Isopods (Isopoda)

Isopods (Isopoda) are one of the biggest orders (by species richness) in the class of crustaceans (Crustacea). They inhabit very diverse habitats, from deep sea and fresh water to land, and include parasitic species. Out of 52 000 described species of crustaceans, more than 10 000 belong to the order of isopods. That includes around 5 000 sea species, around 950 freshwater and more than 4 000 terrestrial species, and each year more species are described. They are very old group of animals – the fossil record dates back to the Carboniferous period (Paleozoic) – at least 300 million years ago.

The name Isopoda derives from the Greek ISO meaning “same” and POD meaning “foot”, owing to very similar legs.

Isopods differ from other crustaceans in having 7 pairs of thoracic legs (walking legs) and 5 pairs of abdominal legs, used for respiration, but also an additional copulatory organ within males. These are relatively small crustaceans, from 0.5 mm and even up to 50 cm (deep sea species giant isopod; Bathynomus giganteus), but most species are from 0.5 to 3 cm in size.

Most species have separated sexes, except for some marine parasites that are hermaphrodites – they live first part of the life cycle as males, and later transform into females. In some species parthenogenesis is observed, meaning that females reproduce asexually, and the young derive from unfertilized egg cells.

The female carries the eggs in a brood pouch, in which the young develop. Young emerge from the brood pouch as independent individuals, very similar to adult crustaceans. Because the outside of the body consists of solid chitinous cuticle, growth demands that they moult several times and so reject the old cuticle (ecdysis).

They feed with almost everything, algae, moss and dead organic matter (detritus), and are an important link in the decomposition of dead matter. They are also an important segment of food for various predators.

Recent studies have identified numerous new species of isopods for Croatia and science. Unfortunately, threats endanger the caves and their inhabitants. The Red Book of Croatian Cave Fauna includes 12 terrestrial and 15 aquatic isopods.

Freshwater isopods (Isopoda aquatica)

There are 34 taxa (species and subspecies) of freshwater crustaceans found in Croatia, together with several undescribed species. Of these, 23 are cave species, 5 species live in underground interstitial habitats, 4 in the brackish waters in the coastal area and only 2 species are known from rivers and lakes.

Cave species completely lost their pigment and sense of sight due to the lack of light. Most cave species feed on dead organic matter (detritus), but some are even predators (Dalmatian giant pill-bug; Sphaeromides virei mediodalmatina).

These species often have very limited distribution, and are therefore very vulnerable. For instance, Dalmatian blind water-louse (Proasellus anophthalmus dalmatinus) is spread in a very narrow area in the southern Dalmatia and Herzegovina, and found in a few caves. Pretner’s spined cave pill-bug (Monolistra pretneri) is spread in several caves along the river Zrmanja around Žegar, within the river Krka and the Vransko jezero lake on the island of Cres (the only island finding of genus Monolistra in Croatia).

Terrestrial isopods (Isopoda Terrestria)

Terrestrial isopods are the only group of crustaceans that are fully adapted to terrestrial life. They inhabit different biotopes, from the sea shore, high mountains and forests to deserts. The terrestrial life is possible due to so-called pseudotrachea or lungs on abdominal legs. But less adapted families still have the same mode of breathing as aquatic isopods with the gills formed of abdominal legs, and are very dependent on high relative air humidity. As a rule, the relative air humidity is always very high in caves, which makes cave habitats suitable for them.

Out of 117 species and 32 subspecies that have been established for the Croatian fauna so far, 20 species are obligate subterranean animals (troglobionts), and 14 species and 7 subspecies are believed to dwell both in caves and surface habitats (troglophiles).

Terrestrial isopods have a number of adaptations to cave habitats, and some have developed a strong ornamentation of the cuticle for defense from predators, or from the pressure when moving through cracks in the sediment, e.g. Herzegovinian Humpback Woodlouse (Cyphoniscellus herzegowinensis). Some species are very widespread, such as white titanic-bug (Titanethes albus) that is spread in Italy, Slovenia and Croatia, while most species are spread over a small area or on a single site, such as Krk’s Iliric Woodlouse (Alpioniscus christiani), found only in Biserujka cave on the island of Krk. Some species have a very interesting distribution, likes Učka’s Tiny Humpback Woodlouse (Thaumatoniscellus speluncae) that was found only in several caves in Učka Mt and Ćićarija Mt., and its only known relative was found in one locality in Romania.

Jana Bedek –


  • Bedek, J., Taiti, S. & Gottstein, S. (2011): Catalogue and atlas of cave-dwelling terrestrial isopods (Crustacea: Oniscidea) from Croatia. Nat. Croat., Vol. 20, No. 2: 237-354.
  • Botosaneanu, L. (ed.): Stygofauna Mundi – A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabitating Subterranean Waters (including the Marine Interstitial). E. J. Brill/Dr. W. Backhuys, Leiden. pp. 740.
  • Schmalfuss, H. (2003): World catalog of terrestrial isopods (Isopoda: Oniscidea). Stuttgarter Beiträge zur Naturkunde, Serie A 654. pp. 341.

Decapods (Decapoda)

Decapods (Decapoda) are the biggest order by species richness in the class of crustaceans (Crustacea). The name Decaspoda derives from the Greek DECA meaning “ten” and POD meaning “foot”. They inhabit marine and fresh waters, but some species are able to survive for a short time on land where they go out in search of food. Decapoda comprises of the most known crustaceans, e.g. shrimps, lobsters, troughs, crabs, river crabs, that is mostly crabs used in human diets.

Most of the decapods are omnivores (both predators and herbivores and detritives), opportunists in the true sense of the word. Sometimes they come in large populations, and then they represent an important prey to marine as well as terrestrial predators. Both marine and freshwater decapods inhabit cave habitats.

