Acantharia

The heaviest known skeleton is a superb adaptation to the planktonic way of life.

Reference: Korsun S, 2011. Acantharia Müller 1855. In: Protista. III. Handbook of Zoology (Ed. S.A. Karpov), Moscow, KMK Scientific Press Ltd., p. 95-120 + 11 color plates [in Russian]

Fig. 41. Phyllostaurus cuspidatus (order Arthracanthida): ко, periplasmic cortex under tension; кш, elastic junction; м, myonemes; о, opening for an axopode; эк, ectoplasm; цк, central capsule with numerous nuclei inside. Magn. 200x. (Drawing from Schewiakoff [1926, Table 38]).

Fig. 42. Scheme illustrating positional relationship of spicules in the acantarean skeleton (Müller’s law). А. There are a total of 20 radial spicules (or 10 diametrical spicules). These 20 spicules comprise five quartets: one “equatorial” (э), two “tropical” (т) and two “polar” (п). Б. In a polar view, the spicules of each quartet make up a Greek cross, while each cross is revolved 45° longitude relative to the neighboring cross.

Fig. 46. Cell organization in Acantharia: ан, axoneme; эc, extrusome; рп, rhizopodia; м, myonemes; пл, plasmalemma; сп, spicule; цк, capsular wall; ко, periplasmic cortex; кош, elastic junction; эк, ectoplasm; эн, endoplasm; фс, zooxanthella; пг, pigment body; мх, mitochondrion; я, nucleus; эпр, er; АГ, golgi; ЦОМТ, microtubule organization center. (After several authors).

Fig. 49. Morphological details of Amphilithium clavarium (order Symphyacanthida): ко, periplasmic cortex under tension; кш, elactic junction; м, myonemes; о, opening for an axopode; р, radial elastic threads; с, meshwork of the inner periplasmic cortex; т, plate-shaped perispicular cytoplasm swelling under the myonemes; цк, capsular wall ripped around the spicule. The elastic junction is unzipped on the left. Magn. 560х. (Drawing from Schewiakoff [1926, Table 13]).

Fig. 50. Diagram showing arrangement of microtubules in an axoneme, transversal section. Two patterns are principally known for Acantharia: А, hexagonal (orders Chaunacanthida and Arthracanthida) and Б, dodecagonal (order Holacanthida). Order Symphyacanthida has an irregular pattern, refrained by two adjacent hexagons (not shown).

Fig. 51. Life cycle of arthracanthid acanthareans: 1, juvenile with the single nucleus, 2,3, multinucleate trophont, 4, gamont, 5, swarmers with two flagella (redrawn after Febvre 1990, Fig. 30a).

Fig. 55. Feeding of Acanthoplegma krohni (А,Б,В,Г,Д) and swarmers of Acanthometra pellucida (Е): ап, axopodium; ж, lipid droplet; з, zooxanthella spore; ко, periplasmic cortex; м, myonemes; пг, pigment; пщ, remnant of food object; эк, ectoplasm; эн, endoplasm; я, nucleus. Е. Magn. 1800x. (Drawings from Schewiakoff [1926]).

Classification

Phylum Acantharia
1. Order Holacanthida: 10 diametrical spicules.
2. Order Symphyacanthida: 20 radial spicules; bases fused in a solid mass.
3. Order Chaunacanthida: 20 radial spicules; bases joint flexibly allowing spicules to sway.
4. Order Arthracanthida: 20 radial spicules; bases pyramidal in shape, tightly packed, and spicules cannot sway; capsular wall thick, clearly visible.
    Suborder Sphaenacantha
    Suborder Phyllacantha

Fig. 54. Representatives of acantharean orders. А. Order Holacanthida, Acanthoplegma krohni. Б. Order Symphyacanthida, Heliolithium aureum. В. Order Chaunacanthida, Stauracon pallidus. Г,Д. Order Arthracanthida, Acanthometra pellucida. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; м, myonemes; пщ, remnants of food object; рп, rhizopodia; слс, mucous layer; цк, capsular wall; эвр, radial elastic fibrillae; эк, ectoplasm; экс, ectoplasmic network; эн, endoplasm. Magn. А 120х, Б 200x, В 80х, Г 120х, Д 900х. (Drawings from Schewiakoff [1926]).

Fig. 53. Order Holacanthida representatives. А. Cell organization in Acanthochiasma rubescens. Skeleton (Б) and the central part of the diametrical spicule (В) of Acanthocyrta haeckeli. The central part of the diametrical spicule of Acanthospira spiralis (Г). Intact (Д) and partially disintegrated (Е) skeleton of Acanthocolla cruciata. ж, lipid droplet; кр, crystalloid; пг, pigment granule; рп, rhizopodia. (Drawings from Schewiakoff [1926]).

