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How Giant Isopods Got Supersized
Survival in the deep sea is inherently challenging: darkness abounds, the temperature is near freezing, and food is hard to come by. And yet, rather than wither in the harsh conditions, many deep-sea animals, from massive spider crabs to giant squid, adapt by growing supersized, dwarfing their shallow-water or terrestrial kin. Why these animals get so big has interested scientists for more than a century. Now, by asking a slightly different question—how do they get so big?—scientists are getting closer to an answer.
A team of researchers recently sequenced the genome of the giant isopod Bathynomus jamesi—a first for a deep-sea crustacean. With round, segmented bodies, giant isopods look like roly-polies—except they can grow as long and as heavy as a chihuahua. The team behind the work, led by Jianbo Yuan, a geneticist at the Chinese Academy of Sciences in Beijing, hopes the details hidden in the animal’s genetic code will help us better understand what’s going on behind the scenes, genetically speaking, with deep-sea gigantism.
Giant isopods, or bathynomids, are the jumbo cousins of the armored crustaceans found milling around under fallen logs. While the smallest isopod species measures less than half a centimeter, bathynomids can grow 80 times longer. The niches isopods occupy are similarly varied: there are more than 10,000 known species, and they’re found everywhere from the bottom of the ocean to caves to mountaintops. This physiological and ecological diversity makes the isopod family tree the perfect place to hunt for clues about what drives adaptation fathoms below.
Among the most interesting questions, says Yuan, is whether or not today’s deep-sea giants simply stem from hefty ancestors—animals like anomalocarids, large arthropod predators that existed some 50 million years ago—or whether they evolved more recently under the pressures of life in the deep sea. In the case of giant isopods, their genome points to the latter explanation.
Like their bodies, bathynomid genomes are incredibly large. B. jamesi, the researchers found, has a high number of so-called jumping genes, transposable elements that can move from one place in the isopod’s genetic code to another. Jumping genes are linked to high rates of mutation, something the researchers think may make the isopod better equipped to deal with environmental stress.
Having a large number of genes is something B. jamesi shares with other deep-sea invertebrates. That invertebrates—organisms generally considered to be less complex than vertebrates—have evolved some of the most complex, adaptable genetic codes has baffled scientists since genome sequencing began.
Beyond revealing the size of its genome, the scientists’ delving into B. jamesi’s biology and genetics have also suggested possible explanations for a number of key adaptations the animal harnesses to thrive in the deep.
B. jamesi’s stomach, for instance, can expand to take up two-thirds of its body. This ensures that, when it can find food, it can gobble as much as possible. Yuan and the team also found in B. jamesi’s genes changes related to thyroid and insulin function, which likely boost the isopod’s ability to grow and absorb nutrients. In addition, they found a tweak that slows the breakdown of fat. Keeping extra junk in the trunk lets giant isopods go years without eating.
Alexis Weinnig, a deep-sea biologist and geneticist at the Leetown Research Laboratory in West Virginia who was not involved in the study, says she likes that Yuan and his team are trying to better understand deep-sea isopods through their genes. Living in the deep sea, isopods are hard to find and harder to study in the field. “I think getting into the basic genetics is going to be a big player in understanding the underlying reasons for gigantism,” she says.
Weinnig hopes the finding reminds people that beyond their potential to help make sense of a scientific quandary, deep-sea species deserve the limelight.
“We lose track of how incredible it is that these animals live on our planet,” says Weinnig. “They have to be resourceful across the board … with reproduction, with metabolic processing. Everything has to be used so that nothing is going to scrap.”
