Growth, Regeneration, and Damage Repair of Spines of the Slate-Pencil Sea Urchin Heterocentrotus mammillatus (L.) (Echinodermata: Echinoidea )

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1988
Authors
Ebert, Thomas A.
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University of Hawaii Press
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Abstract
Spines of sea urchins are appendages that are associated with defense, locomotion, and food gathering. Spines are repaired when damaged, and the dynamics of repair was studied in the slate-pencil sea urchin Heterocentrotus mammillatus to provide insights not only into the processes of healing. but also into the normal growth of spines and the formation of growth lines. Regeneration of spines on tubercles following complete removal of a spine was slow and depended upon the size of the original spine. The maximum amount of regeneration occurred on tubercles with spines of intermediate size (1.6 g), which, on average, developed regenerated spines weighing 0.1, 0.3, and 0.7 g after 4, 8, and 12 months, respectively. Some large tubercles, which had original spines weighing over 3 g, failed to develop a new spine even after 8-12 months. Regeneration of a new tip on a cut stump was more rapid than production of a new spine on a tubercle. Regeneration to original size was more rapid for small spines than for large spines, but large stumps produced more calcite per unit time. In 4 months, a small spine with a removed tip weighing 0.15 g regenerated a new tip weighing 0.09 g, or 63% of its original weight. In the same time, a large spine with 2.35 g of tip removed regenerated 0.40 g of new tip, or 17% of the original weight. Holes were drilled in spines to serve as bench marks to document the production of new growth lines in response to damage. Blind holes drilled in spines caused 65% loss over 4 months, but holes drilled completely through spines caused only a 6% loss. Cutting a spine near its base did not initiate shedding of the stump. Repair of holes drilled in spines showed that major repair originated from the spine surface, and regeneration into the hole was over obtuse angles. For a slanting hole through a spine, filling of the hole carne from the distal edge for the distal opening and from the proximal edge for the proximal opening. Spines were tagged with tetracycline, but of 46 spines examined 4-12 months following treatment, only 1 displayed evidence that the spine surface had been marked. This suggests that spines usually are not growing. Abrasion of spines either by wiping with a cloth or sanding with emery paper failed to induce growth lines to form. Of 57 abraded spines and 64 unabraded spines, only 2 in each treatment category developed new growth lines. Results show that some growth lines in spines develop from trauma, but these tend to be local and do not extend from the base to the tip. Spines of Heterocentrotus mammillatus usually are not growing but occasionally undergo a growth episode that produces a growth line that extends from the milled ring to the spine tip. Such an episode is not related to damage repair from abrasion or breakage and appears to be endogenous. The SPINES OF A SEA URCHIN are morphological parts that serve as the animal's first defense against assaults from the environment (e.g., Fricke 1974, Strathmann 1981), appendages for gathering and manipulating food (e.g., Ebert 1968, Leighton 1968), and braces for wedging into crevices (e.g., Regis and Thomassin 1982). Spines form an important part of an urchin's maintenance mechanisms (Ebert 1982), and it is no surprise to find that they are repaired rapidly once damaged (Ebert 1967, Heat field 1971, Mischor 1975). Spine sizes vary greatly across taxa. For example, in St rongylocentrotus purpuratus (Stimpson 1857), calcite of a large primary spine weighs only about 0.03 g. In slate-pencil sea urchins of the genus Heterocentrotus, spines are substantially more massive. Calcite in a large H. mammillatus (Linneaus 1758) spine may weigh over 5 g, and calcite of a large primary spine of H. trigonarius (Lamarck 1816) may weigh over 11 g. Although spines from Strongylocentrotus purpuratus would be expected to be repaired rapidly (Ebert 1967, Heatfield 1971), it is less clear whether very large spines from Heterocentrotus species also would be repaired in just a few months or whether the benefits of large spines carry a potential burden of a long regeneration time. A second issue associated with damage repair focuses on growth lines in spines. In St rongylocentrotus purpuratus, lines form as a consequence of tip regeneration following breakage (Ebert 1967, Heatfield 1971 ), but growth lines in Heterocentrotus spp. have been interpreted as periodic (Dotan and Fishelson 1985, Weber 1969), an interpretation that has been disputed (Ebert 1985, 1987). The purpose of this paper is to present results of a 1-year study on Heterocentrotus mammillatus (family Echinometridae), a species with massive spines. The focus is on the responses of spines to damage, spine repair and regeneration in response to trauma, and the formation of growth lines. Data are presented on (a) regeneration following total removal of spines; (b) repair rates of calcite on cut spine stubs; (c) responses of spines to small holes drilled into or through them; (d) responses of spines to abrasion; and (e) results of tagging spines with tetracycline.
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Ebert TA. 1988. Growth, regeneration, and damage repair of spines of the slate-pencil sea urchin Heterocentrotus mammillatus (L.) (Echinodermata: Echinoidea ). Pac Sci (3-4): 160-172.
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