Over 250 species of decapods have been recorded in Croatia, the vast majority marine species. Seven species of the genus Troglocaris have been found in caves, in freshwater but also in anchialine caves. Some taxa are widespread, such as Troglocaris anophthalmus periadriatica, which is found from Otočac to Vjetrenica in Herzegovina, while some are narow endemics, such as Troglocaris anophthalmus intermedia, which was recorded only in the cave Mikašinovića špilja near Ogulin.

Jana Bedek –


  • Botosaneanu, L. (ed.): Stygofauna Mundi – A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabitating Subterranean Waters (including the Marine Interstitial). E. J. Brill/Dr. W. Backhuys, Leiden. pp, 740.

Amphipods (Amphipoda)

Amphipods are small, fast moving (mostly swimming) crustaceans with laterally compressed bodies. Amphipods are characterized and differentiated from crustaceans by the absence of a carapace or shell over the front part of the body. The name “amphipoda” is derived from the Greek roots AMPH meaning “different”, and POD meaning “foot“, and refers to the superficial appearance that these animals have two distinct types of leg-like appendages – swimming and feeding appendages.

Amphipods can grow up to 60 millimeters, but they are typically less than 15 millimetres long. The body of an amphipod is divided into a head, a thorax and an abdomen. They possess large compound eyes on either side of the head. Unlike the eyes of shrimps or crabs, these eyes are not on stalks. On the head they have two pairs of antennae, with one pair usually very small, the mandibles, two pairs of maxillae and one pair of maxilliped. The thorax and abdomen are usually quite distinct and bear different kinds of legs. Thoracic legs are adapted for swimming, while modified hind legs are adapted for both rowing and leaping. Gills are present on the thoracic segments, where also the glands may be found. First four pairs of female’s pereiopods form a chamber in which the eggs are carried between laying and hatching.

Around 9,000 species of amphipods have so far been described. Alicella gigantea (up to 14 cm) and Eurythes gryllus (up to 11 cm) are the largest species of amphipods ever observed. Amphipods are placed in four suborders: Gammaridea, Ingolfiellidea, Hyperiidea and Caprellidea. Most of the species are classified into one suborder Gammaridea (approximately 80% of all species). There are more than 140 freshwater and marine amphipod species in Croatia.

Amphipods live in an enormous variety of habitats, mostly sea, but also in freshwater environments (streams, rivers, ponds, wetlands, bractic waters). The coarse sand or moist ground also offer a suitable habitat for amphipods. Parasitic amphipods are known from surface waters (Cyamidae, Hyperiidea). Most species of whale louse (Cyamidae) are associated with a single species of whale. Around 10 000 whale louse could live on a single whale.

The stygofauna of entire continents is sometimes dominated by amphipods. Like other stygobiotic organisims, amphipods also show various degrees of adaptation to their particular habitats, usually expressed by loss of eye pigment and body pigmentation, relatively greater slenderness of the body, elongation of certain body appendages (antennae, pereiopods, uropods), reduced number of offspring, etc. Stygobiotic amphipods often feed on microorganisms, such as yeasts, growing on cave loam, bat guano, etc., but certain species feed on rotting plant debris as well, and some are predators.

Most of the stygobiotic species are classified into suborder Gammaridea. In caves, pits and wells we can often find genus Niphargus, which is the most diverse genus of freshwater amphipods in the world with more than 300 described species. They inhabit subterranean waters and constitute a substantial part of Europe’s groundwater biodiversity. 60 species of the genus Niphargus are recorded in Croatia. In Croatia lives large number of narrowly endemic Niphargus species such as Niphargus croaticus, N. aquilex, N. buturovici, N. echion, N. numerus, N. pectencoronatae, etc.

Order Gammaridae consists from many genera which encompass many endemic species from Croatia, like Accubogammarus algor jalzici, Bogidiellla glacialis, Echinogammarus acarinatus, Fontogammarus dalmatinus and Melita valesi.

Petra Kutleša –


  • Botosaneanu L. (1986): Stygofauna mundi: a faunistic, distributional, and ecological synthesis of the world fauna inhabiting subterranean waters (including the marine interstitial). E.J. Brill, Leiden. pp. 740.
  • Fišer, C., Sket, B. & Stoch, F. (2006): Distribution of four narrowly endemic Niphargus species (Crustacea: Amphipoda) in the western Dinaric region with description of a new species. Zoologischer Anzeiger – A Journal of Comparative Zoology, 245 (2): 77-94.
  • Fišer, C., Sket, B. & Trontelj, P. (2002): The taxonomic complexity of the genus Niphargus (Crustacea: Amphipoda) and an approach to its elucidation. In: Latella, L., Mezzanotte, E. & Tarroco, M. (eds.): XVI International Symposium of Biospeleology, 8–15 September 2002, Verona, Italy. Museo Civico di Storia Naturale di Verona, Verona, pp. 39-40.
  • Gottstein Matočec, S. (ed.), Bakran-Petricioli, T., Bedek, J., Bukovec, D., Buzjak, S., Franičević, M., Jalžić, B., Kerovec, M., Kletečki, E., Kralj, J., Kružić, P., Kučinić, M., Kuhta, M., Matočec, N., Ozimec, R., Rađa, T., Štamol, V., Ternjej, I. & Tvrtković, N. (2002): An overview of the cave and interstitial biota of Croatia. Nat. Croat., Vol. 11, Suppl. 1: 1-112.
  • Matoničkin I., Habdija, I. & Primc-Habdija, B. (1999): Beskralješnjaci – Biologija viših avertebrata. Školska knjiga, Zagreb. pp 609.
  • (Contents created and maintained by researchers from Dept. of biology, Biotechnical faculty,University of Ljubljana, Cene Fišer, Peter Trontelj and Boris Sket.)