Fig. 56. Family Acanthocollidae representatives. А. Acanthospira spiralis. Б. Acanthocolla cruciata. В. Acanthocyrta haeckeli. ап, axopodium; ко, periplasmic cortex; м, myonemes; ппб, whip-like pseudopodium; рп, rhizopodia; сл, mucus; эк, ectoplasm; эн, endoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 57. Order Symphyacanthida (В,Г) and Chaunacanthida (А,Б) representatives. А. Incystment (litholophus stage) in Heteracon biformis. Б. Cell organization in Heteracon biformis. В. Amphilithium clavarium. Г. Cell organization in Dicranophora buetschlii. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; кош, elastic junction; кр, concretion; м, myonemes; рп, rhizopodia; ск, discoidal skeletal element; цк, capsule; эвр, radial elastic fibrillae; эк, ectoplasm; экс, ectoplasmic meshwork; эн, endoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 58. Order Symphyacanthida representatives. А. Amphilithium concretum. Б. Amphibelone hydrotomica. В,Д. Haliommatidium muelleri. Г. Pseudolithium bifidum. ап, axopodium; з, zooxanthella; ко, periplasmic cortex; кр, concretion; м, myonemes; пг, pigment body; рп, rhizopodia; цк, capsule; эк, ectoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 60. Family Astrolithidae representatives. А. Acantholithium stellatum. Б. Astrolithium bulbiferum. В. Astrolonche serrate. Г. Cell organization in Astrolithium bulbiferum. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; м, myonemes; оч, colored particles; пг, pigment body; рп, rhizopodia; цк, capsule; эк, ectoplasm; экс, ectoplasmic meshwork; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 61. Order Chaunacanthida (А,Б,В) and Arthracanthida (Г) representatives. А. Amphiacon denticulatus. Б. Conacon foliaceous. В. Gigartacon fragilis. Г. Amphilonche elongata. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; м, myonemes; пг, pigment body; рп, rhizopodia; слс, gelatinous layer; цк, capsule; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 62. Order Arthracanthida representatives. А,Б. Lithoptera muelleri. В. Dictyaspis solidissima. Г. Hexaconus ciliatus. Д. Coleaspis vaginata. Е. Stauracantha orthostaura. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; кош, elastic junction; м, myonemes; о, opening in periplasmic cortex for axopodium; пщ, prey; поа, aspinal pore; пош, sutural pore; рп, rhizopodia; ш, spine; эвр, radial elastic fibrillae; экс, ectoplasmic meshwork; я, nucleus; яа, aspinal pit; яш, sutural pit. (Drawings from Schewiakoff [1926]).

Fig. 63. Order Arthracanthida representatives. А. Phractopelta dorataspis. Б. Hexalaspis heliodiscus. В. Diploconus fasces. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; м, myonemes; поа, aspinal pore; пок, coronal pore; пош, sutural pore; р, shell; рп, rhizopodia; цк, capsule; эк, ectoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 64. Family Dorataspidae representatives. А. Pleuraspis costata. Б. Lychnaspis giltschi. В. Dorataspis loricata. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; м, myonemes; рп, rhizopodia; цк, capsule; ш, spine; эк, ectoplasm; эн, endoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 65. Skeleton of family Dorataspidae representatives. А,Б. Dorataspis loricata: full-grown specimen (А) and juvenile specimen without serrate rims encircling pores and without spines (Б). В. Hystrichaspis dorsata. Г. Pleuraspis costata. Д. Icosaspis ornata. Е. Stauraspis staurantha. поа, aspinal pore; пок, coronal pore; пош, sutural pore; р, shell plate; ш, spine; яа, aspinal pit; яш, sutural pit. (Drawings from Schewiakoff [1926]).

Fig. 66. Spicules and apophyses in suborder Phyllacantha. А. Acanthostaurus purpurascens. Б. Stauracantha orthostaura. В. Phatnacantha icosaspis. Г. Phyllostaurus cuspidatus. Д. Pristacantha multidentata. Е. Pristacantha octodon. Ж. Xiphacantha quadridentata. З. Dictyacantha tabulata. И. Phatnacantha icosaspis. (Drawings from Schewiakoff [1926]).

Fig. 67. Suborder Phyllacantha representatives. А. Amphistaurus complanatus. Б,Г,Д,Е. Acanthostaurus purpurascens: myonemes relaxed (Б), myonemes contracted (Д); gametes (Г), zygotes (Е). В. Lonchostaurus rhobmicus. ап, axopodium; ж, lipid droplet; з, zooxanthella; ко, periplasmic cortex; кош, elastic junction; м, myonemes; рп, rhizopodia; цк, capsule; эвр, radial elastic fibrillae; эк, ectoplasm; я, nucleus. (Drawings from Schewiakoff [1926]).

Fig. 68. Suborder Phyllacantha representatives. А. Xiphacantha alata. Б. Pristacantha multidentata. В. Phatnacantha icosaspis. Г. Dictyacantha tabulata. ап, axopodium; з, zooxanthella; ко, periplasmic cortex; м, myonemes; слс, gelatinous layer; цк, capsule; эк, ectoplasm; экс, ectoplasmic meshwork. (Drawings from Schewiakoff [1926]).

Fig. 59. Light micrographs of acantharians. А. Acanthostaurus sp. Note photosymbionts and axopodia with extrusomes. Б. Lychnaspis sp. Note radial spines on the shell. В. ?Amphibelone sp. Г. Amphilonche elongate. Д. ?Heliolithium aureum. Е. Amphistaurus sp. Note perispicular cones with wavy myonemes. Ж. ?Gigartacon sp. З. Diploconus fasces. Note pseudopodia concentrated around main spicules. (Equatorial Atlantic; inverted compound microscope Carl Zeiss Axiovert; images courtesy of K. Hausmann and A. Kudryavtsev).

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