Iron-rich octahedral crystals have been described by the senior author in the gut caeca cells of the amphipod
Stegocephaloides christianiensis
. The present investigation revealed their presence in other species in the family Stegocephalidae (
Bathystegocephalus inflatus
,
Euandania ingens
,
Parandania boecki
,
Stegocephaloides auratus
,
S. vanhojfeni
,
Stegocephalus inflatus
,
Phippsiella
spp. and
Parandaniexis
sp. (cf.
mirabilis
). Crystals were not found in
Andaniopsis nordlandica
, T
etradeion crassum
or
Andaniexis abyssi
, although the latter gave a tissue reaction for iron. Fe cells contain only a single crystal each in all species and crystals consistently increased in size proximally in each caecum. The most distal region of the caecum was devoid of crystals. Detailed work was confined to
Stegocephaloides christianiensis
,
Stegocephalus inflatus
and
Parandaniexis
sp. (cf.
mirabilis
). Caecum ultrastructure of
S. christianiensis
is described: two cell facies (R /F and B cells) are distinguishable. R /F cells (=Fe cells) are columnar, with a terminal brush-border of long microvilli. Lipid globules, glycogen, Fe crystals and Ca granules are found in these cells. B-cells have a luminal border of short, stubby microvilli with an apical complex of membrane-bound vesicles of varying degrees of coalescence. The composition of the Fe crystals has been described using X -ray microprobe analysis. Strong Fe peaks were revealed together with minor peaks for Si, P, S, Cl, K, Ca, Cu and Zn. These elements were identified in the surrounding cytoplasm also. Crystal composition is homogeneous with no separate core. The crystal consists of hexagonally arranged, electron dense cores of 5.8 + 0.3 nm diameter at intercore distances of 10.5 ± 0.5 nm, 7.5 + 0.5 nm and 9.5 + 0.5 nm. Wide angle electron diffraction analysis of the cores gave four rings with
d
spacings of 0.250, 0.223, 0.191 and 0.145 nm (all + 0.003 nm). On these bases the substance of the crystals is identified as ferritin. Ferritin crystals are voided in the faeces of
Stegocephaloides christianiensis
, suggesting a role in iron excretion, perhaps as part of a body content regulation process. The content of iron in
S. christianiensis
and a variety of other inshore Amphipoda has been investigated using atomic absorption spectrophotometry. Iron content was non-linearly related to body dry mass in
S. christianiensis
and cannot simply be explained as a consequence of surface adsorption. Iron levels in
S. christianiensis
were higher than in many other species investigated. The morphology of the mouthparts of
S. christianiensis
has been investigated using scanning electron microscopy. Analyses of fresh stomach contents revealed cnidarian nematocysts which corresponded in size and form with those from
Adamsia carciniopados
,
Pennatula phosphorea
and
Hydractinia echinata
. Behavioural observations on live
S. christianiensis
suggested that
Pennatula
was a likely prey item. Investigations of a range of Cnidaria and of a few known predators of cnidarians (
Pycnogonum
,
Hyperia
) confirmed that the discharged acontia of
Adamsia
and the soft tissues of
Pennatula
contained unusually high concentrations of iron. It is proposed that the production and expulsion of ferritin crystals by
S. christianiensis
and other cnidarian-consuming species in the family Stegocephalidae represents an iron regulation system in animals experiencing a dietary iron challenge.
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