注释：
isopod: n
表示" 等足类的动物"，means "any of various small terrestrial or aquatic crustaceans with seven pairs of legs adapted for crawling"，如：The isopod pictured above was one of these needles, found by a team surveying waters around Antarctica. 图中在南极洲附近海域发现的小家伙就是这样的“一根针”。
abound: v
表示" 富于；充满"，means ""，如：Most of her poems abound in imagery. 她的诗歌大多数富于形象。
wither: v
表示"凋谢"，means "wither, as with a loss of moisture"，如：Truth is a flower in whose neighborhood others must wither. 真理是一朵花，在它附近的花必然凋谢。
sequence: v; n
表示" 按顺序排好; 顺序"，means "arrange in a sequence；the order in which things or events follow one another"，如：Please sequence the names in alphabetical order. 请按字母顺序排列好名单。
genome：n
表示" 基因组；染色体组"，means "the ordering of genes in a haploid set of chromosomes of a particular organism; the full DNA sequence of an organism;"，如：Scientists have decoded the dog genome. 科学家已经译解了狗的基因组。
crustacean: n
表示" 甲壳类动物"，means "any mainly aquatic arthropod usually having a segmented body and chitinous exoskeleton"，如：Every crustacean has a layer of shell, which is also called the carapace. 甲壳动物体外都有一层几丁质外壳, 称为甲壳。
roly-poly: n; adj
表示" 果酱布丁卷;矮胖子或圆滚滚的东西; "，means "pudding made of suet pastry spread with jam or fruit and rolled up and baked or steamed;short and plump"，如：She's a real roly-poly. 她可真是又矮又胖。
chihuahua: n
表示" 吉娃娃（一种产自墨西哥的狗)"，means "an old breed of tiny short-haired dog with protruding eyes from Mexico held to antedate Aztec civilization"，如：How could a chihuahua kill a rottweiler? 吉娃娃怎么可能会要了罗特韦尔的命？
bathynomids: n
表示"巨大等足目"
jumbo: n; adj
表示" 喷气式飞机; 特大的"，means "of great mass; huge and bulky"，如：Jumbo jets somehow lack the glamour of the transatlantic liner. 大型喷气式飞机不像横渡大西洋的客轮那么引人注目。He bought a jumbo packet of washing-powder. 他买了一包特大型包装的洗衣粉。
fathom: n
表示" 英寻（=6英尺)"，means "a linear unit of measurement (equal to 6 feet) for water dept"，如：The ship sank in four fathoms.这条船沉下四英寻。
hefty: adj
表示" 重的；肌肉发达的"，means "(of a person) possessing physical strength and weight; rugged and powerful;"，如：He aimed a hefty kick at the door.他重重地踢在门上。Richard is a really hefty guy. 理查德是肌肉发达的男孩。
anomalocarid: n
表示"异甲酰亚胺"
arthropod: n
表示"节肢动物"，means "invertebrate having jointed limbs and a segmented body with an exoskeleton made of chitin"，如：The arthropod species and individuals at the jujube yard with weeds were considerably higher than those without weeds. 有杂草的枣园，节肢动物各类群的物种数明显高于无杂草的冬园，并且优势种不明显。
transposable: adj
表示"可换位的,"，means "capable of changing sequence"，如：transposable code 可易位密码
invertebrate: n
