Order HARPACTICOIDA Sars, 1903

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Introduction

Harpacticoid copepods are minute crustaceans in the order Harpacticoida, subclass Copepoda (Huys & Boxshall 1991), class Maxillopoda. Worldwide, there are approximately 50 families and 460 genera of Harpacticoida. The total number of described species has been estimated to be between 3000 (Huys et al. 1996) and 4000 to 4500 (Giere 1993). Presently only 163 described species are recorded for Australia.

Harpacticoid copepods exhibit a diverse range of body shapes, but in general they are linear and range in length from 0.2 to 2.5 mm (Giere 1993); they represent a significant component of the meiofauna. Mare (1942) coined the word meiofauna to define an the assemblage of benthic invertebrates smaller than the macrobenthic fauna. Prior to the introduction of the word meiofauna, researchers had referred to small invertebrates as microfauna, however this term now refers largely to Protozoa. Meiofauna are defined as animals that pass through a 500 µm mesh sieve but are retained on 45 µm mesh.

Harpacticoids are found in marine, estuarine and freshwater environments and also inhabit the terrestrial realm occurring in mosses and leaf litter. The majority are free-living and benthic, although there are a few pelagic and symbiotic species (Hicks & Coull 1983). In the harpacticoid community there are family and generic level associations linked with particular habitat types. In the marine environment, small harpacticoids with elongate vermiform bodies live in the interstitial spaces between sand grains. Larger species that have a broad cephalothorax are known to burrow into sediments with special spade-shaped appendages (Hicks & Coull 1983). Epibenthic species that live on the sediment surface tend to be large and exhibit a variety of body shapes (Hicks & Coull 1983). Species living in algae and seagrasses, referred to as phytal or epiphytic species, also tend to be large and frequently have appendages modified for grasping plant material. Families that include phytal species exhibit various forms, some being flat and shield-shaped (e.g. Porcellidiidae and Peltidae) and others that have modified mouthparts such as a 'sucker-disk' which assists in adhesion to the plant surface (Giere 1993). Other phytal species are fusiform in shape and cling to plants with strongly prehensile first legs or mouthparts. Many phytal species are also good swimmers (Noodt 1971). More unusual marine habitats exploited by harpacticoids include deep-sea hydrothermal vents (Conroy-Dalton & Huys 1999) and the body cavities of other organisms such as sea-urchins (Huys 1995).

Harpacticoid copepods feed on diatoms, bacteria and protozoans which they strip (with their mouthparts) from phytal material, detritus and sand grains (Giere 1993). Food availability and water temperature are thought to be the prime factors influencing the reproduction and growth of harpacticoid copepods (Giere 1993). Reproduction takes place through copulation and involves the transfer of sperm from the male's spermatophore to the female oviduct. Prior to reproduction a male harpacticoid will grasp a female with his modified antennules. Coupling arrangements tend to be family specific. Males may grasp the third or fourth legs of the females (Hicks & Coull 1983; Huys et al. 1996), the caudal rami (Huys et al. 1996), the caudal setae (Dürbaum 1995), the posterolateral margin of the cephalothorax or around the genital double-somite (as in Porcellidiidae) (Huys et al. 1996). Precocious coupling of males and juvenile females is also common in harpacticoids. In these situations, a male attaches himself to a juvenile female (usually a copepodite IV or V) and stays with her until her final moult, when she is ready to mate. Males have been observed to release their hold on moulted exoskeletons of copepodite Vs, then reclasp the body of the newly matured female (Feller 1980). This behaviour ensures that the 'trailing' male is the first to mate with the female when she reaches maturity. Post-copulatory coupling has also been observed in harpacticoids and this is thought to prevent subsequent matings of the female with other males (Dürbaum 1995).

During copulation a spermatophore is extruded from the urosome of the male and attached externally to the female's urosome, close to the gonopore. An adhesive substance secreted from the spermatophore aids attachment in the genital region (Hosfeld 1994). The neck of the spermatophore grows to become the fertilisation tube (Dürbaum 1995) which enters the gonopore (also commonly referred to as the copulatory or genital pore) of the female. Sperm are then discharged from the spermatophore into the seminal receptacles of the female (Dürbaum 1995). Eggs are fertilised as they emerge into the genital antrum (Huys et al. 1996). Female harpacticoids carry their eggs in an external egg sac (ovisac) and these may be paired.

After an incubation period of one to eight days eggs hatch (Hicks & Coull 1983), and nauplii are released. In general, harpacticoids go through six naupliar stages and six copepodite (post-naupliar) stages, the last one being the adult form (Huys et al. 1996). The time this cycle takes varies and developmental rates are influenced by temperature, food supply and salinity (Hicks & Coull 1983). Some harpacticoid species reach maturity in just six days after hatching (Harpacticus sp.; Walker 1981) while others can take up to 62 days (Tigriopus fulvus (Fischer, 1860); Fraser 1936).

