当只有坚固的贝壳才能幸存：考古学将牡蛎壳转化为保护工具的新方法

随着全球牡蛎数量减少和渔业崩溃，考古学家或许能够从人类与牡蛎之间长达数千年的联系这一宝贵的长期视角，为有效的管理提供信息。

　　

要点

随着全球牡蛎蛎数量减少和渔业崩溃，考古学家或许能够从人类与牡蛎蛎之间长达数千年的联系这一宝贵的长期视角，为有效的管理提供信息。

由于牡蛎蛎壳形状不规则且易碎，因此很难精确测量考古遗址中发现的牡蛎蛎壳。传统上，研究人员只测量完整的左壳，而忽略碎片。

佛罗里达自然历史博物馆和史密森国家自然历史博物馆的研究人员在一项新研究中表明，贝壳碎片很重要。将它们纳入计算可以提高考古学家对过去牡蛎蛎种群的了解。

我们曾过度捕捞牡蛎蛎，围绕牡蛎蛎建立经济，在某些地方甚至几乎将它们从海岸线上彻底抹去。如今，全球85%的牡蛎蛎礁已经消失。许多渔业正在崩溃，佛罗里达州也不例外：该州许多河口的牡蛎蛎礁损失高达90%。科学家和环保人士正争分夺秒地重建可持续的牡蛎蛎种群，而原住民社区曾世代守护着这片土地。通过研究古代牡蛎蛎种群，考古学家或许能够指导当今的牡蛎蛎礁修复工作。

“我们知道，过去发生的事情会影响现在和未来，无论是五分钟前、五年前还是五千万年前。时间中存在着有形和无形的联系，而这些关于牡蛎蛎的考古视角可以作为我们了解现状的基准，”佛罗里达自然历史博物馆南佛罗里达考古与民族学以及加勒比考古收藏的副馆长米歇尔·勒费弗尔说道。

北美东部沿海咸淡水交汇处出产的东部牡蛎蛎（学名： Crassostrea virginica）如今已成为美味佳肴，但它曾是佛罗里达州墨西哥湾沿岸原住民的主食。散布在佛罗里达州各地的贝丘遗址——古代社群堆积的贝壳、骨骼和其他杂物——便是这段历史的见证。通过研究这些贝丘遗址，考古学家可以了解数百或数千年前牡蛎蛎种群的健康状况，以及当时人们将贝壳堆放在贝丘上的情景。

考古学家最常用的指标之一是贝壳大小，它可以作为年龄的替代指标（年龄的测量更为复杂且耗时）。通过追踪贝壳大小随时间的变化，科学家可以了解当时人们捕捞牡蛎蛎的强度。例如，平均贝壳尺寸的减小可能表明牡蛎蛎的捕捞速度超过了其种群的自我维持能力。

但这种方法也有其局限性——主要原因在于牡蛎蛎壳本身。东部牡蛎蛎壳呈淡白色至灰色，表面布满脊状突起，形状多为泪滴状，但发育并不均匀。

“牡蛎蛎之所以如此引人入胜，部分原因在于它们具有很强的生态可塑性，”史密森学会北美考古馆馆长托本·里克说道。“它们能够适应环境，因此它们的生长方式会受到水流、沉积物、栖息地和水中营养物质的影响。所有这些变化都会颠覆人们对牡蛎蛎的测量和认知。”

如果贝壳完好无损地被发现，这不成问题，但对于考古学家来说，这种情况往往并不常见。 

无论是加工过程中还是在贝丘中埋藏数千年后，牡蛎蛎壳往往都会出现裂纹或破碎。如果牡蛎蛎壳的发育过程是均匀的，考古学家只需测量破碎的牡蛎蛎壳，将数据代入数学模型，就能估算出完整牡蛎蛎壳的大小。但由于牡蛎蛎形状的不规则性，科学家们至今仍无法找到破碎牡蛎蛎壳与其完整尺寸之间的对应关系。

如果没有这种模型，考古学家面对一堆破碎的贝壳时，通常会试图通过只收集和测量完好的贝壳来规避问题。但这样做可能会带来一系列完全不同的挑战。

“考古学家通常会保留完整的贝壳，而忽略碎片化的问题。但很多藏品中，超过一半的贝壳都是破碎的。这意味着你面对的样本远非完整。我们想知道这是否是一个有偏差的样本，”佛罗里达自然历史博物馆佛罗里达考古、生物考古和陶瓷技术实验室馆长尼尔·沃利斯说道。

There is a risk that by focusing only on the whole shells that “survived” intact, scientists are overlooking important data in the fragments. In reality, size and breakage might be correlated. It’s possible, for example, that larger shells are more likely to remain intact, while smaller ones tend to break apart and are left out of the analysis.

“It’s like we’re trying to reconstruct a 1,000-piece puzzle, but we only have five of them to work with. There are a lot of holes,” Rick said.

To test whether fragmentation does affect research outcomes, archaeologists conducted a case study using oyster shells excavated from two midden sites in Florida: Calusa Island Midden and Garden Patch.

