Monstrilloida is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...

s with a

cosmopolitan distribution In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of the world in appropriate habitats. Such a taxon, usually a species, is said to exhibit cosmopolitanism or cosmopolitism. The extr ...

in the world's oceans. The order contains a single family, Monstrillidae. The name of the first ever described genus ''Monstrilla'' is derived from

latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

, meaning "tiny monster", because the lack of usual diagnostic features of copepods puzzled early taxonomists.

Description

The family Monstrillidae is characterised by having a well-developed fourth pair of swimming legs, but a rudimentary or absent fifth pair. Adults have no oral

appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including ante ...

s, and the mouth leads only to a short, blind

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

. They also lack second antennae, but show large, multiramous and setaceous antennulae. These antennulae are rigid and anteriorly oriented. Females carry a long pair of spines to which the eggs are attached, while males have a "genital protuberance, which is provided with lappets"; in both sexes, the genitalia are very different from those of all other copepods. Some species have large, well-developed nauplius eyes, while others are basically blind. Larval nauplial stages do not possess any discernible antennae, antenullae or mouth parts, but paired tube-shaped nourishing appendages to absorb nutrients from their host, which are also present in later copepodite stages that resemble the adult morphology; in adults, scars of these now discarded appendages remain as small processes on the cephalothorax. Taxonomy of genera and species descriptions are normally based on a few key characteristics: the length and setation-pattern of the antennulae, presence/absence and morphology of the eyes, number and shape of caudal setae, and structure and setation-pattern of the swimming leg in females/genital complex in males, respectively. Right_antennule_of_Monstrillopsis_planifrons

Right_antennule_of_Monstrillopsis_planifrons

Distribution

Monstrilloids are distributed worldwide (including both the

Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...

and

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and othe ...

), inhabiting coastal-neritic waters (0-200m depth). Adults are regularly caught with plankton nets and are clearly

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

organisms; however, the endoparasitic larval stages infect sessile benthic organisms and therefore are part of the

epibenthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.

Biology

Biologically and ecologically, our knowledge of the order is limited, although the

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...

differs from that of all other copepods: Members of the Monstilloida are protelean parasites, meaning that their larval stages are

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

s that kill their host to emerge as free-living subadults. Apparently, some hosts recover after the final subadult monstrilloid exits their body. It is hypothesized, that the host's relative body size and the number and location of copepods parasitizing the same host determine whether it survives an infection. The detailed life cycle may vary between different species, but generally follows a certain sequence: after a free-swimming, infective nauplius stage, the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e develop inside benthic

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. The ...

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...

s and bivalve mussels (They may be a pest in commercially important bivalve aquaculture), from where the planktonic adults emerge. They do not possess functioning mouth-parts, their sole purpose is to reproduce. In Contrast to holoplanktonic

calanoid Calanoida is an order of copepods, a group of arthropods commonly found as zooplankton. The order includes around 46 families with about 1800 species of both marine and freshwater copepods between them. Description Calanoids can be distinguis ...

and cyclopoid copepods, Monstrilloids do not use their largest cephalic appendages, the antennulae, for locomotion, but to create a stream-line shaped corridor, rather using their four pairs of swimming legs to move in the water column. Sex determination depends on conspecifics infecting the same host individual. In case of 2-3 coexisting monstrilloids they become males, when there is only one parasite in a host, it develops into a female. FMIB 47702 Stages in the life-history of Haemocera danae, one of the Monstrillidae

FMIB 47702 Stages in the life-history of Haemocera danae, one of the Monstrillidae

Taxonomy

The

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

of the order and family is undergoing several revisions, for instance, the family Thaumatopsyllidae was formerly included in the order, but is now usually placed in the Cyclopoida. and the genus ''Strilloma'' is now considered a

taxonomic synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Lin ...

of '' Monstrilla'', the largest genus. In General, the Monstrilloida are taxonomically challenging, both regarding their relation to other copepod groups and species assignment within the order. Monstrilloida was placed as a sister taxa to the Siphonostomatoida, but a lack of mouth parts makes comparison based on homologies difficult. A more recent analysis placed the order nested within the fish-parasitizing caligiform groups of Siphonostomatoida. Consequently, they would have evolved from an ectoparasitic ancestor associated with fish; most parasitic copepods are not free-living as adults, so Monstrilloids presumably underwent a change in life cycle strategy, host selection and body morphology. Yet the unique nature of the order Monstrilloida sister group of the Siphonostomatoida has been corroborated using modern molecular approaches. So far, no ultimately satisfying copepod phylogeny has been proposed, and the placement of the monophyletic Monstrilloida remains unresolved. Because of their enigmatic life cycle, the morphology of most postnaupliar and copepodite stages is not known. Monstrilloids are not abundant in planktonic samples, and often only single specimens can be collected. Since many species occupy overlapping geographic ranges, males and females of different species collected in the same sample may be mistaken for conspecifics. Until now, both sexes were described for less than 25% of all known species. To link males and females of a species, taxonomists have started to use molecular methods such as

DNA Barcoding DNA barcoding is a method of species identification using a short section of DNA from a specific gene or genes. The premise of DNA barcoding is that by comparison with a reference library of such DNA sections (also called " sequences"), an indi ...

recently. As of 2019, the order Monstrilloida contains seven accepted genera with more than 160 species: * '' Monstrilla'' Dana, 1849 * '' Australomonstrillopsis'' Suárez-Morales & McKinnon, 2014 * '' Caromiobenella'' Jeon, Lee & Soh, 2018 * '' Cymbasoma'' Thompson, 1888 * '' Maemonstrilla'' Grygier & Ohtsuka, 2008 * '' Monstrillopsis'' Sars, 1921 * '' Spinomonstrilla'' Suárez-Morales, 2019

References

{{Taxonbar, from=Q3480154 Copepods Parasitic crustaceans