Harpacticoids in Australia
Little is known of the composition of the harpacticoid fauna of Australia. In a comprehensive book on meiobenthology, Giere (1993: p. 4) noted the lack of knowledge of Australian Harpacticoida stating that '…wide areas in Africa, South America, Asia and Australia remain terra incognita in the field of meiobenthology; their coast and inland meiobenthos still awaits description'. Only 163 species of Harpacticoida have been documented as occurring in Australia.

Harpacticoids as food for fish
In Port Phillip Bay, Victoria, Australia harpacticoids are the major food source for a range of juvenile fish including commercially important species such as King George whiting (Sillaginodes punctata (Cuvier, 1829; Jenkins et al. 1996). However, fisheries scientists have been restricted in their investigations of the relationship between meiofauna and juvenile fish because of difficulties associated with identifying harpacticoids beyond the taxonomic level of order. There are no keys or guidebooks to the harpacticoid fauna of Australia, and with so few Australian species described these would be of little use anyway. The major monograph on the Harpacticoida (Lang 1948) contains familial and generic diagnoses; it is written in German, and in many instances it is outdated. The recent contribution by Huys et al. (1996) of a key to the world's families has provided some identification assistance, but non-specialists may feel overwhelmed by the size of the key (147 couplets) and the need to examine mouthparts in some instances.{CONROYDAS1999A}{DRBAUMJ1995A}{HOSFELDB1994A}{HOSFELDB1994A}

• Harpacticoida Sars, G.O. 1903. Copepoda. Harpacticoida. Part I & Part II. Misophriidae, Longipediidae, Cerviniidae, Ectinosomatidae (part). pp. 1-28 in Sars, G.O. (ed.). An Account of the Crustacea of Norway, with short descriptions and figures of all the species. Bergen : Bergen Museum Vol. V pp. 489 [1].

  Compiled from secondary source:

   Boxshall, G.A. & Halsey, S.H. 2004. An Introduction to Copepod Diversity. London : The Ray Society 966 pp. [224-414] (excellent family level diganoses; lists included genera for each family and a key to genera within each family; this two-volume set is a must for anyone studying copepods); Huys, R. & Boxshall, G.A. 1991. Copepod Evolution. London : The Ray Society [113-151]; Huys, R., Gee, J.M., Moore, C.G. & Hamond, R. 1996. Marine and Brackish Water Harpacticoid Copepods. Shrewsbury : Field Studies Council Vol. 1 352 pp.

ID Keys

See Huys et al. (1996) and Boxshall & Halsey (2004) for keys to the families and genera of Harpacticoida.

References

Fischer, S. 1860. Beiträge zur Kenntniss der Entomostraceen. Abhandlungen der Bayerischen Akademie der Wissenschaften. Mathematisch-Naturwissenschaftliche Abteilung 8(3): 645-682

Fraser, J.H. 1936. The occurrence, ecology and life history of Tigriopus fulvus (Fischer). Journal of the Marine Biological Association, United Kingdom 20: 523-536

Giere, O. 1993. Meiobenthology. The Microscopic Fauna in Aquatic Sediments. Berlin : Springer-Verlag 328 pp.

Hicks, G.R.F. & Coull, B.C. 1983. The ecology of marine meiobenthic harpacticoids. Annual Review of Oceanography and Marine Biology 21: 67-175

Huys, R. 1995. A new genus of Canuellidae (Copepoda, Harpacticoida) associated with Atlantic bathyal sea-urchins. Zoologica Scripta 24(3): 225-243

Huys, R. & Boxshall, G.A. 1991. Copepod Evolution. London : The Ray Society

Huys, R., Gee, J.M., Moore, C.G. & Hamond, R. 1996. Marine and Brackish Water Harpacticoid Copepods. Shrewsbury : Field Studies Council Vol. 1 352 pp.

Jenkins, G.P., Wheatley, M.J. & Poore, A.G.B. 1996. Spatial variation in recruitment, growth and feeding of post-settlement King George whiting, Sillaginodes punctata, associated with seagrass beds of Port Phillip Bay, Australia. Canadian Journal of Fisheries and Aquatic Sciences 53: 350-359

Lang, K. 1948. Monographie der Harpacticiden. Lund, Sweden : Håkan Ohlssons Boktrycheri pp. 1682

Mare, M.F. 1942. A study of a marine benthic community with special reference to the micro-organisms. Journal of the Marine Biological Association, United Kingdom 25: 517-554

Walker, L.M. 1981. Reproductive biology and development of a marine harpacticoid copepod reared in the laboratory. Journal of Crustacean Biology 1: 376-388