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Dicyemids are the most common and characteristic endosymbionts found in the renal sac of benthic cephalopods. In this chapter, we introduce biology and diversity of dicyemids of European typical cephalopods, and . The diphasic life cycle of dicyemids consists of vermiform stages formed asexually and an infusoriform stage developed sexually. Their morphology varies depending on the development stage. Recent molecular studies suggested that dicyemids belong to lophotrochozoans. In Europe, 16 dicyemid species were described from 17 cephalopod species.

Cephalopods and their metazoan parasites have coevolved in wild fisheries for many years. In fact, helminth larvae and parasitic copepods have been recorded in cephalopods worldwide. This is not surprising considering the important role cephalopods play in the transfer of energy and contaminants in marine food webs. Nerito-oceanic ommastrephid squids are by far the most noticeable trophic bridge for helminth parasites in the marine realm, coastal octopus, and cuttlefish serving as primary host for crustaceans. Although it is highly likely that parasitic infections occurred, relatively little is known about the pathogenic potential of metazoan parasites in naturally infected cephalopods. It is stated that heavy parasitic infections may probably cause host morbidity or poor condition but signs of disease are singularly rare with very few specimens exhibiting disease conditions. Unfortunately, neither robust scientific evidence nor available material is available to support this statement. It is more likely that metazoans may deplete energy stores of infected cephalopods, which are directed toward tissue repair and the host’s defense mechanisms. Parasitic infection may thus be considered an environmental stressor and as such a source of uncertainty in the evaluation of the potential productivity of cephalopod populations.

This chapter reviews the mechanical (physical) and chemical (water quality) related pathologies that have been reported since cephalopods are maintained, reared or cultured in captivity. For the first time, it builds up on the existing knowledge from researchers and aquarists (which are represented as authors of the chapter) in order to provide the most updated and inclusive revision on this theme. It is organized in terms of pathologies that are reported and eventual described for one or more species, which are commonly kept for research and display purposes, and exemplified with photos when possible. It includes pathologies of the mantle, arms, eyes, shell; egg infections, malformations of the shell and eggs; and causes of disease or mortality related with water quality focusing on pH and trace elements.

Cephalopods are animals endowed with a high regenerative potential. They can regenerate missing or injured structures such as the cornea, the shell, the arms and tentacles and even peripheral nerves and brain centers. As much as regeneration and healing cannot be considered as pathologies sensu stricto, they are always accompanied by conditions such as inflammation, tissue degeneration and potentially by infection. No treatments are currently available for any of these cases. Although they often get resolved autonomously, the early identification and monitoring of post-traumatic events is fundamental in the context of animal welfare.

Neoplasia are growth disturbances characterized by excessive, abnormal proliferation of cells, independent of normal-regulating mechanisms of the animal and persisting after termination of the stimulus that initiated growth. Over lasts years there has been an increase in research into tumors of invertebrates. In cephalopods, reports on the incidence of the tumors or neoplasia are scarce. They have been described as hard compact and homogeneous nodules of connective tissue located in the mantle or at the base of suckers. The aetiological origin of tumors observed in cephalopods is unknown, but it could be related to aquarium maintenance. Other injurious agents, including infectious (virus, bacteria, or parasites) and xenobiotics, may also have produced the lesions. In occasions inflammatory processes have been associated with tumors with severe oedema associated. Inflammation is part of the biological response to body tissues to harmful stimuli such as pathogens, damaged cells or effect of xenobiotics. The inflammatory focus is characterized by exudation with interstitial fluid changes and hemocytic migration. Some inflammatory lesions include fibrosis and necrotic cells in the affected area, with loss of histological features and even organ architecture. All this changes have been observed in cephalopods in inflammatory reactions originated by infection with different pathogens. The chapter covers a selection of the reported cases of disorders related to neoplasia and inflammation.

In the majority of the shallow-water cephalopod species, senescence is a short stage of their lifespan, which takes place at the end of sexual maturity. Senescence is not a disease, although senescent cephalopods can be frequently mistaken with diseased animals. Senescence is accompanied by physiological, immunological and behavioural changes, which are briefly exposed in this chapter. A suppressed immune system may increase susceptibility to parasite infection in senescent cephalopods. High prevalence of infection by was observed in and sp. in . In both cases, the infection was found in post-spawned females and was infecting different organs. Cestode larvae sp. were found in the buccal mass of post-spawned females.

Despite the economic and ecologic importance of Mexican four-eyed and the jumbo squid , the pathogens and diseases in both species remain largely unknown. This chapter covers the pathogens and diseases, mainly protozoan and metazoan parasites, of these two non-European cephalopods.

Parasitic diseases and other abnormalities play critical roles in causing morbidity in the majority of Cephalopoda. However, to date, reports of cephalopod diseases from Asia are scarce and lack detailed information on the description of specific characters. This paper presents a brief overview of various pathogens and produced diseases in Asian cephalopods, including coccidiosis by Anisakiasis, infection by the copepods and other abnormalities such as edema and broken skin. The coccidian sp. that infects the definitive host is a heteroxenous parasite transmitted through the food web. Anisakids play an important role in Asia as parasitic disease for cephalopods and it is even transmitted to humans. Concerning the infection by copepods, sp. is the only species of the family Octopicolidae reported from North Pacific waters. Other abnormalities like edema or broken skin may have been the result of bacterial infections, so that abnormalities could cause the degeneration and death observed in .