramous thoracic legs, which appear to be used for grasping, and the fifth segment bears the elongate caudal rami. The adult male resembles the sexual female in size and basic body plan, with a large cephalothorax and a six-segmented trunk, but it has more pairs of limbs: vestigial sensory antennules, six pairs of biramous swimming legs, a well-developed median penis, and the caudal rami. Adults of both sexes develop within posteriorly located, sac-like expansions of the trunk of the attached tantulus larva.

Distribution

Knowledge of tantulocaridan distributions is incomplete, in part because they are often overlooked due to their minute body size. Species have been reported from the North and South Pacific and North and South Atlantic oceans, as well as from both Arctic and Antarctic waters.

Habitat

Tantulocaridans spend most of their lives attached to their hosts, which include isopod, tanaid, amphipod, cumacean, ostracod, and copepod crustaceans. The dispersal and infective stage in the life cycle, the tantulus larva, has also been found living free in marine sediments. The sexual adults have never been collected away from the host, but probably inhabit the hyperbenthic zone, just above the sea bed.

Behavior

Little is known of tantulocaridan behavior. After release from the mother, infective larvae spend some time in the sediment before encountering a suitable benthic or hyperbenthic host. Host location and attachment mechanisms are poorly understood in these forms, which lack eyes and antennules, the main sensory interfaces of other crustaceans.

Feeding ecology and diet

Tantulocaridans are ectoparasitic and do not appear to feed away from their hosts. They attach to the external skeleton of their hosts by means of an adhesive oral disc. The host surface is punctured by a stylet, which is protruded out through a minute pore in the center of this disc. Nutrients are obtained via the puncture into the host. There is evidence of an absorptive rootlet system extending from the oral disc of the tantulocaridan and penetrating through the tissues of the host. Tantulocaridans exhibit varying degrees of host specificity: for example, members of the family Deoterthridae occur on cumacean, isopod, tanaid, amphipod, ostracod, and copepod hosts, members of the Microdajidae on tanaid hosts only, members of the Doryphallophoridae on isopods only, and members of the Basipodellidae on copepods only.

Reproductive biology

Tantulocaridans have a bizarre double life cycle, involving a sexual phase and an asexual phase. The asexual phase is encountered much more frequently than the sexual phase. The sac-like asexual female releases fully formed tantulus larvae, which are capable of infecting a new host and developing directly into another asexual female, without mating and without even molting. The tantulus larva is minute, ranging from 0.00335 in (85µm) to about 0.00709 in (180 µm) in length. It comprises a head, which has an oral disc but lacks any limbs, and a trunk of six leg-bearing segments and a maximum of two limb-less abdominal segments. The swimming legs are biramous and have reduced endites. After successfully infecting a host, the tantulus larva develops into the asexual female, and the postcephalic trunk of the larva is shed, so the female remains attached to the host by the adhesive oral disc of the preceding larval stage. The trunk of the female expands to accommodate the growing larvae until they are released.

In the sexual phase, the cycle again begins with the infective tantulus larva attaching to its host by the oral disc. A saclike expansion begins to grow near the back of the trunk, within which either an adult male or an adult female then develops. The precise location of this expansion varies according to family. Both are supplied with nutrients from the host, transported via an umbilical cord originating in the still-attached larval head. Fully formed adults develop within the sac, which remains attached to the host by the oral disc of the larva. On reaching maturity these sexual adults are released by rupturing of the sac wall. These sexual stages have never been observed alive, but it is assumed that the male, which has well-developed swimming legs and a large cluster of antennulary chemosensors, actively searches out and locates the receptive female. The male carries a large penis and presumably inseminates the female by the single ventral copulatory pore. The fertilized eggs are presumed to develop within the expandable cephalothorax of the female until ready to hatch, as a fully formed tantulus.

Conservation status

Information on the abundance and distribution of tantulocaridans is fragmentary. No species are listed by the IUCN.

Significance to humans

Tantulocaridans have no known significance to humans.

Species accounts

List of Species

No common name

Itoitantulus misophricola

family

Deoterthridae

taxonomy

Itoitantulus misophricola Huys, Ohtsuka, and Boxshall, 1992, Okinawa, Japan.

other common names

None known.

physical characteristics

Tantulus larva have well-developed rami, with endites on second to fifth pairs of swimming legs; and sixth leg with recurved apical spine. Trunk sac containing developing sexual male formed posterior to sixth thoracic tergite. Adult male with recurved penis, unsegmented abdomen, and distinct caudal rami.

distribution

Okinawa in southern Japan to the Philippines.

habitat

Parasitic on benthic or hyperbenthic copepods living at depths of 550–6,725 ft (167–2,050 m).

behavior

Nothing is known.

feeding ecology and diet

Parasitizing copepods of the orders Harpacticoida (family Styracothoracidae) and Misophrioida (family Misophriidae).

reproductive biology

This was the first tantulocaridan for which the complete double life cycle was elucidated.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Microdajus langi

family

Microdajidae

taxonomy

Microdajus langi Greve, 1965, Norway.

other common names

None known.

physical characteristics

Tantulus larva have reduced rami and lacking endites on swimming legs. Trunk sac containing developing sexual male formed posterior to sixth thoracic tergite. Adult male with slender stylet-like penis, with unsegmented abdomen and with distinct caudal rami.

distribution

Southern Norway to the west coast of Scotland.

habitat

Parasitic on tanaid crustaceans in marine sediments, at depths of 70–430 ft (22–130 m).

behavior

Nothing is known.

feeding ecology and diet

Specific to tanaid hosts of the family Leptognathiidae.

reproductive biology

Sexual female stage unknown.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Periodicals

Boxshall, G. A., and R. J. Lincoln. "The Life Cycle of the Tantulocarida (Crustacea)." Philosophical Transactions of the Royal Society of London B315 (1987): 267–303.

Huys, R., G. A. Boxshall, and R. J. Lincoln. "The Tantulocaridan Life Cycle: The Circle Closed?" Journal of Crustacean Biology 13 (1993): 432–442.

Geoffrey Allan Boxshall, PhD