Calusa Island, composed largely of its midden, is a small stretch of conservation land north of Pine Island. This area — Pineland — was once home to one of the largest Calusa communities in southwest Florida. With deposits here dating back about 4,000 years, it provides a unique look at the Late Archaic period all the way until the most recent deposits, around A.D. 1200.

While in some cases, middens were simple refuse heaps to toss waste, others held strategic or ceremonial importance. At some sites, the Calusa used shells to build elevated platforms on which they constructed their homes. Living atop these shell mounds would have offered both a defensive vantage point and protection from storm surges.

Farther north in the state, Garden Patch served as a ceremonial center for about three centuries. The early village features several mounds arranged in a horseshoe shape around an open plaza. The 6-foot-tall midden analyzed in this study was formed about 1,600 years ago and seems to have been used seasonally. For some 200 years, local community members and guests likely gathered for winter feasts and rituals, tossing shells and bone scraps into the pile.

At both sites, archaeologists collected samples from the middens, systematically digging bagfuls of whole and fragmented shell to take back to the lab for analysis. During fieldwork, one practical reason for favoring whole shells — a metric that is often collected while still in the field — quickly became obvious. Bags of midden deposit are heavy, cumbersome and time consuming to analyze. Researchers hauled the samples nearly 2 miles from the Garden Patch site to a paved road, sometimes through shin-deep water. The Calusa Island midden is surrounded by water, which meant researchers had to juggle weight limits while cramming onto small boats. Getting the shells back to the lab was one thing; analyzing them was another.

“The amount and volume within an oyster shell midden can be remarkable. We took years to sort through and do the physical analysis,” LeFebvre said. “Logistics and capacity are big challenges. It may sound obvious, but it takes time, money and people power to do everything.”

After two years of meticulously sorting through the samples — identifying the eastern oysters, counting whole shells and fragments with hinges, and weighing everything for each site — the researchers could begin their calculations. The goal was to determine whether including fragmented shells would significantly alter the average size of the oyster population compared with calculations based solely on whole shells. To test this, they found the sample’s average shell size following the traditional method of only measuring whole shells. Then, they did the calculations again, this time using both the whole shells and the fragments (plugging the weight of the fragments into a mathematical model gives scientists an estimated average size of the shells before they were broken).

In the end, their suspicions were confirmed. Including the fragments shifted the sample’s average shell size, which means whole shells alone weren’t enough to accurately represent the entire population. In one case, the difference was significant enough to flip the interpretation on its head. Rather than increasing, oyster size had decreased over time. If archaeologists only analyzed whole shells at the site, they would draw false conclusions about how past communities were managing oysters.

“This is the first study I know of that has taken the question of fragmentation head-on, showed some concrete results and pointed to the future,” Rick said. “We all measure size. We know it’s an important metric, but we also all know it’s flawed. Now we’re going to take a crack at trying to understand those flaws and how bad they might be."

The results are a call to action for archaeologists. “What we came up with is not a solution to the problem. It’s a way to recognize the problem,” Wallis said. “It is a call for archaeologists to listen up, discuss, reflect and get to work on better metrics.”

Oysters are keystone species, actively shaping their habitat and supporting fellow marine life. In optimal conditions, a single oyster can filter nearly 2 gallons of water per hour, removing bacteria, minerals and excess nutrients. Clustered together in reefs, oysters form complex structures that provide habitat for fish and other animals. Over 30 species classified as Florida’s Species of Greatest Conservation Need are linked to the habitat or food sources provided by oysters. They play an outsized role not only in their ecosystem, but also in the culture and economy of the communities that rely on them. Their reefs also serve as natural barriers, anchoring the shoreline and protecting towns from powerful hurricane swells. In some of these towns, oyster harvesters have worked the reefs for generations; their jobs and income disappear along with the animal.

恢复牡蛎蛎种群意味着更清洁的海湾、繁荣的野生动物、可持续的渔业，以及一个既能享用牡蛎蛎又能保护牡蛎蛎的未来。在某些情况下，它还能为原住民社区提供机会，让他们重新掌控这片土地。这些原住民社区的祖先曾经生活在如今考古学家正在研究的海岸和河口地区。

“对许多社区来说，他们的祖先捕捞牡蛎蛎的历史已有数千年之久，”里克说道，他近期的研究主要集中在切萨皮克湾。“在这些地点，人们正努力确保这些社区不仅参与保护他们的历史，也参与未来的牡蛎蛎管理。这些地点及其中的牡蛎蛎可以帮助部落民族成为这些地区环境管理和保护的利益相关者。”

但恢复牡蛎蛎种群绝非易事。这项工作需要自然资源保护主义者、当地社区和科学家之间的合作——部分工作还需以考古研究为指导。

尽管如此，里克依然保持乐观。“我从一件非常简单的事情中看到了成功的迹象：人们在倾听，而且他们很关心。如果考古学能够激励人们成为更好的守护者，并将自己视为这个更大系统的一部分，那么从某种意义上说，我们已经成功了。”

这项研究发表在《考古科学杂志》上。

佛罗里达自然历史博物馆的 Cristina Oliveira、Alisa Luthra、Jennifer Green 和 Aditi Jayarajan 也是这项研究的共同作者。