表示" 无脊椎动物"，means "any animal lacking a backbone or notochord"，如：Endemism of vertebrates and invertebrates is also very high.公园里的本地特有的脊椎动物和无脊椎动物也很多。
vertebrate: n
表示" 脊椎动物"，means "animals having a bony or cartilaginous skeleton with a segmented spinal column and a large brain enclosed in a skull or cranium"，如：All mammals are umbilicate vertebrates. 所有的哺乳类动物都是有肚脐的脊椎动物。
baffle: v
表示"难倒；阻碍"，means " cause to have difficulty in understanding and confuse so much that effective action is impossible"，如：One of the exam questions baffled me completely. 有一道试题把我完全难住了。
gobble: v
表示" 狼吞虎咽"，means "eat hastily without proper chewing;"，如：As he was still hungry, he gobbled up a second sandwich. 因为还饿，他又狼吞虎咽地吃了第二块三明治。
thyroid: n
表示" 甲状腺"，means "located near the base of the neck"，如：Iodine tends to localize in the thyroid.碘容易集于甲状腺。
insulin: n
表示" 胰岛素"，means "hormone secreted by the isles of Langerhans in the pancreas;"，如：They were all dependent on supplemental insulin.他们都依赖于外源胰岛素。
tweak: v
表示" 拧；扭"，means "pinch or squeeze sharply"，如：She tweaked his ear playfully. 她拧他的耳朵玩儿。
scrap: n; v
表示"废品；报废"，means " worthless material that is to be disposed of；dispose of (something useless or old)"，如：The cars have been broken down for scraps. 那些汽车已经拆掉当废铁。You ought to scrap the old bike and buy a new one. 你应该扔掉这辆旧自行车，买一辆新的。
中文简要说明：
等足目（Isopods）大多小小的，那深海的大王具足虫是如何变得如此巨大，中国大陆科学家们深入研究了大王具足虫属（Bathynomus）的基因组，找寻深海生物为何能变得如此巨大的原因。研究发现大王具足虫基因组特别的大，而且拥有大量的跳跃基因，这就是巨大化的关键。因为，跳跃基因越多表示突变率越高，使它们更能应对深海环境压力。而其它的深海生物也有相似的庞大基因组，因此这个发现也成为其他大型深海生物演化的关键线索。
据《HAKAI Magazine》报导，生物要在深海中的生存十分具有挑战性：黑暗的环境、温度接近冰点，食物来源稀少。然而，许多深海生物并未在恶劣环境中凋零，相反地，它们进化出超大型的体型，与淡水或陆地上的亲属相形见绌，从巨大的蜘蛛蟹到巨型乌贼。为什么这些生物会变得如此巨大已经引起科学家们一个多世纪的兴趣。
如今，科学家们正逐渐接近答案。去年8月，北京中国科学院海洋研究所袁剑波院士所领导的团队，对大王具足虫属的基因组进行研究，这也是首次对一种深海甲壳类生物进行此项研究。巨型等足目具有圆形分节体，看起来就像圆圆小小的球潮虫，但是，大王具足虫却可以长到和一只吉娃娃犬一样长、一样重。
透过大王具足虫的基因码，可以让科学家们更好地了解深海巨大化现象在基因层面上发生了什么。
巨大等足目（Bathynomids）是朽木下面徘徊的甲壳类生物的巨型表亲。虽然最小的等足目物种只有不到半厘米长，但是巨大等足目可以长到它们的80倍大。已知的等足目物种超过1万种，从海底到洞穴再到山顶的各个地方。这样的生态多样性，使得等足目族谱成为寻找适应深海生活的最佳线索。
袁剑波表示，巨大等足目的基因组，显示出还是它们是在最近在深海生活的压力下进化而来，而不是远古大型节肢动物演化而来。
而巨大化的关键就在于，巨大等足目的基因组也非常庞大。研究人员发现，该研究对象詹姆斯深水虱（Bathynomus jamesi）拥有大量的跳跃基因（Jumping Genes），这些转位子从一个位置跳到另一个位置，因此，跳跃基因越多，表示突变率越高，研究人员认为就是因为这样，等足目更能应对环境压力。
拥有大量基因是詹姆斯深水虱与其他深海无脊椎动物共同特点。无脊椎动物——一般被认为是比脊椎动物更简单的生物，但是他们却已经进化出一些最为复杂、适应力强的基因组，这让科学家们自基因定序开始就感到困惑不解。
除了发现其基因组特别的大之外，科学家们对詹姆斯深水虱的生物学和基因学的探索还提出了它们在深海中繁荣生存的一些关键适应能力。詹姆斯深水虱的胃可以扩展到占据其身体的3分之2的空间。这确保了当它能找到食物时，可以尽可能地吃到饱。袁剑波和团队还在基因中发现了与甲状腺和胰岛素功能相关的变化，这增强了它们生长和吸收营养物质的能力。此外，他们还发现了一个可以减缓脂肪分解的调整。保留额外的储备能量可以让大王具足虫属（Bathynomus）多年不进食。
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