Monstrilloids are one of the most intriguing groups of copepods. Their complex life cycle represents the successful evolutionary outcome of dealing with three distinct kinds of habitat, viz., planktonic, benthic, and endoparasitic, each of which presents particular challenges that have been overcome by monstrilloids. These copepods combine a unique set of strategies and adaptations to complete their life cycle. The non-feeding planktonic adult phase lacks mouthparts and their antennules are fixed, thus limiting their swimming abilities but they compensate for this handicap by having powerful swimming legs and probably generate a very distinct hydrographic signal that may be useful in avoiding predators and allowing sexual recognition between adult males and females. Parasitizing exclusively on abundant, gregarious sessile or sedentary benthic organisms represents an advantage in that potential hosts can be found without the need for long-distance dispersal. The endoparasitic stages of monstrilloids are unique; after infection by an early planktonic nauplius, successive nauplioid stages feed on their own vitellum while developing feeding tubes to absorb nutrients from their hosts. They grow within the host's body as successive copepodite stages that are contained in a protective sheath. Preadult individuals exit through the host body wall causing significant host damage or death, behaving in these instances as parasitoids. The diversity of the group appears to be underestimated, and extensive geographic areas remain completely unknown for this group of copepods. More effort will be required to advance our knowledge of monstrilloid diversity and biology that are yet to be revealed.

In symbiotic copepods, most naupliar stages are typically planktonic, playing a primary role in dispersal, while the first copepodid usually represents the infective stage. Later copepodid stages, including adults, are associated with host organisms. Many symbiotic copepods have abbreviated life cycles, with a reduced number of naupliar stages and two different feeding habits. These patterns are presumably related to distinct life cycles strategies. Exceptional cases are exemplified by members of the Monstrillidae and Thaumatopsyllidae, both of which are protelean parasites, with infective nauplii and non-feeding planktonic adults. In the Caligidae, the life cycle follows a generalized pattern, but adults of many species like Caligus undulatus seem to exhibit a dual mode of life involving host switching. Adults leaving the first host become temporarily planktonic before attaching to the final host. This dual mode of life is also found in adults of the Ergasilidae. Abbreviation of the planktonic phase is characteristic for some symbiotic taxa, thus suggesting that they have evolved to become highly efficient in locating and infecting new hosts without needing long-distance larval dispersal. The life cycle of copepods associated with zooplankters is also briefly reviewed. Zooplankters are clearly less used as hosts by copepods than benthic invertebrates. It is likely that symbiotic copepods dynamically utilize planktonic phases in their life cycle, thus maintaining the balance between dispersal, host location, reproduction, and predator-avoidance strategies.

Female harpacticoid copepod specimens representing an undescribed species of Peltidium (Peltidiidae) were found from an unidentified species of Sargassum during a series of samplings carried out in 2014 in Tampa Bay, Florida, U.S.A. The new species, Peltidium camilae, is similar to P. nichollsi Geddes, 1968 and P. lerneri Geddes, 1968. These species share the female exopod of leg 5 with two inner and three apical setae, the second endopodal segment of leg 1 with three setae, and the third endopodal segment of legs 2-4 with three, five and four setae, respectively. Peltidium camilae n. sp. can be distinguished from P. nichollsi and P. lerneri by having a shorter endopod relative to the exopod on legs 2-4, apical claws on the terminal exopodal segment of leg 1 that are as long as the first exopodal segment of leg 1, and subequal middle and inner apical setae on the exopod of leg 5. This is the first record of the family Peltidiidae from the Gulf of Mexico.

The locomotion, feeding, excretion, and oviposition of a member of the copepod family Misophriidae were observed based on a live specimen collected from a sandy bottom at a depth of 52 m off Nagannu Island, Okinawa, Japan. This species is related to Arcticomisophria Martínez Arbizu and Seifried, 1996 in the armature of leg 1, but the fifth leg is much more reduced. The combination of morphological characters strongly suggests that it represents an undescribed genus. The maxillipeds played a major role in attaching to the bottom and in crawling, while the antennae and mandibular palps were involved in slow swimming along the bottom. It fed on small-sized cultured phytoplankters, and excreted numerous fecal pellets. The female carried 4-5 eggs of 0.09 mm diameter that were loosely attached to the urosome. Nearly complete nuclear 18S and 28S rRNA gene sequences and a partial mitochondrial cytochrome c oxidase subunit 1 (CO1) gene sequence were obtained and are made available for future phylogenetic and systematic work.

The phylogenetic relationships of four dioecious populations and one parthenogenetic population of the harpacticoid Canthocamptus staphylinus (Jurine, 1820) were studied. Analysis of the mtCOI gene revealed two main clades as a phylogenetic tree and a network of haplotypes: a clade with Fennoscandian populations in Lake Pääjärvi (Finland) and Lake Vänern (Sweden), and a second clade with populations in Lake Võrtsjärv (Estonia), Orlov Pond in Saint Petersburg (Russia), and the type locality of the species in Lake Geneva (Switzerland). The parthenogenetic population of C. staphylinus showed the smallest nucleotide and haplotype polymorphisms and could have evolved as a reaction to the changing environmental conditions following the Last Glacial Maximum, 20K YBP.