Tantulocarida

Phylum Arthropoda Subphylum Crustacea Class Maxillopoda Subclass Tantulocarida Number of families 4

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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

Photo: The tantulus larva and globular adult female of Microdajus langi, a tantulocaridan parasite, attached to its host, a tanaidacean crustacean. (Photo by G. A. Boxshall. Reproduced by permission.)

(Tantulocaridans)

Photo: The tantulus larva and globular adult female of Microdajus langi, a tantulocaridan parasite, attached to its host, a tanaidacean crustacean. (Photo by G. A. Boxshall. Reproduced by permission.)

Tantulocarida

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 tantulo-caridan 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 (12-15pm) 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 post-cephalic 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, os-tracod, 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 en countered 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 (85pm) to about 0.00709 in (180 pm) 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 an-tennulary 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.

Millipedes Have Two Penises
1. Microdajus langi; 2. Itoitantulus misophricola. (illustration by Bruce Worden)
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